Download Recent zoonoses caused by influenza A viruses

Rev. sci. tech. Off. int. Epiz., 2000, 19 (1), 197-225
Recent zoonoses caused by influenza A viruses
D.J. Alexander & I.H. Brown
Virology Department, Veterinary Laboratories Agency-Weybridge, New Haw, Addlestone, Surrey KT15 3NB,
United Kingdom
Summary
Influenza is a highly contagious, acute illness which has afflicted humans and
animals since ancient times. Influenza viruses are part of the
Orthomyxoviridae
family and are grouped into types A, B and C according to antigenic
characteristics of the core proteins. Influenza A viruses infect a large variety of
animal species, including humans, pigs, horses, sea mammals and birds,
occasionally producing devastating pandemics in humans, such as in 1918, when
over twenty million deaths occurred world-wide. The two surface glycoproteins
of the virus, haemagglutinin (HA) and neuraminidase (NA), are the most important
antigens for inducing protective immunity in the host and therefore show the
greatest variation. For influenza A viruses, fifteen antigenically distinct HA
subtypes and nine NA subtypes are recognised at present; a virus possesses one
HA and one NA subtype, apparently in any combination. Although viruses of
relatively f e w subtype combinations have been isolated from mammalian species,
all subtypes, in most combinations, have been isolated from birds. In the 20th
Century, the sudden emergence of antigenically different strains in humans,
termed antigenic shift, has occurred on four occasions, as follows, in 1918 (H1N1),
1957 (H2N2), 1968 (H3N2) and 1977 (H1N1), each resulting in a pandemic. Frequent
epidemics have occurred between the pandemics as a result of gradual antigenic
change in the prevalent virus, termed antigenic drift. Currently, epidemics occur
throughout the world in the human population due to infection with influenza A
viruses of subtypes H1N1 and H3N2 or with influenza B virus. The impact of these
epidemics is most effectively measured by monitoring excess mortality due to
pneumonia and influenza. Phylogenetic studies suggest that aquatic birds could
be the source of all influenza A viruses in other species. Human pandemic strains
are thought to have emerged through one of the following three mechanisms:
- genetic reassortment (occurring as a result of the segmented genome of the
virus) of avian and human influenza A viruses infecting the same host
- direct transfer of whole virus from another species
- the re-emergence of a virus which may have caused an epidemic many years
earlier.
Since 1996, the viruses H7N7, H5N1 and H9N2 have been transmitted from birds to
humans but have apparently failed to spread in the human population. Such
incidents are rare, but transmission between humans and other animals has also
been demonstrated. This has led to the suggestion that the proposed
reassortment of human and avian viruses occurs in an intermediate animal with
subsequent transference to the human population. Pigs have been considered the
leading contender for the role of intermediary because these animals may serve
as hosts for productive infections of both avian and human viruses and, in
addition, the evidence strongly suggests that pigs have been involved in
interspecies transmission of influenza viruses, particularly the spread of H1N1
viruses to humans. Global surveillance of influenza is maintained by a network of
laboratories sponsored by the World Health Organization. The main control
measure for influenza in human populations is immunoprophylaxis, aimed at the
epidemics occurring between pandemics.
Keywords
Birds - Ecology - Influenza - Interspecies transmission - Mammals - Pandemics Prophylaxis - Public health - Reassortment - Zoonoses.
Rev. sci. tech. Off. int. Epiz., 19(1)
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Introduction and history
Influenza in h u m a n s is a highly contagious, acute, primarily
respiratory illness. T h e s y m p t o m s and epidemiological
characteristics are sufficiently distinct that accounts of
e p i d e m i c s c a n b e dated b a c k to ancient times. T h e s e accounts
s h o w clearly the occurrence of fairly regular e p i d e m i c s in
m o s t populations, with occasional severe e p i s o d e s in w h i c h
the disease not only a p p e a r e d to b e m o r e serious and to infect
a larger proportion of the population, but also a p p e a r e d to
spread world-wide, thus reaching p a n d e m i c status. T h e three
types of influenza virus, namely: A, B and C, are all capable of
causing infections of h u m a n s . Influenza type C viruses are
relatively mild, causing only c o m m o n cold-like s y m p t o m s .
Influenza type B viruses cause significant flu-like illnesses
w h i c h are nevertheless milder than influenza type A
infections. Only influenza A viruses h a v e b e e n k n o w n to
cause the devastating infections w h i c h spread throughout the
world and this review is limited to these viruses.
The first influenza p a n d e m i c for w h i c h m o r e than descriptive
reports exist, was that of 1889. Retrospective research h a s
partially identified the virus responsible b y testing for
influenza antibodies in the serum of p e o p l e w h o w e r e alive at
that time (174). By far the worst p a n d e m i c of those recognised
to date b e g a n in 1918. During the p a n d e m i c , an estimated 2 0
to 4 0 million p e o p l e died. In well-developed countries, s u c h
as the United States of America (USA), approximately 0.5% of
the population died. However, in s o m e communities in
Alaska and the Pacific islands, half the population p e r i s h e d
(85). Subsequent influenza p a n d e m i c s occurred in 1957,
1968 and 1977; while these resulted in far less loss of life, in
world-wide terms, than the 1918 p a n d e m i c , significant
deaths did occur and the impact o n society in terms of
morbidity and consequent e c o n o m i c factors was e n o r m o u s .
During the interval b e t w e e n these p a n d e m i c s , lesser
epidemics occurred in m o s t h u m a n populations, often with
significant impacts o n those populations in terms of
morbidity, mortality and e c o n o m y .
Despite the importance of influenza as a disease of humans,
the earliest recognition of influenza virus in animals related to
infections of poultry. A disease capable of causing extremely
high mortality in infected domestic fowl was first defined in
1878 and b e c a m e k n o w n as 'fowl plague'. As early as 1 9 0 1 ,
the causative organism of this disease was s h o w n to b e an
ultra-filterable agent (i.e. a 'virus'), although not until 1955
was the close relationship b e t w e e n this agent (and other,
milder, viruses isolated from birds) and m a m m a l i a n influenza
A viruses demonstrated (137).
Isolation of influenza virus as the causative organism of a
disease termed 'swine influenza', w h i c h was applied to a 'new'
disease of pigs with clinical signs similar to those in h u m a n s
first described at the time of the 1918 h u m a n p a n d e m i c (39),
also p r e c e d e d the isolation of h u m a n influenza virus. In the
late 1920s, S h o p e demonstrated the ability to transmit swine
influenza b e t w e e n pigs using ultrafiltered material (148). A
virus demonstrated to b e related to the pig virus was
eventually isolated from a h u m a n patient in 1 9 3 3 , b y
inoculating a filtrate of throat washings into the n o s e s of
ferrets (160).
The demonstration, in 1955, that 'fowl plague' virus was an
influenza A virus, was followed in 1956 b y e v i d e n c e that an
emerging respiratory disease in h o r s e s was also caused b y an
influenza type A virus (69, 137, 162). In the next two years,
respiratory disease caused b y this virus in h o r s e s b e c a m e
w i d e s p r e a d in Europe. T h e similarities and association of
these viruses was n o t e d b y influenza scientists, a n d the W o r l d
Health Organization ( W H O ) encouraged and co-ordinated
w o r k o n the e p i d e m i o l o g y of animal viruses, particularly in
relation to h u m a n influenza (84). However, the vast reservoirs
of influenza viruses that exist in animals, particularly birds,
w e r e not fully discovered until the 1970s.
Importance for public and
animal health
Public health
Although infection with influenza A viruses m a y b e
considered a serious illness with potentially
fatal
c o n s e q u e n c e s for susceptible individuals, the incidence of
influenza A infections in h u m a n populations varies
significantly from year to year. Incidence ranges from years of
devastating p a n d e m i c s to years w h e n the n u m b e r of cases in a
given population d o e s not reach e p i d e m i c status.
O n average, e a c h individual case of influenza in the USA is
associated with an estimated five to six days of restricted
activity, three to four days of b e d disability and approximately
three days lost from w o r k or s c h o o l (140). T h e overall effect
o n public health will therefore b e generally related to the
n u m b e r of individuals that b e c o m e infected, although this in
turn m a y b e related to the degree of immunity of individuals
in the population w h i c h c a n also m o d i f y the severity of the
clinical signs. Clinical attack rates during an e p i d e m i c are
difficult to estimate d u e to the p r o b l e m s in m a k i n g a clinical
diagnosis, especially w h e n cases of influenza A infection occur
at a time w h e n other respiratory viruses, including influenza
type B, m a y b e circulating. In the United K i n g d o m (UK)
during the 1918 p a n d e m i c , an estimated 2 3 % of the
population d e v e l o p e d influenza, in 1957 and 1958, 17%
(8 million cases) and in 1969 and 1970, 8% (10). During the
p a n d e m i c of 1969 and 1970, serological evidence suggested
that a larger proportion of the population of the UK was
infected, presumably often subclinically. In interpandemic
years, clinical strike rates are highest in the elderly, although
the highest infection rates are in s c h o o l children. However, in
the p a n d e m i c s of 1918-1919 and 1957, the groups showing
Rev. sci. tech. Off. int. Epiz., 19 (1)
the highest proportions of clinical disease w e r e children u n d e r
fourteen years o l d (10).
Influenza A infections significantly affect overall mortality in
human populations, and until the occurrence of h u m a n
immunodeficiency virus (HIV) infections, w e r e the only
viruses to h a v e this effect. Excess mortality d u e to p n e u m o n i a
and influenza (P&I) is usually a reliable m a r k e r for assessing
the beginning and e n d of an influenza e p i d e m i c , although this
is probably an underestimation of influenza-associated deaths
(158). Although mortality rates during the 1918 p a n d e m i c
were extremely h i g h (3,000 per 1,000,000 in 1918 and 1,170
per 1,000,000 in 1919 in England and W a l e s [10]),
significant mortality also occurs during the interpandemic
epidemics. T h e c o m b i n e d p a n d e m i c s of 1957 and 1968, in
the USA, a c c o u n t e d for approximately 9 8 , 0 0 0 excess deaths;
however, the e p i d e m i c s from 1957 to 1975, excluding the
pandemic years, accounted for over twice that n u m b e r of
excess deaths (41). T h e impact of the p a n d e m i c s is amplified
by the difference in the distribution of deaths b y age group
c o m p a r e d to the interpandemic e p i d e m i c s . Simonsen et al.
observed that during the p a n d e m i c years of 1 9 6 8 - 1 9 7 0 ,
approximately 5 0 % of the excess P&I deaths in the USA w e r e
recorded in p e o p l e less than 65 years old. In contrast, over the
entire d e c a d e , only 0%-7% of the excess P&I deaths occurring
each year w e r e in the u n d e r 65 age group (158). Similar
figures w e r e obtained for the p a n d e m i c of 1957. Thus, while
the 1918 p a n d e m i c was particularly n o t e d for greater
mortality in young adults than in the m o r e elderly, this m a y
be a feature of p a n d e m i c s in general.
Human influenza e p i d e m i c s m a y also affect public health as a
result of the s o c i o - e c o n o m i c impact and the m o n o p o l i s a t i o n
of healthcare resources during an e p i d e m i c . In the past,
pandemics and significant e p i d e m i c s have h a d e n o r m o u s
effects o n hospital resources. During the e p i d e m i c caused b y
the 1957 p a n d e m i c virus, hospital admissions in the UK rose
by as m u c h as 2 5 0 % in s o m e areas, while at the p e a k of the
epidemic, u p to o n e third of nurses w e r e absent from w o r k
(10).
Influenza infections m a y leave a legacy o n c e the e p i d e m i c or
pandemic is over. Ravenholt and F o e g e h a v e a d v a n c e d a
compelling argument that the global p a n d e m i c of a
Parkinson-like disease t e r m e d encephalitis lethargica, affecting
more than 1,000,000 p e o p l e b e t w e e n 1919 and 1928, was a
direct c o n s e q u e n c e of the influenza p a n d e m i c of 1919 (128).
Animal health
Pigs
The disease caused b y influenza viruses in pigs is essentially
similar to that r e c o r d e d in h u m a n s , although generally
somewhat milder, consisting of an acute febrile, respiratory
disease characterised b y fever, apathy, anorexia, coughing,
laboured breathing, sneezing, nasal discharge, l o w mortality
rate and rapid recovery, usually five to s e v e n days after the
onset of clinical signs (148, 115, 4 3 ) . T h e extremely h i g h
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morbidity h a s serious e c o n o m i c c o n s e q u e n c e s d u e to the
increased time n e e d e d to attain slaughter weight as a result of
the influenza virus infection. T h e cost h a s b e e n estimated at
u p to £7 per pig, accounting for a financial loss to the pig
industry in the U K of approximately £ 6 5 million e a c h year
(89).
Horses
Equine influenza is a clinical disease of horses, d o n k e y s and
m u l e s caused b y influenza A viruses of subtypes H 7 N 7 and
H3N8. T h e disease in fully susceptible animals is similar to
that s e e n in pigs and h u m a n s , presenting as a severe
respiratory infection with a harsh c o u g h , nasal discharge and
pyrexia. Infections with subtype H 3 N 8 are generally
considered to b e m o r e severe than H7N7 infections (65).
W h e n e q u i n e influenza has b e e n introduced into susceptible
populations, characteristic rapid spread and h i g h morbidity
h a v e occurred.
In highly d e v e l o p e d countries, equine species are largely k e p t
for sport or as c o m p a n i o n animals. Thus, while influenza
infections can b e a nuisance or, for the h o r s e racing industry,
an e c o n o m i c p r o b l e m , the disease is m a n a g e d largely b y
vaccination and b y resting clinically infected animals (111).
However, in m a n y p o o r e r parts of the world, horses, d o n k e y s
and m u l e s remain the principal w o r k i n g animals. As a
c o n s e q u e n c e , vaccination and rest m a y not b e an option, and
this in turn, results in m o r e severe disease, often with
secondary bacterial infections. In relatively recent years, s u c h
severe infections have occurred three t i m e s , in India, in 1987
(176), and in the People's Republic of China from 1989 to
1990 (60) and from 1993 to 1 9 9 4 (154). In e a c h case, the
disease spread rapidly and widely. In the 1 9 9 3 - 1 9 9 4 o u t b r e a k
in the People's Republic of China, an estimated 2,245,000
horses w e r e affected clinically and 2 4 , 6 0 0 (1%) died (154). In
the e p i d e m i c of 1 9 8 9 - 1 9 9 0 in the north-east of the People's
Republic of China, morbidity was estimated at 8 1 % , with
mortality varying in different h e r d s , but reaching 2 0 % or
m o r e in s o m e (60).
Birds
In birds, the clinical signs and disease o b s e r v e d following
infection with influenza A viruses vary according to the host
species, age, the presence of o t h e r micro-organisms and
environmental
factors. In susceptible avian species,
u n c o m p l i c a t e d infections can b e divided too t w o forms b a s e d
o n the severity of the clinical disease p r o d u c e d . T h e very
virulent viruses cause a disease formerly k n o w n as fowl plague
and n o w t e r m e d highly pathogenic avian influenza (HPAI), in
w h i c h mortality m a y b e as h i g h as 1 0 0 % . T h e s e virases have
b e e n restricted to subtypes H5 and H7, although not all
viruses of these subtypes cause HPÄ1. Seventeen primary
isolates of such viruses h a v e b e e n reported in domestic
poultry since 1959 (5). Apart from s u d d e n onset of mortality,
clinical signs associated with HPAI vary enormously and n o n e
are p a t h o g n o m o n i c . Generally, all or s o m e of the following
m a y b e o b s e r v e d : cessation of egg laying, respiratory signs,
200
excessive lachrymation, sinusitis and, m o r e characteristically,
o e d e m a of the h e a d face and n e c k and cyanosis of the
unfeathered skin (46). Infections of poultry with HPAI are
often self-limiting or rapidly controlled b y slaughter.
However, o n three occasions in recent years, in Pennsylvania,
USA, from 1983 to 1984 (187), in Mexico from 1994 to 1995
(31, 179) and in Pakistan from 1994 to 1995 (113), the
viruses have b e c o m e w i d e s p r e a d in c h i c k e n s and have h a d
severe implications for animal health and the e c o n o m y of the
country or region in w h i c h they occurred. T h e impact of these
outbreaks is best m e a s u r e d in losses of birds and financial
terms. For example, in the e p i d e m i c in Pennsylvania, m o r e
than 17,000,000 birds w e r e slaughtered or died. Lasley
estimated that this cost the Government of the USA
US$60,000,000, with a further US$15,000,000 b o r n e b y the
farmers, and that subsequent rises in f o o d prices cost the
consumer US$349,000,000 (99).
All other viruses cause a m u c h milder disease consisting
primarily of respiratory disease, depression and egg
production p r o b l e m s in laying birds. In addition, these n o n
pathogenic avian influenza viruses m a y replicate in the
epithelial cells of the intestine of birds without inducing signs
of disease (83), although virus m a y b e s h e d in high
concentrations in the faeces (159, 8 3 , 90). Infections such
as Pasteurella
multocida,
avian pneumovirus,
avian
paramyxovirus type 3, infectious bronchitis virus and e v e n
live bacterial or virus vaccines or environmental conditions
m a y result in significant severe disease, s o m e t i m e s with
mortalities sufficiently h i g h to b e mistaken for HPAI. Typical
of these extremes was the infected turkey b r e e d e r flock
reported b y Alexander and S p a c k m a n as infected with virus of
H7 subtype, in w h i c h the only sign was 2 % white, m i s s h a p e n
eggs (7); c o m p a r e d to the outbreaks caused b y a virus of
H4N8 subtype in flocks of c h i c k e n s in Alabama, USA, in
1975, in w h i c h u p to 6 9 % mortality was reported, including
one flock with 3 1 % mortality in a single day (80). Viruses of
low pathogenicity m a y b e c o m e w i d e s p r e a d in a given area,
incurring substantial e c o n o m i c losses. During 1995, 178
turkey farms in Minnesota, USA, were affected b y influenza A
virus of H 9 N 2 , resulting in the worst e c o n o m i c loss due to
influenza infections r e c o r d e d in a single year in Minnesota
(approximately US$6,000,000) (63).
Aetiology
Taxonomy
The influenza viruses are m e m b e r s of the
Orthomyxoviridae
family w h i c h includes three genera, o n e consisting of types A
and B influenza viruses, a s e c o n d containing type C influenza
viruses, and a third containing 'Thogoto-like viruses' (78).
Influenza viras types A, B and C infect h u m a n s but, e x c e p t for
occasional reports, infections of other animals are restricted to
type A influenza viruses. Only influenza A viruses have b e e n
isolated from birds.
Rev. sci. tech. Off. int. Epiz., 19(1)
Influenza A virus particles appear roughly spherical or
filamentous, with diameter of 80 n m to 120 n m . The
nucleocapsid s h o w s helical symmetry and is e n c l o s e d within a
protein matrix. External to the matrix is a lipid m e m b r a n e , the
surface of w h i c h is c o v e r e d b y two types of glycoprotein
projections, or spikes, with w h i c h haemagglutinin (HA) and
neuraminidase (NA) activities are associated. T h e s e two
surface glycoproteins, particularly the HA, appear to b e the
most important antigens in terms of stimulation of protective
immunity in the host. Consequently, considerable antigenic
variation is s e e n in these p o l y p e p t i d e s while other
polypeptides are antigenically m o r e stable.
T h e classification and nomenclature of influenza viruses take
account of the antigenic variation that exists (193). Influenza
viruses are g r o u p e d into types A, B and C o n the basis of the
antigenic nature of the internal nucleocapsid or the matrix
protein. B o t h these antigens are c o m m o n to all viruses of the
s a m e type (138). Viruses of influenza A type are further
divided into subtypes o n the basis of the HA and NA antigens.
At present, fifteen HA (H1-H15) and nine NA (N1-N9)
subtypes are k n o w n ; a virus possesses o n e HA and o n e NA
subtype, apparently in any c o m b i n a t i o n (72). T h e system of
nomenclature for influenza viruses consists of several
c o m p o n e n t parts. T h e information is presented as follows:
type, host of origin (for n o n - h u m a n species), geographical
origin, strain n u m b e r and year of isolation. This is followed by
the antigenic subtype in parentheses, e.g. A/duck/Alberta/
3 5 / 7 6 (H1N1).
Molecular biology
The g e n o m e s of influenza A and B viruses consist of eight
unique segments of single-stranded ribonucleic acid (RNA)
w h i c h are of negative polarity. Influenza C viruses possess
seven segments of RNA. T h e viral RNA is transcribed to
complementary messenger RNA b y a virus-associated
polymerase c o m p l e x (designated PB1, PB2 and PA). T o b e
infectious, a single virus particle must contain e a c h of the
eight unique RNA segments. It is likely that the incorporation
of RNA into the virion is at least partly r a n d o m . T h e r a n d o m
incorporation of RNA segments allows the generation of
progeny viruses containing novel combinations of genes w h e n
cells are concurrently infected with two different parent
viruses. This p h e n o m e n o n is referred to as genetic
reassortment ( 7 4 , 1 8 2 ) .
T h e HA protein is an integral m e m b r a n e protein and the
major surface antigen of the influenza virus virion. This
protein is responsible for binding of virions to host cell
receptors and for fusion b e t w e e n the virion e n v e l o p e and the
host cell. Newly synthesised HA is cleaved to r e m o v e the
amino-terminal h y d r o p h o b i c s e q u e n c e w h i c h is the signal
s e q u e n c e for transport to the cell m e m b r a n e . Carbohydrate
side chains are a d d e d ; the n u m b e r and position of these
chains vary with the virus strain. Fusion activity requires
post-translational cleavage of HA b y cellular proteases into the
disulphide-linked fragments HAI and HA2. This cleavage of
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Rev. sci. tech. Off. int. Epiz., 19(1)
the HA d o e s not affect the antigenic or receptor-binding
properties of the protein (105), but is essential for the virus to
be infectious (93) and is an important determinant in
pathogenicity (134). Molecules of H A form h o m o t r i m e r s
during maturation. T h e three-dimensional structure of the
complete trimer h a s b e e n determined, consisting of a globular
head (HA1) o n a stalk (HA1/2). T h e h e a d contains the
receptor-binding cavity in addition to m o s t of the antigenic
sites of the m o l e c u l e . T h e carboxy-terminus of HA2 anchors
the glycoprotein in the cell or virion m e m b r a n e . T h e HA is
subject to a h i g h rate of mutation d u e to error p r o n e viral RNA
polymerase activity. Selection for a m i n o acid substitutions in
the variants p r o d u c e d is driven at least in part b y i m m u n e
pressure, as the HA is the major target of the host i m m u n e
response. T h e amino acids m a k i n g u p the receptor binding
site are highly conserved but the remainder of the HA
molecule is highly mutable. T h e fifteen subtypes of HA
recognised currently differ b y at least 3 0 % in the a m i n o acid
sequence of HA1 and are not cross-reactive serologically.
Subtypes m a y include several variant strains w h i c h are only
partially cross-reactive in serological assay.
The NA is the s e c o n d major surface antigen of the virus
which, like HA, is an integral m e m b r a n e glycoprotein. T h e
protein functions to free virus particles from host cell
receptors, to enable progeny virions to e s c a p e from the cell in
which they arose, and s o facilitate virus spread. This activity
destroys the HA receptor o n the host cell (27), preventing
progeny virions reabsorbing to the host cell (123). Like HA,
the NA is highly mutable with variant selection driven b y host
immune pressure. T h e nine subtypes identified in nature to
date are not cross-reactive serologically, although variants
within subtypes are partially cross-reactive serologically.
Ecology
Influenza A viruses infect a large variety of animal species,
including h u m a n s , pigs, h o r s e s , sea m a m m a l s and birds
(Tables I and II) (2, 189). Recent phylogenetic studies of
influenza A viruses h a v e revealed species-specific lineages of
viral genes and h a v e demonstrated that the prevalence of
interspecies transmission is d e p e n d e n t o n the animal species
involved (189). In the early 1970s, the W H O initiated
long-term global studies o n the influenza viruses of l o w e r
animals and birds to determine as far as possible w h e t h e r a
finite n u m b e r of influenza A viruses existed in nature and
w h e t h e r it w a s possible to isolate a future p a n d e m i c strain of
virus from a m o n g these viruses, in advance of the appearance
of a n e w strain in h u m a n s . A vast n u m b e r of viruses h a v e b e e n
isolated from a w i d e variety of birds and a range of terrestrial
and sea m a m m a l s . Although an e n o r m o u s diversity of animal
species h a v e b e e n s h o w n to b e susceptible to influenza A virus
infections, three groups of animals appear to b e far m o r e
important in terms of n u m b e r s a n d the e p i d e m i c / e n d e m i c
nature of the disease than other animals, namely: birds, pigs
and horses.
Influenza viruses in humans
Despite the recognition of fifteen distinct haemagglutinin
subtypes (H1-H15) and nine neuraminidase subtypes
(N1-N9), the n u m b e r of subtype combinations found to b e
Table I
Haemagglutinin (H) subtypes of influenza A viruses isolated from humans, pigs, horses and birds
Subtype
Examples(a) of viruses of the subtype isolated from the specified host group
Humans
Pigs
Horses
Birds
m
PR/8/34 (H1N1)
Swìne/lowa/15/30 (H1N1)
_(b)
Duck/Alberta/35/76 (H1N1)
H2
Singapore/1/57 (H2N2)
-
-
Duck/Gennany/1215/73 (H2N3)
H3
Hong Kong/1/68 (H3N2)
Swine/Taiwan/70 (H3N2)
Equine/Miami/1/63 (H3N8)
H4
-Hong Kong/156/97 (H5N1)
-Swine/HK/9/98 (H9N2)
-
Duck/Utaine/1/63fH3H8)
Duck/CzeehoslovakR/56 (H4N6)
H5
H6
H7
H8
-England/268/96 (H7N7)
H9
-Hong Kong/1073/99 (H9N2)
H10
-
H11
-
H12
H13
H14
H15
-
-
-
-
a) The reference strains of influenza viruses, or the first isolates of the subtype from the host group
b) Not found in this host group
FPV: fowl plague virus
-
Tem/S.Africa/61 (H5N3)
Turkey/Massacfousetts/3?40'/S5 (H6N2)
Equine/Prague/1/56(H7N7)
FPV/Dutch/27 (H7N7)
-
Turkey/0ntario/8118/6B (H8N4)
-
Duck/England/56 (H11N6)
-
Duck/Albcrta/60/76(H12N5)
-
Gull/Maryland/7Q4/77 (H13N6)
-
Duck/Gurjev/263/82 (H14N5)
Turkey/Wiscoosin/1/66 (H9N2)
Chicken/Germany/N/49 (H10N7)
Duck/AtJStralia/341/83 (H15N8)
Rev. sci. tech Off. int. Epiz.. 19 (1)
202
Table II
Neuraminidase (N) subtypes of influenza A viruses isolated from humans, pigs, horses and birds
Examples
Subtype
(a)
of viruses of the subtype isolated from the specified host group
Humans
Pigs
Horses
N1
PR/8/34 (H1N1)
Swine/lowa/15/30(H1N1)
-(b)
Chicken/Scotland/59 (H5N1)
N2
Singapore/1/57 (H2N2)
Swine/Taiwan/70 (H3N2)
N3
N4
_
_
_
N6
_
--
Turkey/0ntarlo/6118/68 (H8N4)
N5
-
--
Turkey/Massachusetts/3740/65(H6N2)
N7
England/268/96 (H7N7)
Swine/England/92(H1N7)
Equine/Prague/1/56 (H7N7)
FPV/Dutch/27 (H7N7)
N8
-
Duck/Ukraine/1/63 (H3N8)
N9
-
-
Equine/Mlaml/1/63 (H3N8]
-
Duck/Memphis/546/74 (H11N9)
-
Birds
Tern/S.Africa/61 (H5N3)
Shearwater/Australla/1/72 (H6N5)
Duck/Czechoslovakia/56 (H4N6)
a) The reference strains of influenza viruses, or the first isolates of the subtype from the host group
b) Not found in this host group
FPV: fowl plague virus
prevalent in h u m a n s and other m a m m a l s at any o n e time
appears to b e severely limited. Pandemics of influenza occur
w h e n a virus is introduced w h i c h is fully capable of
replication and spread, and for w h i c h all or a large proportion
of the population have h a d n o immunological experience of at
least the functionally important haemagglutinin. This is
termed antigenic shift and has occurred four times during the
20th Century, e a c h time resulting in a true p a n d e m i c . T h e
first of these shifts was, almost certainly, in 1918, w h e n virus
of H1N1 entered the h u m a n population, resulting in the most
devastating p a n d e m i c k n o w n to m a n . T h e s e c o n d was in
1957, w h e n H 2 N 2 virus appeared. T h e third, in 1968, with a
H 3 N 2 virus and the fourth in 1977, with the re-emergence of
the H1N1 virus. In 1957 and 1968, the introduction of a n e w
influenza virus coincided with the disappearance of the earlier
subtype, but this did not occur in 1977, and currently b o t h
H 3 N 2 and H1N1 viruses are circulating.
The emergence of n e w subtypes of influenza virus and the
ensuing p a n d e m i c s are unmistakable, but the e p i d e m i c s
occurring b e t w e e n the p a n d e m i c s m a y nevertheless b e severe.
These are a result of the gradual change of the antigenicity of
the circulating virus b y the accumulation of point mutations
until the virus is antigenically sufficiently different from earlier
strains that a large proportion of the population is susceptible
and cases reach e p i d e m i c levels. This accumulative variation
is termed antigenic drift, and the size, severity and spread of
the virus will d e p e n d o n the degree to w h i c h the virus is
antigenically different from viruses already experienced b y the
population.
Examination of serum samples taken from elderly p e o p l e
prior to the p a n d e m i c s of 1957 or 1968 has b e e n u s e d to
assess virus subtypes present in h u m a n s before H 1 N 1 .
'Sero-archaeology' studies h a v e indicated that the two
prevalent viruses preceding 1918 w e r e H 2 N 2 in 1889 and
H 3 N 8 in 1900 (189), further confirming the limited range of
subtypes k n o w n to cause influenza e p i d e m i c s in humans.
Influenza viruses in pigs
Swine influenza (SI) was first o b s e r v e d at the time of the
p a n d e m i c in h u m a n s in 1918 and the viruses responsible are
k n o w n to b e closely related, possibly having derived from a
c o m m o n ancestor (130). Although the disease was described
in pigs during the years w h i c h followed, not until 1930 was
the virus isolated and identified. This H 1 N 1 virus was the
prototype strain of a group of viruses n o w k n o w n as classical
viruses, w h i c h h a v e b e e n reported in pig populations
world-wide.
Influenza A viruses of subtypes H 1 N 1 and H 3 N 2 h a v e b e e n
reported widely in pigs and h a v e b e e n frequently associated
with clinical disease. T h e s e include classical swine H1N1,
avian-like H1N1 and h u m a n - and avian-like H 3 N 2 viruses.
Influenza is widespread and e n d e m i c in pig populations
world-wide and is responsible for o n e of the most prevalent
respiratory diseases in pigs. Occasionally, influenza infections
of pigs m a y result in e p i d e m i c s through the introduction of
virus to an immunologically naïve population or due to
significant antigenic drift in the HA or NA of e n d e m i c viruses.
Swine influenza is related to the m o v e m e n t of animals from
infected to susceptible herds, clinical disease generally
appears with the introduction of n e w pigs into a herd. Once a
h e r d is infected, the virus is likely to persist through the
production of young susceptible pigs and the introduction of
n e w stock. Outbreaks of disease occur throughout the year
but usually p e a k in the colder m o n t h s (45). Infection is
frequently subclinical, and often typical signs are s e e n in only
2 5 % to 3 0 % of a herd. Swine husbandry practices influence
directly the evolution of SI viruses through r e d u c e d immune
pressure and constant availability of susceptible hosts,
generally leading to r e d u c e d genetic drift in the genes
encoding HA and NA, c o m p a r e d to those of similar viruses in
the h u m a n population. However, mixing of pigs from
multiple-sources and the h i g h frequency of contact with other
203
Rev. sci. tech. Off. int. Epiz., 19 (1)
species, particularly h u m a n s , provides an opportunity for
co-circulation of viruses and genetic reassortment.
Serological studies of pigs w o r l d - w i d e have s h o w n that
classical swine H1N1 influenza virus is prevalent throughout
the major pig populations, with approximately 2 5 % of
animals demonstrating e v i d e n c e of infection (24, 70).
Classical swine H 1 N 1 influenza virus isolates in the USA h a v e
remained conserved b o t h genetically (104) and antigenically
(146), but viruses w h i c h are antigenically distinguishable,
although closely related, h a v e b e e n reported b y Olsen e£ al.
(120) and W e n t w o r t h et al. (190). In Canada, an H1N1 virus
was associated with a n e w and distinctive pathology in pigs
which a p p e a r e d in 1990. However, the virus was m o s t closely
related to classical swine influenza viruses (131). In Europe,
classical viruses r e a p p e a r e d in 1976 (116), but h a v e b e e n
largely r e p l a c e d since 1979 b y 'avian-like' swine H 1 N 1 viruses
which are antigenically and genetically distinguishable from
the classical swine H 1 N 1 influenza viruses of North America
(125, 142).
Influenza A viruses of H 3 N 2 subtype, related closely to early
human strains, h a v e b e e n widely reported from pigs,
particularly in E u r o p e and Asia w h e r e they continue to
circulate long after disappearing from the h u m a n population.
These viruses, following years of adaptation to pigs, are
associated with clinical signs in pigs w h i c h are typical of SI
(61). Seroprevalence is usually in the range 3 0 % to 5 0 % (98,
132), but can b e as h i g h as 7 5 % (175). This apparently h i g h
level of H 3 N 2 infections in Europe is in sharp contrast to the
low prevalence in pigs in North America. S o m e of the H 3 N 2
viruses isolated from pigs in Asia are entirely avian-like (91),
but repeated introductions from h u m a n s appear to occur
regularly.
The prevailing h u m a n H1N1 strains are transmitted
frequently to pigs and are occasionally associated with
respiratory epizootics in pigs (24, 8 7 ) . However, most strains
are not readily transmitted a m o n g pigs in the field and are not
therefore maintained entirely independently of the h u m a n
population (70).
Influenza A H 1 N 2 viruses, derived from classical swine H 1 N 1
and 'human-like' swine H 3 N 2 viruses h a v e b e e n isolated in
Japan (169) a n d France ( 5 5 ) . In J a p a n , these viruses a p p e a r to
have spread widely within pig populations and are associated
frequently with respiratory epizootics (121). Subsequently, an
H1N2 influenza virus related antigenically to h u m a n and
'human-like' swine viruses h a s e m e r g e d and b e c o m e e n d e m i c
in pigs in the UK, often in association w i t h respiratory disease
(23).
Influenza viruses in horses
Influenza is a c o m m o n disease of h o r s e s throughout the
world. Apart from rare reports of isolations or serological
evidence of infection with other subtypes, only two subtypes
of influenza A virus, H 7 N 7 and H3N8, have b e e n identified as
infecting a n d causing disease in horses. Subtype H 7 N 7
viruses m a y h a v e d i s a p p e a r e d largely from the h o r s e
population, as n o substantiated reports of virus isolated from
horses h a v e b e e n r e c o r d e d since 1980 (180). However, recent
w o r l d - w i d e serological surveillance has suggested that H 7 N 7
m a y b e circulating in Eastern Europe ( 1 1 1 , 106) and Central
Asia (189). Phylogenetic analyses of H 3 N 8 (equine-2) viruses
h a s revealed the existence of two distinct evolutionary
lineages, E u r o p e a n and American, the circulation of w h i c h
was originally centred largely o n the geographical origin
(112). However, in Europe, b o t h lineages n o w appear to b e
co-circulating, with the American type viruses predominant in
s o m e areas (122). T h e s e findings h a v e importance for control,
since vaccines including o n e H 3 N 8 virus type m a y not
provide protection against infection with viruses from the
heterologous group. Recently, equine influenza o u t b r e a k s d u e
to infection with H 3 N 8 virus w e r e o b s e r v e d in South Africa,
India, the People's Republic of China, Hong K o n g a n d
Nigeria, w h e r e e q u i n e influenza viruses w e r e not k n o w n to b e
circulating. Recent outbreaks in the People's Republic of
China h a v e b e e n d u e to b o t h conventional strains of H 3 N 8
virus, and viruses w h i c h contained the s a m e surface antigens
as the other e q u i n e viruses of this subtype, but w h i c h h a d
genetic features that w e r e avian-like, indicating that the virus
h a d b e e n introduced from birds (60, 188). Sporadic
outbreaks typically occur and may, in part, b e d u e to
antigenic drift, w h i c h m a y c o m p r o m i s e the efficacy of the
vaccines available ( 1 8 , 1 1 2 ) .
Influenza viruses in birds
W i l d birds
T h e first isolation of influenza virus from feral birds was in
1961, from c o m m o n terns (Sterna hirundo) in South Africa
(14). However, the e n o r m o u s p o o l s of influenza viruses
w h i c h were present in the wild bird population w e r e not
revealed until systematic investigation of influenza in feral
birds was u n d e r t a k e n in the 1970s, following the 1968
p a n d e m i c in h u m a n s .
Isolations of virus from other wild birds h a v e b e e n completely
o v e r s h a d o w e d b y t h e n u m b e r , variety and w i d e s p r e a d
distribution of influenza viruses i n waterfowl, order
Anseriformes. In the surveys listed b y Stallknecht a n d Shane,
a total of 2 1 , 3 1 8 s a m p l e s from all s p e c i e s resulted i n the
isolation of 2,317 (10.9%) viruses ( 1 6 3 ) . Of these s a m p l e s ,
14,303 w e r e from b i r d s of the o r d e r Anseriformes a n d y i e l d e d
2,173 (15.2%) isolates. T h e next highest isolation rates w e r e
2.9% and 2 . 2 % from the Passeriformes a n d Charadriiformes.
T h e overall isolation rate from all b i r d s o t h e r than d u c k s and
geese was 2 . 1 % . In a three-year study of waterfowl
congregating o n lakes in Alberta, Canada, prior to migration,
Hinshaw et al. found an overall isolation rate of 2 6 % , with
60% of juveniles excreting virus in the first year of the life of
the bird (71). T h e large n u m b e r s and concentration of
waterfowl o n s u c h lakes, c o u p l e d with the facts that d u c k s
h a v e b e e n reported to excrete u p to 1 0 m e a n egg infectious
d o s e s of virus p e r g r a m of faeces (184), and that this virus
8 . 7
Rev. sci. tech. Off. int. Epiz., 19 (1)
204
remains viable for considerable periods in l a k e water (164),
must represent a unique level of viral contamination of a large
environment.
C a g e d ' p e t ' birds
Since 1975, w h e n the first isolates from caged birds w e r e
r e c o r d e d , isolates from all sources have b e e n mainly of H 4 or
H 3 subtypes. T h e majority of influenza viruses from caged
birds c o m e from passerine species and only rarely are
psittacines infected. Although the presence of influenza
viruses in birds h e l d in quarantine is monitored continually in
several countries around the world, periods w h e n n o
isolations have b e e n m a d e h a v e b e e n r e c o r d e d , often lasting
several years.
Africa in 1995, and the Republic of K o r e a in 1996 (9). Since
1997, serious p r o b l e m s associated with H 9 N 2 virus have
b e e n reported in Iran, Saudi Arabia, Pakistan, the People's
Republic of China and other countries of Asia. In Minnesota,
USA, during 1 9 9 5 , 1 7 8 turkey farms w e r e infected with virus
of subtype H 9 N 2 (63).
The influenza status of c o m m e r c i a l d u c k s in m o s t countries is
poorly understood or h a s not b e e n investigated fully.
However, c o m m e r c i a l d u c k s , especially fattening d u c k s , are
generally reared o n p o n d s or fields outdoors and w h e n
surveillance of c o m m e r c i a l d u c k s h a s b e e n undertaken,
e n o r m o u s p o o l s of virus and m a n y subtype combinations
h a v e b e e n detected (3).
D o m e s t i c poultry
At the e n d of the 19th Century and in the early 20th Century,
'fowl plague', the highly pathogenic form of avian influenza,
was often reported in c h i c k e n s and was probably enzootic in
several countries. However, in the s e c o n d half of the 20th
Century, reports of influenza infections of c h i c k e n s have b e e n
rare c o m p a r e d to infections of other domestic poultry, despite
the m u c h higher populations of chickens. For e x a m p l e in the
USA, despite frequent influenza epizootics in turkeys in s o m e
States, b e t w e e n 1964 and 1982, only three outbreaks w e r e
r e c o r d e d in c h i c k e n s (127).
Ratites
T h e increase in trade in ostriches and other ratites during the
1990s led to the m o v e m e n t of large n u m b e r s of s u c h birds
around the world. T h e testing for viruses, including influenza,
has resulted in the regular isolation of influenza viruses from
these birds. Since these birds are mainly k e p t in o p e n fields,
influenza infections p r o b a b l y represent spread from feral
birds.
Despite the l o w incidence of influenza infections of c h i c k e n s
throughout the world, twelve outbreaks of HPAI h a v e
occurred since 1959, with significant spread of infection in
Pennsylvania and neighbouring States of the USA during
1983 and 1984, and in Mexico and Pakistan in 1994 and
1995. Outbreaks since 1959 have s h o w n n o or extremely
limited spread. Eight HPAI outbreaks in b a c k y a r d poultry
flocks infected with H 5 N 2 virus w e r e reported in Italy in
1997 and 1998. Outbreaks of H 5 N 1 HPAI occurred o n three
farms in H o n g Kong from March to May 1997, with
7 0 % - 1 0 0 % mortality and subsequent spread to live bird
markets (35).
Differences in pathogenicity a m o n g influenza viruses result in
the production of a w i d e spectrum of clinical diseases that
range in severity from fatal systemic to mild, sometimes
inapparent, respiratory disease (43, 108). Severity of the
disease is also determined b y the host species infected (6, 8,
77) and in part b y factors s u c h as host age and sex, virus dose,
environment and concurrent infections with other pathogens
(46).
Turkeys tend to b e reared in less substantial a c c o m m o d a t i o n
than c h i c k e n s or o n o p e n range and m o s t of the major
turkey-producing countries h a v e h a d disease p r o b l e m s
associated with influenza infections. In the USA, in California
and Minnesota, w h e r e turkey farms are not only heavily
concentrated but are also situated o n migratory waterfowl
flyways and are reared o n range, influenza virus infections
have b e e n s e e n regularly and m a y represent a significant
e c o n o m i c loss (63). In other countries w h e r e these birds are
generally reared indoors, infections of turkeys are not s e e n
regularly and in the years w h e n infection h a s occurred, these
have usually b e e n restricted to o n e or two isolated incidents.
During the period b e t w e e n 1994 and 1999, infections of
poultry with influenza viruses of H 9 N 2 subtype appear to
have b e e n c o m m o n world-wide. Outbreaks occurred in Italy
in 1994, Germany from 1995 to 1996, Ireland in 1997, South
Pathogenesis
T h e pathogenesis of infection with influenza virus is difficult
to define precisely, since it is c o m p l i c a t e d frequently b y the
effects of infection with secondary bacteria. Severe damage to
the epithelium of the respiratory tract is the main feature in
infections of m a m m a l s with influenza virus. T h e resulting
d a m a g e to the epithelium facilitates secondary infection by
bacterial respiratory pathogens.
Humans
In h u m a n s , the disease incubation p e r i o d will vary b e t w e e n
one and three days, d e p e n d i n g o n the virus strain and dose.
Typically, the onset of clinical signs is rapid, characterised by
malaise, fever, nasal s y m p t o m s , an unproductive cough,
myalgia and h e a d a c h e . Initially, typical s y m p t o m s are rhinitis
followed b y tracheobronchitis and, infrequently, an interstitial
pneumonitis (40). T h e disease p r o c e s s usually damages the
respiratory tract from the n o s e to the small b r o n c h i , but rarely
damages alveolar cells (100). T h e illness is rapidly prostrating
and usually lasts from three to five days, b e i n g m o s t severe in
children and the elderly. Complications include primary viral
or secondary bacterial p n e u m o n i a (68, 101) and these often
205
Rev. sci. tech. Off. int. Epiz.. 19 (1)
confuse the respiratory pathology following infection with
influenza virus. Significant pathological changes in other
organs have not b e e n o b s e r v e d consistently.
Influenza A virus infection of h u m a n s and other m a m m a l s
can result in gross lung lesions w h i c h are patchy and
randomly distributed throughout the l o b e s . T h e altered lung
areas are d e p r e s s e d and consolidated, d a r k red or purple red
in colour, contrasting sharply with n o r m a l lung tissue.
Infection of h u m a n s with an avian influenza A virus of H 5 N 1
subtype in H o n g K o n g in 1997, a p p e a r e d to result in a higher
rate of complications in patients hospitalised w h e n c o m p a r e d
to infections with the prevailing h u m a n subtypes H 1 N 1 and
H3N2. F e w of the severely affected patients h a d active
underlying disease, but additional manifestations s u c h as
gastrointestinal and renal failure w e r e prominent, an unusual
feature of influenza infections in h u m a n s (197).
Other mammalian species
Disease signs in pigs and h o r s e s a p p e a r suddenly in all or a
large n u m b e r of animals of all ages k e p t within a single unit,
following an incubation p e r i o d of o n e to three days. In pigs,
morbidity is high, resulting in serious e c o n o m i c implications
for the farmer. Secondary bacterial infections in b o t h pigs and
horses can often increase the severity of the illness and m a y
result in complications s u c h as p n e u m o n i a , in addition to
strangles in horses (81).
In pigs, the b r o n c h i and b r o n c h i o l i are dilated a n d filled with
exudate. Bronchial and mediastinal l y m p h n o d e s are usually
hyperaemic and enlarged (148, 177). Histologically,
widespread degeneration and necrosis of the epithelium
occurs in the b r o n c h i and bronchioli. T h e l u m e n of b r o n c h i ,
bronchioli and alveoli are filled with exudate containing
desquamated cells and neutrophils progressing to mainly
monocytes. Furthermore, dilatation of the capillaries and
infiltration of the alveolar septae w i t h l y m p h o c y t e s ,
histiocytes and plasma cells occurs. W i d e s p r e a d interstitial
pneumonia and e m p h y s e m a a c c o m p a n y these lesions,
although the severity of the former is d e p e n d e n t o n the
infecting strain (21). In North America, a proliferative
necrotising p n e u m o n i a of pigs is characterised b y w i d e s p r e a d
hyperplasia of type II epithelial cells (51).
Influenza infection of other animals, s u c h as ruminants, sea
mammals, d o g s and mustelids usually results in subclinical
disease. However, H7N7 and H 4 N 5 influenza A viruses w e r e
associated with a h i g h mortality in Harbour seals at C a p e C o d ,
USA, in 1979 (97) and 1982 (167), respectively, and w e r e
isolated from the lungs and brains of d e a d animals.
Avian species
In birds, the disease signs can vary considerably as d e s c r i b e d
earlier. Infection of birds with highly virulent avian viruses is
characterised b y haemorrhagic, necrotic, congestive and
transudative changes. Haemorrhagic changes are frequently
severe in the oviducts a n d intestines. S o m e t i m e s , despite a
h i g h titre of virus, n o lesions occur in the lung (135).
Encephalitis m a y d e v e l o p in the c e r e b r u m and c e r e b e l l u m ,
especially in broilers (1). Alterations to myocardial tissues
h a v e b e e n o b s e r v e d following infection with highly
pathogenic strains (126).
T h e haemagglutinin glycoprotein for influenza viruses is
produced
as
a
precursor,
HAO,
which
requires
post-translational cleavage b y host proteases b e f o r e it is
functional and virus particles are infectious (134). T h e HAO
precursor proteins of avian influenza viruses of l o w virulence
for poultry have a single arginine at position - 1 from the
cleavage site and another at position - 4 . T h e s e viruses are
limited to cleavage b y h o s t proteases s u c h as trypsin-like
e n z y m e s and are thus restricted to replication at sites in the
host w h e r e s u c h e n z y m e s are found, i.e. the respiratory and
intestinal tracts. T h e HPAI viruses possess multiple basic
a m i n o acids (arginine and lysine) at the HAO cleavage sites,
either as a result of apparent insertion or apparent substitution
(145, 178, 191), and appear to b e cleavable b y a ubiquitous
protease(s), p r o b a b l y o n e or m o r e proprotein-processing
subtilisin-related e n d o p r o t e a s e s of w h i c h furin is the leading
candidate (165). T h e s e viruses are able to replicate
throughout the bird, damaging vital organs and tissues w h i c h
results in disease and d e a t h ( 1 3 4 ) . Typical cleavage site amino
acid s e q u e n c e s for H5 viruses of h i g h and l o w virulence are
s h o w n in T a b l e III.
Table III
Amino acid sequences at the HAO cleavage site of H5 influenza A
viruses in relation to their virulence for chickens (4)
Virus
-
Amino acids at HAO
cleavage site
(marked by * )
H5 viruses low pathogenicity
Various sources > 100 isolates
-PQRETR*GLF-
H5 viruses high pathogenicity
1994/1995 Mexican isolates (H5N2)
-PORKRKTR*GLF-
Chicken/Scotland/59(H5Nt!
-PORKKR*GLF-
Tern/S.Africa/61 (H5N3)
-PORETRRROKR*GLF-
Chicken/Pennsylvania/1370/83 (H5N2)
-PQKKKR*GLF-
Turkey/England/50-92/911H5N1)
-PQRKRKTR*GLF-
HK/156/97 (H5N1) (human)
-PQRERRRKKR*GLF-
Ck/HK/97 (H5N1)
—PQRERRRKKR*GLF—
Poultry/ltaly/97 (H5N2)
-PQRRRKKR*GLF-
HAO: uncleaved haemagglutinin gene
Data taken from: Genbank or viruses sequenced at the Veterinary Laboratory
Agency-Weybridge
Arginine (RI and lysine (K) ara basic amino acids
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Rev. sci. tech. Off. int. Epiz., 19(1)
a) Direct transmission
Phylogenetic evidence suggests that an influenza A virus possessing eight gene segments from avian influenza reservoirs may have been transmitted directly
humans and pigs before 1918. This virus is believed to have caused the severe Spanish influenza pandemic of 1918. Direct transmissions of H5N1 and H9N2
viruses from chickens to humans, whilst not resulting in a pandemic to date, demonstrate that avian species may be a direct source of virus for humans.
b) Genetic reassortment
In 1957, the Asian pandemic virus H2N2, appears to have acquired three genes (PB1, HA and NA) from the avian influenza gene pool in wild ducks by genetic
reassortment with the circulating human strain from which it retained five other genes. Following the appearance of this virus, the H1N1 strains disappeared
from humans. In 1968, the Hong Kong pandemic virus H3N2 probably originated by a similar mechanism. It has been suggested that the reassortment event
leading to the production of Asian and Hong Kong pandemic viruses may have occurred in an intermediate host such as the pig, since the latter is receptive to,
and allows productive replication of, both human and avian influenza viruses.
Fig. 1a & Fig. 1b
Theoretical origin of influenza A viruses circulating in humans since 1918
Rev. sci. tech. Off. int. Epiz., 19(1)
207
Epidemiology
Theories of pandemic human influenza
Three major pandemics occurred in the 20th Century, caused
by viruses antigenically 'new' to the host population (antigenic
shift), interspersed with both minor and major epidemics.
Pandemic strains generally appear through genetic
reassortment. Because viral RNA is segmented, genetic
reassortment can readily occur in mixed infections with
different strains of influenza A viruses (182,183). This means
that when two viruses infect the same cell, progeny viruses
may inherit sets of RNA segments made up of combinations of
segments identical to those of either of the parent viruses. This
gives a theoretically possible number of 2 (256) different
combinations that can form a complete set of RNA segments
from a concurrent infection, although in practice, only a few
progeny virions possess the correct gene constellation
required for viability (74,189). The new subtypes of influenza
viruses which appeared in humans in 1957 (Asian influenza),
1968 (Hong Kong influenza) and 1977 (Russian influenza),
8
had several features in common. The appearance of these
subtypes was sudden, the viruses were antigenically distinct
from the influenza viruses then circulating in humans, they
were confined to HI, H2 and H3 subtypes and the first
outbreaks occurred in South-East Asia. Phylogenetic evidence
suggests that these pandemic strains were derived from avian
influenza viruses either after reassortment (Fig. la) or by
direct transfer (Fig. lb) (189). The appearance of the H2N2
and H3N2 subtypes was paralleled by the disappearance from
the human population of the subtypes circulating previously,
H1N1 and H2N2, respectively. This phenomenon probably
occurred in 1918 when emerging H1N1 viruses replaced
H3-like viruses. The reasons for the sudden disappearance of
human strains circulating previously are unknown, but the
earlier strain is possibly disadvantaged compared with the
new strain because it has already elicited widespread
immunity in the human population. This may explain the
failure of the H1N1 virus to replace H3N2 on re-emergence in
1977 (Fig. le), as a large proportion of the population would
have been infected with H1N1 prior to 1957, and would have
retained some immunity.
c) Re-emergence
In 1977, the Russian influenza virus H1N1 that had circulated in humans prior to 1950 reappeared and has continued to co-circulate with H3N2 viruses in the
human population. The origin of the virus is a mystery and a number of sources have been proposed, including the possibility that the virus re-emerged following
escape from a laboratory.
Fig. 1c
Theoretical origin of influenza A viruses circulating in humans since 1918
208
Rev. sci. tech. Off. int. Epiz., 19 (1)
T h e e m e r g e n c e of p a n d e m i c strains f o l l o w i n g g e n e t i c
reassortment
The detection of vast p o o l s of influenza virases of m a n y
different subtypes a m o n g animals, particularly aquatic birds,
gave considerable impetus to research a i m e d at determining
w h e r e n e w subtypes, particularly those that cause p a n d e m i c s ,
emerge. A n u m b e r of theories has b e e n suggested, of w h i c h
the m o s t widely a c c e p t e d is that b y adaptation, s o m e t i m e s
involving genetic reassortment, transference of virus from
animals to h u m a n s occurs, resulting in an antigenically novel
virus with the ability to infect and spread in h u m a n s
(incidents are summarised in Table IV). Following genetic
reassortment, viruses m a y arise that possess the genes
necessary to enable infection of h u m a n s but m a y have surface
antigens n e w to the host i m m u n e system.
Table IV
Summary of influenza A hosts and the potential for recombination
and/or spread to humans
Influenza
host
Contact
with
humans
Infections w i t h
human influenza
viruses
Spread from
host to
humans
Pigs
Yes
Yes: H1N1, H3N2
Yes: natural
Horses
Yes
None known
None known
None known
None known
Birds
Waterfowl
lndirectly
Caged birds
Yes
None known
None known
Domestic
poultry
Yes
None known
Yes: H5N1,
H9N2, H7N7
(a)
(b)
(b)
Mustelids
Yes
Yes: laboratory
Whales
No
None known
None known
Seals
No
None known
Yes: laboratory
Yes: laboratory
a) Contact may occur via lake water and other media contaminated by infective faeces
b) The H7N7 virus was isolated from the eye of a woman who kept ducks with which feral
waterfowl mingled
Genetic and b i o c h e m i c a l studies h a v e s h o w n that the
p a n d e m i c viruses of 1957 and 1968 arose b y genetic
reassortment. T h e 1957 Asian H 2 N 2 strain obtained HA, NA
and PB1 genes from an avian virus and the remaining five
genes from the preceding h u m a n H 1 N 1 strain (49, 88, 141).
The 1968 Hong K o n g H 3 N 2 strain contained HA and PB1
genes from an avian d o n o r ( 4 7 , 8 8 ) and the NA and other five
genes from the Asian H 2 N 2 strain.
T h e role of t h e pig
Pigs serve as major reservoirs of H 1 N 1 and H 3 N 2 influenza
viruses and are frequently involved in interspecies
transmission of influenza viruses. T h e maintenance of these
viruses in pigs and the frequent introduction of viruses from
other species m a y b e important in the generation of 'new'
strains of influenza, s o m e of w h i c h m a y h a v e the potential to
transmit to other species, including h u m a n s .
Pigs h a v e b e e n c o n s i d e r e d the logical intermediate host for
reassortment of influenza A viruses, for these animals can
serve as hosts for viruses from either birds or h u m a n s (144).
This susceptibility is d u e to the p r e s e n c e of receptors for b o t h
avian (a2-3-galactose sialic acid) and h u m a n (a2-6-galactose
sialic acid) influenza viruses w h i c h , following infection of pigs
with an avian virus, can result in receptor modification o n the
HA to a 2 - 6 , thereby providing a potential link from birds to
h u m a n s (Fig. 2) (79). Furthermore, pigs are susceptible to
infection with all of the avian subtypes tested to date (H1 to
H13) and those avian viruses w h i c h d o not replicate in pigs
can contribute genes in the generation of reassortants w h e n
co-infecting pigs with a swine influenza virus (92).
Evidence for the pig as a mixing vessel of influenza viruses of
n o n swine origin has b e e n demonstrated in Europe b y
Castrucci et al., w h o detected reassortment of h u m a n and
avian viruses in pigs in Italy (32). Novel reassortant viruses of
H 1 N 2 subtype derived from h u m a n and avian viruses (26) or
H1N7 subtype derived from h u m a n and equine viruses (22)
h a v e b e e n isolated from pigs in the UK. T h e H 1 N 2 viruses
derived from a multiple reassortant event and spread widely
within pigs in the UK. Other studies of influenza viruses
isolated from pigs in North America (195) and the south of
the People's Republic of China (156) failed to detect any
reassortant viruses containing internal gene segments of non
swine origin, although genetic heterogeneity of the HA of
swine H3 influenza viruses occurs in nature in the People's
Republic of China (91). Since 1998, H 3 N 2 viruses isolated
from pigs in the USA have contained combinations of human,
swine and avian genes. Furthermore, the HA gene of these
viruses was derived from a h u m a n virus circulating in the
h u m a n population in 1993 (181).
Occassional transmission of influenza viruses from pigs to
h u m a n s h a s b e e n demonstrated (see the sub-section below,
entitled 'Pigs' in the section 'Spread b e t w e e n h u m a n s and
m a m m a l i a n species'). T h e internal protein genes of h u m a n
influenza viruses share a c o m m o n ancestor with the genes of
s o m e swine influenza viruses. A n u m b e r of authors have
p r o p o s e d the nucleoprotein (NP) gene as a determinant of
host range w h i c h can restrict or attenuate virus replication,
thereby controlling the successful transmission of virus to a
'new' host (143, 172, 161). T h e s e observations support the
potential role of the pig as a mixing vessel of influenza viruses
from avian and h u m a n sources. T h e pig appears to have a
b r o a d e r host range in the compatibility of the NP gene in
reassortant viruses than b o t h h u m a n s and birds (143).
Direct transfer from another species
Alternatively, n e w p a n d e m i c viruses c o u l d occur in the
h u m a n population if an avian strain or a strain from another
m a m m a l b e c a m e infectious for h u m a n s . T h e appearance of
the 'Spanish' influenza virus (H1N1) in 1918 almost certainly
involved this m e c h a n i s m . Later, retrospective serological
investigations confirmed that the disease in h u m a n s and pigs
h a d b e e n caused b y closely related influenza A viruses. The
209
Rev. sci. tech. Off. int. Epiz., 19 (1)
a2-3
cc2-6
a2-6
Pigs have been considered the logical intermediate host for reassortment of influenza A viruses, for they can serve as hosts for viruses from either birds or
humans. This susceptibility is due to the presence of receptors for both avian (a2-3-galactose sialic acid) and human (a2-6-galactose sialic acid) influenza
viruses which, following infection of pigs with an avian virus, can result in receptor modification on the HA to that characteristic of human viruses, thereby
providing a potential link from birds to humans. The segments in the centre of each virus particle represent the genome. The subtype of some emerging
reassortant viruses may differ from their progenitors through exchange of HA (white in reassorted virus) and/or NA genes. These viruses which are distinct
antigenically may be able to spread in an immunologically naïve human population if transmission from pigs occurs.
HA: haemagglutinin
NA: neuraminidase
Fig. 2
The potential role of the pig as an intermediate host
causative agent was an H1N1 influenza A viras which was
possibly derived from the same avian ancestor (52, 83, 130).
For this reason, the avian-like H1N1 viruses circulating in
pigs in Europe since 1979 (125), and in South-East Asia since
1993, have been implicated as the precursors of the next
human pandemic virus (56, 103). Although a pandemic has
not resulted to date, the transmissions of H5N1 and H9N2
viruses from chickens to humans in Hong Kong in 1997 and
1999, respectively, further demonstrate that avian species
may directly be the source of human pandemic strains (35,
151). Initially, following the transmission of these viruses
from aquatic birds to pigs or chickens, the viruses appeared
genetically unstable (103, 198), and in the case of
transmission of avian H1N1 virus to pigs, apparently became
stable after approximately twelve years (103).
R e - e m e r g e n c e of p a n d e m i c s t r a i n s
The appearance of pandemic virus may be in fact the
re-emergence of a virus which caused an epidemic many years
earlier. The appearance of Russian influenza (H1N1) provides
support for this concept. The virus, which reappeared in the
People's Republic of China in 1977, and subsequently spread
world-wide, was genetically identical to the virus which
caused a human influenza epidemic in 1950 (114). Webster
et al. suggested that this virus was most likely reintroduced to
humans from a frozen source, possibly from a laboratory
(189), whilst Shoham has proposed a biotic mechanism for
the preservation of influenza viruses (147). Influenza viruses
of the H3N2 subtype persist in pigs many years after their
antigenic counterparts have disappeared from humans (61,
210
152), providing a reservoir of virus w h i c h m a y in the future
infect a susceptible h u m a n population. However, these
viruses have accumulated many mutations and are therefore
genetically distinct from the precursor viruses. Pandemic
strains m a y also b e antigenically conserved in the avian
reservoir, since counterparts of the Asian p a n d e m i c strain of
1957 continue to circulate with increased prevalence in wild
d u c k s , d o m e s t i c fowl and live bird markets and are c o m i n g
into closer proximity to susceptible h u m a n populations
(136).
A n 'influenza e p i c e n t r e '
T h e majority of p a n d e m i c strains have apparently originated
in the People's Republic of China, raising the possibility that
this region is an influenza epicentre ( 1 5 3 , 1 5 0 ) . In the tropical
and subtropical regions of the People's Republic of China,
h u m a n influenza occurs throughout the year (129). In the
People's Republic of China, influenza viruses of all subtypes
are prevalent in d u c k s and in water frequented b y d u c k s (59).
Agricultural practices provide close contact b e t w e e n wild
aquatic birds, d o m e s t i c d u c k s , pigs and humans, thereby
presenting the opportunity for interspecies transmission and
genetic exchange a m o n g influenza viruses, with the pig acting
as an intermediary b e t w e e n domestic d u c k s and h u m a n s .
Aquatic birds migrating or over-wintering in the region might
provide a source of virus for domestic d u c k s . Yasuda et al.
h a v e s h o w n that d o m e s t i c d u c k s harbour H 3 influenza
viruses antigenically and genetically similar to those in pigs,
suggesting these birds m a y play a role in the transfer of avian
influenza viruses from feral d u c k s to pigs (196).
Rhythm of o c c u r r e n c e
T w o h y p o t h e s e s h a v e b e e n p r o p o s e d for the rhythm of
occurrence of h u m a n influenza A viruses, namely: an
influenza circle or cycle, or an influenza spiral ( 1 4 9 ) . T h e
circle or cycle theory suggests a recycling of H l , H 2 a n d H 3
subtypes. If this is s o , the HA subtype of the next p a n d e m i c
virus w o u l d b e e x p e c t e d to b e H 2 . T h e spiral theory
p r e s u p p o s e s that h u m a n s are capable of being infected with
all k n o w n HA subtypes of influenza A viruses; these presently
n u m b e r fifteen. T h e r e are serological grounds for this in that
rural dwellers in the influenza epicentre have b e e n found to
possess antibodies to the avian subtypes H 4 to H 1 3 e x a m i n e d
(149). It is possible that the h y p o t h e s e s are not mutually
exclusive, as there is n o reason that recycling should not occur
within the spiral.
Spread between humans and mammalian
species
Pigs
Early theories suggesting that the p a n d e m i c of 1918 was a
result of the transmission of virus from pigs to h u m a n s were at
the time speculative and it was not until 1976 that further
evidence for such transmissions b e c a m e available. Pigs w e r e
implicated as the source of infection w h e n an H1N1 virus was
isolated from a soldier w h o h a d d i e d of influenza at Fort Dix,
Rev. sci. tech. Off. int. Epiz., 19(1)
N e w Jersey, USA. T h e virus was identical to viruses isolated
from pigs in the USA. Furthermore, virus isolation
demonstrated that five other s e r v i c e m e n w e r e infected, and
serological evidence suggested that s o m e 500 personnel at
Fort Dix were, or h a d b e e n , infected with the s a m e virus (76,
173).
The incident at Fort Dix cannot b e regarded as e v i d e n c e of
zoonosis, since although pigs w e r e the likely source of the
virus, this was never established. However, considerable
evidence exists to suggest that transmission from pigs to
h u m a n s d o e s occur, following the detection of antibodies to
swine H1 viruses in p e o p l e w h o h a v e h a d close contact with
pigs (95, 139). Final confirmation of the zoonotic nature of
H 1 N 1 influenza viruses from pigs c a m e in 1976 w h e n ,
following an influenza epizootic in pigs, viruses isolated from
the pigs and a h u m a n contact w e r e s h o w n to b e b o t h
antigenically and genetically identical swine H 1 N 1 influenza
viruses (44, 70). Subsequently, there h a v e b e e n several
reports from North America of swine virus b e i n g isolated from
h u m a n s with respiratory illness (38, 4 2 ) , occasionally with
fatal c o n s e q u e n c e s ( 1 3 3 , 190). All cases e x a m i n e d followed
contact with sick pigs and w e r e d u e to viruses related most
closely to classical swine H 1 N 1 influenza virus. Perhaps of
greater significance for h u m a n s is a report in the Netherlands
during 1993 of two distinct cases of infection of children with
H 3 N 2 viruses w h o s e genes e n c o d i n g internal proteins w e r e of
avian origin (34). Genetically and antigenically related viruses
h a d b e e n detected in pigs in Europe, raising the possibility of
-potential transmission of avian influenza virus genes to
h u m a n s following genetic reassortment in pigs (32). These
concerns w e r e substantiated further b y the results of
serological studies in Italy w h i c h indicated that these 'swine'
H 3 N 2 viruses h a d apparently b e e n transmitted to young,
immunologically naïve p e o p l e (30).
Influenza viruses of subtype H 3 N 2 are ubiquitous and
e n d e m i c in m o s t pig populations w o r l d - w i d e , but no
apparent evidence exists of pigs b e i n g infected with this
subtype prior to the p a n d e m i c in h u m a n s in 1968. Indeed,
the appearance of a H 3 N 2 subtype variant strain in the pig
population of a country appears to coincide with the epidemic
strain infecting the h u m a n population at that time ( 1 1 , 24,
117).
Further evidence of the spread of influenza viruses from
h u m a n s to pigs was the appearance in pigs of H1N1 viruses
(or antibodies to H 1 N 1 ) , related to those circulating in the
h u m a n population since 1977 ( 1 1 , 24, 5 3 , 118). Genetic
analysis of two strains of H1N1 virus isolated from pigs in
J a p a n revealed that the HA and NA genes w e r e m o s t closely
related to those of h u m a n H1N1 viruses circulating in the
h u m a n population at that time (87). In addition, reassortant
viruses with s o m e characteristics of h u m a n H1 viruses have
b e e n isolated from pigs in the UK (23, 2 6 ) .
Rev. sci. tech. Off. int. Epiz., 19(1)
211
Horses
Horses
In historical accounts of p a n d e m i c s in h u m a n s , frequent
reference is m a d e to similar disease in horses, w h i c h occurs
either simultaneously or p r e c e d i n g that in h u m a n s . Beveridge
noted s u c h references in the accounts of twelve p a n d e m i c s
w h i c h o c c u r r e d during the 18th and 19th Centuries (17).
Apart f r o m occasional isolated reports of e v i d e n c e of infection
of h o r s e s with viruses of subtypes H 1 N 1 , H 2 N 2 and H 3 N 2
(174), influenza infections of h o r s e s h a v e b e e n restricted to
H7N7 and H 3 N 8 subtypes of influenza A and these viruses
form distinct lineages i n phylogenetic studies (see earlier).
However, examination of H 3 N 8 viruses isolated from severe
e p i d e m i c s in h o r s e s occurring in the Jilin a n d Heilongjiang
Provinces in the north-east of the People's Republic of China
in 1989 and 1990, s h o w e d these to b e antigenically a n d
genetically distinguishable from other e q u i n e H 3 N 8 viruses
circulating in the world. Guo et al. c o n c l u d e d that this virus
was of recent avian origin a n d h a d p r o b a b l y spread direcdy to
horses without reassortment (60). Despite the rapid
geographical spread locally in 1989 and 1990, and the h i g h
morbidity, this virus d o e s not appear to h a v e b e c o m e
established in the h o r s e population; m o r e w i d e s p r e a d
e p i d e m i c s w h i c h occurred in the People's Republic of China
from 1993 to 1994 w e r e caused b y 'classical' H 3 N 8 virus
(154).
Experimental infection of h u m a n volunteers with H 3
(equine-2) viruses h a s p r o d u c e d a n influenza-like illness with
virus-shedding and seroconversion (36, 86). N o evidence
exists of infection of h u m a n s with the other subtype of
influenza, H7 (equine-1), w h i c h has caused w i d e s p r e a d
epizootics in h o r s e s .
Several isolated cases of infection of h o r s e s with subtypes
H 1 N 1 , H 2 N 2 and H 3 N 2 h a v e b e e n reported, usually
associated with h u m a n infections (174). Experimental
infections of h o r s e s w i t h h u m a n - d e r i v e d H 3 N 2 virus h a v e
confirmed the susceptibility of h o r s e s to this virus (19).
Cattle
Marine mammals
Influenza infections of cattle with h u m a n viruses of H 3 N 2 a n d
H1N1 subtypes h a v e b e e n reported regularly, b a s e d largely
on the detection of antibodies (82), occasionally in association
with respiratory epizootics in cattle (25). T h e prevailing
h u m a n strains appear to transmit to cattle frequently a n d
viruses h a v e b e e n isolated from cattle infected naturally (48,
110), although w h e t h e r or not these viruses are able to persist
in cattle remains unclear. B r o w n et al. reported the apparent
maintenance of h u m a n influenza viruses in cattle s o m e ten
years after the disappearance of the viruses from the h u m a n
population, raising the possibility that cattle m a y provide a
reservoir for virus w h i c h m a y b e a b l e to transmit b a c k to
humans w h e n a susceptible p o p u l a t i o n b e c o m e s available
(25).
During 1 9 7 9 a n d 1980, approximately 5 0 0 deaths o c c u r r e d
in harbour seals (Phoca. vitulina) around the C a p e C o d
Peninsula in the USA, representing about 2 0 % of the
population. Deaths w e r e primarily d u e to acute haemorrhagic
p n e u m o n i a , a n d influenza A viruses of H 7 N 7 subtype w e r e
isolated repeatedly from the lungs or brains of d e a d seals (97).
T h e virus infecting the seals was s h o w n to b e closely related
b o t h antigenically a n d genetically to avian influenza viruses
(185) a n d w o u l d a p p e a r to represent direct transmission to
the seals without reassortment.
Avian influenza infections of mammals
Pigs
The p r o b a b l e introduction of classical swine H 1 N 1 influenza
viruses to turkeys from infected pigs has b e e n reported from
North America, and in s o m e cases, influenza-like illness in
pigs has b e e n followed immediately b y disease signs in
turkeys ( 6 2 , 1 0 9 , 1 2 7 ) . Genetic studies of H 1 N 1 viruses from
turkeys in the USA h a s revealed a h i g h degree of genetic
exchange and reassortment of influenza A viruses from
turkeys and pigs in the former species (195). In Europe, avian
H1N1 viruses w e r e transmitted to pigs, b e c a m e established,
and have subsequently b e e n reintroduced to turkeys from
pigs, causing e c o n o m i c losses ( 1 0 2 , 192). An independent
introduction of H 1 N 1 virus from birds to pigs occurred in
Asia in the early 1990s; these viruses are genetically distinct
from the viruses in Europe (56). Recently, H 9 N 2 viruses h a v e
b e e n introduced to pigs in South-East Asia, apparently from
poultry, although the potential of these viruses to spread and
persist in pigs remains u n k n o w n (20).
In 1983, further deaths ( 2 % - 4 % ) occurred in harbour seals o n
the N e w England coast of the USA and, o n this o c c a s i o n ,
another influenza A virus of subtype H 4 N 5 was isolated.
W h i l e distinguishable from the H 7 N 7 virus, o n c e again, all
eight genes of this virus w e r e demonstrably of avian origin
(189). Further surveillance of seals o n the Cape C o d
peninsula was u n d e r t a k e n and Callan et al. reported isolates
of two influenza A viruses of H 4 N 6 subtype m a d e in 1991
and three of H 3 N 3 subtype in 1 9 9 2 from seals found d e a d
with apparent viral p n e u m o n i a (28). Following antigenic and
genetic characterisation, the authors c o n c l u d e d these too w e r e
avian viruses that h a d entered the seal population.
Two viruses of H 1 3 N 2 and H 1 3 N 9 subtypes w e r e isolated
from a single b e a c h e d pilot whale, and genetic analysis
indicated that the viruses h a d b e e n introduced recently from
birds (33, 73).
Mink
In October 1984, outbreaks of respiratory disease-affected
approximately 100,000 m i n k o n 3 3 farms situated in close
proximity along a coastal region of southern S w e d e n , with
100% morbidity a n d 3 % mortality (94). Influenza A viruses of
H 1 0 N 4 subtype were isolated from the m i n k a n d genetic
Rev. sci. tech. Off. int. Epiz., 19 (1)
212
analysis indicated that the virases w e r e of avian origin and
w e r e very closely related to a virus of the same subtype
isolated from c h i c k e n s and a feral d u c k in England in 1985
(16). Earlier experimental infections h a d suggested that m i n k
w e r e susceptible to infection with avian influenza viruses
(119).
Humans
Historical
background
Until 1996, n o reports existed of naturally acquired infections
of humans with avian influenza viruses b y direct transmission
from birds. Evidence of antibodies to influenza subtypes
k n o w n only to infect birds h a d b e e n d e t e c t e d in s e r u m
samples t a k e n from families with h o u s e h o l d d u c k s and pigs in
the south of the People's Republic of China (149, 157), but
this m a y have occurred following passage of the virus through
pigs. Beare and W e b s t e r p e r f o r m e d experimental infections
o n h u m a n volunteers and reported only transitory infections
with little or n o clinical signs and n o antibody production
(13). However, reports of ostensibly avian viruses infecting
h u m a n s or infections as a result of laboratory accidents have
b e e n m a d e o n several occasions.
C a m p b e l l et cd. reported the isolation of a virus of H 7 N 1
subtype from a m a n suffering from hepatitis, but with n o
antibody response to that virus (29). A female laboratory
w o r k e r in Australia d e v e l o p e d kerato-conjunctivitis after
accidentally splashing infective allantoic fluids containing
A/chicken/Victoria/76 (H7N7) o n h e r face. Virus was isolated
b y swabbing the conjunctiva, but significant antibody titres
were not obtained (171). A harbour seal infected
experimentally with A/seal/Massachusetts/1/80 (H7N7), and
k n o w n to b e shedding that virus, s n e e z e d directly into the
face and right eye of an investigator w h o subsequently
d e v e l o p e d conjunctivitis (186). T h e virus was isolated from
the affected eye u p to four days after the incident, but n o
antibody response was detected. As discussed a b o v e , the virus
was k n o w n to b e of avian origin. Four field w o r k e r s
autopsying seals during the o u t b r e a k of 1980 also d e v e l o p e d
conjunctivitis, but n o virology was p e r f o r m e d (185). Since
1996, three accounts h a v e b e e n r e c o r d e d of avian influenza
viruses apparently affecting h u m a n s as a result of spread
directly from birds.
H7N7 in England
Kurtz et al. reported the isolation of an influenza virus,
A/England/268/96 (H7N7), from the eye of a fortythree-year-old w o m a n with conjunctivitis (96). Partial
sequencing of the internal protein and the neuraminidase
genes of isolate 2 6 8 / 9 6 s h o w e d all s e v e n genes to have closest
h o m o l o g y with avian influenza viruses and the entire
nucleotide s e q u e n c e of the haemagglutinin gene of 2 6 8 / 9 6
h a d close ( 9 8 . 2 % ) h o m o l o g y with an H 7 N 7 virus isolated
from turkeys in Ireland in 1995 (12). Kurtz et al. considered
the most likely source of the virus to b e waterfowl, as the
w o m a n tended a collection of twenty-six d u c k s of various
b r e e d s w h i c h m i x e d freely with feral waterfowl o n a small l a k e
(96). T h e infected w o m a n did not have measurable antibodies
to H7 five w e e k s after the appearance of conjunctivitis.
Cloacal swabs t a k e n from h e r d u c k s o n e m o n t h after the
isolation of virus from the affected eye did not yield any virus.
H5N1 in Hong Kong
Outbreaks of HPAI, c a u s e d b y virus of H 5 N 1 subtype,
occurred o n three c h i c k e n farms in H o n g K o n g from March to
May 1997, with mortality ranging from 7 0 % - 1 0 0 % , and
subsequent spread to live b i r d markets in H o n g K o n g (35,
155). In May 1997, a three-year-old child d i e d of apparent
viral p n e u m o n i a with severe complications after admission to
hospital in H o n g K o n g (197). A n influenza virus of H5N1
subtype was isolated from respiratory s p e c i m e n s t a k e n from
this patient and similar virus infections w e r e confirmed by
virus isolation and/or serology in seventeen other cases in
h u m a n s w h i c h occurred in H o n g K o n g during N o v e m b e r and
D e c e m b e r 1997; a further five patients d i e d (155). Nucleotide
sequencing of the virus from the index case s h o w e d all eight
genes to b e of avian origin (35), and demonstrated 9 9 %
h o m o l o g y of all eight genes with those of the H 5 N 1 virus from
c h i c k e n s in H o n g K o n g (168). Analysis of the haemagglutinin
and neuraminidase proteins of the sixteen viruses isolated
from the eighteen cases indicated that all w e r e essentially
similar and a p p e a r e d to have spread directly from poultry
with n o cumulative changes that w o u l d indicate adaptation to
the h u m a n host (15). All h u m a n isolates h a d the same
multiple basic amino acids as the c h i c k e n isolates (15).
Investigations of the h u m a n infections with H 5 N 1 in Hong
K o n g revealed n o e v i d e n c e of h u m a n to h u m a n transmission
and e a c h case was assumed to represent spread from domestic
poultry (155). One of the surprising features of these
infections was the severe clinical disease caused in humans.
Both h u m a n and poultry H 5 N 1 viruses, in c o m m o n with
most avian influenza viruses, w e r e s h o w n to bind
preferentially to the N-acetylneuraminic acid-a2,3,-galactose
linkages o n sialyloligosaccharides ( 1 0 7 ) , whilst conventional
h u m a n influenza viruses b i n d to
N-acetylneuraminic
acid-a2,6Gal linkages; apparently this did not prevent
penetrative virus replication in s o m e of the infected humans.
There was considerable fear that the virus w o u l d mutate to
b e c o m e transmissible either b y mutation in o n e of the infected
p e o p l e or b y reassortment with h u m a n influenza in a person
infected with h u m a n influenza earlier or subsequently. To
prevent this occurring, it was d e c i d e d to r e m o v e the apparent
source of the H 5 N 1 virus, and all c h i c k e n s in H o n g Kong,
approximately 1,500,000, were slaughtered.
H9N2 in Hong Kong and other regions of the People's
Republic of China
Although n o H5N1 viruses have b e e n isolated from humans
since D e c e m b e r 1997, the p r o b l e m s of bird to h u m a n spread
in the area w e r e raised again in March 1999, w h e n two young
girls aged o n e and four years w e r e admitted to hospital in
H o n g Kong with typical influenza-like s y m p t o m s . In b o t h
213
Rev. sci. tech. Off. int. Epiz, 19 (1)
cases, influenza A viruses of H 9 N 2 subtype w e r e isolated from
respiratory tract samples. Both children m a d e uneventful
recoveries.
Subsequent to these reports from Hong Kong, it was revealed
that five cases of h u m a n H 9 N 2 infection h a d b e e n r e c o r d e d
on the mainland of the People's Republic of China in August
1998.
Genetic analyses of the two H o n g K o n g H 9 N 2 isolates have
s h o w n that these, like the H 5 N 1 viruses, appear to b e avian
viruses w h i c h h a v e infected h u m a n s directly (57, 124). As
described earlier, influenza A viruses of H 9 N 2 subtype h a v e
b e e n w i d e s p r e a d in d o m e s t i c poultry throughout the w o r l d in
recent years, but especially in Asia, including the People's
Republic of China. Transfer to h u m a n s c o u l d quite possibly
have occurred in other areas, but g o n e u n o b s e r v e d , as the
level of monitoring in H o n g K o n g and the People's Republic
of China h a d b e e n considerably raised as a result of the H 5 N 1
virus isolations from h u m a n s . T h e greatest threat of these
infections is p r o b a b l y the risk of a dual infection with a
conventional h u m a n virus, resulting in a reassortant virus
with H9 subtype haemagglutinin, c o m b i n e d with all or s o m e
of the other genes from the h u m a n virus, thereby allowing
transmission b e t w e e n h u m a n s .
Surveillance
Humans
Global surveillance of influenza is maintained b y a n e t w o r k of
laboratories s p o n s o r e d b y the W H O . T h e two primary goals
of the p r o g r a m m e are to gain an understanding of the
epidemiology of influenza and to promptly isolate influenza
viruses from n e w e p i d e m i c s and distribute t h e m for vaccine
production (64). T h e p r o g r a m m e is i m p l e m e n t e d through
three W H O Collaborating Centres for Influenza Reference
and Research located in L o n d o n , Atlanta and Melbourne. T h e
functions of these centres are as follows:
a) to collect a n d distribute information o n the types of
influenza virus w h i c h prevail in various countries world-wide
Morbidity and mortality associated with influenza is u s e d as
an indicator of influenza activity and impact. Precise
quantification of the impact of influenza morbidity and
mortality is problematic, since laboratory confirmation is
required for exact diagnosis. Many m e t h o d s h a v e b e e n
d e v e l o p e d to p r o v i d e estimates of the mortality or morbidity
associated with influenza. T h e s e m e t h o d s usually use baseline
rates of mortality or morbidity to calculate excess rates of
mortality or morbidity attributed to circulating influenza
virus.
During recent years, the operation of the n e t w o r k h a s b e c o m e
increasingly focused o n the annual consultation o n influenza
vaccine
formulation
(see
the
subsection
entitled
'Immunoprophylaxis' b e l o w ) . In addition, surveillance of
h u m a n a n d animal influenza in the p r o p o s e d 'influenza
epicentre' h a s b e e n intensified a n d i m p r o v e d , thereby
contributing to the early identification of two subtypes of
influenza virus not r e c o r d e d previously in h u m a n s .
Animals
Demonstration that the H 2 N 2 and H 3 N 2 p a n d e m i c viruses
contained genes from an influenza virus of avian origin l e d to
systematic, long-term global surveillance studies o n the
influenza viruses of birds and m a m m a l s , to determine the
diversity of influenza A viruses in nature and w h e t h e r a future
p a n d e m i c strain of virus c o u l d b e isolated from these animals
in advance of the appearance of the strain in humans. A vast
n u m b e r of viruses have n o w b e e n isolated from a w i d e variety
of birds and a range of terrestrial and sea m a m m a l s . T h e s e
studies are ongoing but are generally m o r e localised and less
formally structured than the equivalent s c h e m e s for the
h u m a n population. However, international and national
reference laboratories with a similar remit to those in the
h u m a n sector h a v e b e e n established for avian and equine
influenza viruses. T h e s e facilities have p r o v i d e d invaluable
information for dealing with emergencies such as the early
identification of a putative vaccine strain from birds following
the infection of p e o p l e in Hong K o n g with H5N1 virus, and
after vaccine failure in controlling equine influenza.
b) to advise o n the strains to b e included in influenza vaccines
c) to collect, store and study strains from various outbreaks
and distribute these strains to interested laboratories
Prophylaxis and treatment
d) to educate visiting w o r k e r s in research techniques.
The surveillance p r o g r a m m e h a s n o w g r o w n to include 110
designated National Influenza Centres in 80 countries.
National centres are involved in the isolation and preliminary
characterisation of influenza viruses from suspect cases of
infection in h u m a n s and, if appropriate, in collection of
viruses from regional laboratories within the country. T h e
Centres p r o v i d e the W H O with regular epidemiological
reports and forward virus isolates for further analysis to o n e of
the three Collaborating Centres.
Two control measures are currently available for influenza in
h u m a n s , namely: immunoprophylaxis with vaccines and
c h e m o p r o p h y l a x i s or therapy with antiviral drugs. Since the
late 1940s, the principal preventive measures against
influenza have b e e n inactivated virus vaccines. T h e efficacy of
vaccines has varied b e t w e e n 6 0 % and 9 0 % and has b e e n
d e p e n d e n t o n the closeness of the 'antigenic m a t c h ' b e t w e e n
the vaccine virus and the e p i d e m i c virus. However, e v e n for
those not completely protected, vaccination r e d u c e s the
severity of the disease, thereby reducing costs and mortality.
Rev. sci. tech. Off. int. Epiz., 19 (1)
214
Immunoprophylaxis
Vaccination of p e o p l e at h i g h risk before e a c h annual
influenza season is currently the m o s t effective measure for
reducing the impact of h u m a n influenza. W h e n vaccine and
e p i d e m i c strains of virus are well m a t c h e d , achieving h i g h
vaccination rates a m o n g c l o s e d populations can reduce the
risk of outbreaks b y inducing population immunity. This
occurs w h e n the overall n u m b e r of susceptible p e o p l e in a
population b e c o m e s t o o l o w for virus to spread and infect a
significant n u m b e r of the susceptible individuals.
To maximise protection of p e o p l e at h i g h risk, they and their
close contacts s h o u l d b e targeted for organised vaccination
p r o g r a m m e s . Influenza vaccination is strongly r e c o m m e n d e d
for any p e r s o n over six m o n t h s of age w h o , b e c a u s e of age or
an underlying m e d i c a l condition, is at increased risk for
complications of influenza. T h e high-risk group consists of
the following: p e o p l e over sixty-five years of age, residents of
nursing h o m e s and other chronic-care facilities, p e o p l e with
chronic disorders of the pulmonary or cardiovascular
systems, and p e o p l e w h o have suffered chronic m e t a b o l i c
diseases or i m m u n o s u p p r e s s i o n
(including
immuno­
suppression as a result of medication) within the past year. In
addition, p e o p l e w h o m a y transmit influenza to those at h i g h
risk, i.e. hospital and nursing h o m e personnel and h o u s e h o l d
m e m b e r s of p e o p l e in high-risk groups, s h o u l d also b e
vaccinated. Other groups m a y b e included o n individual
merit, such as pregnant w o m e n , p e o p l e infected with HIV and
foreign travellers.
Currently, in the UK and the USA, less than 3 0 % of p e o p l e in
high-risk groups are vaccinated e a c h year and m o r e effective
strategies are required for delivering vaccine to m e m b e r s of
high-risk groups. In general, successful vaccination
p r o g r a m m e s have c o m b i n e d education for healthcare
workers, publicity and education targeted towards potential
recipients, and a plan for identifying p e o p l e at h i g h risk.
Two basic a p p r o a c h e s for immunisation h a v e b e e n pursued,
namely: the use of inactivated virus preparations and the use
of live, attenuated viruses. At present, only vaccines p r e p a r e d
with inactivated or killed virus particles are licensed for use in
the European Union and the USA. N u m e r o u s refinements in
the production processes over m a n y years have resulted in
vaccines that are m o r e highly purified and m o r e predictable in
their reactogenicity and immunogenicity (194).
E a c h year, the influenza vaccine is redefined to reflect changes
in the antigenicity of circulating virus strains, thereby
containing virus strains representative of influenza viruses
believed likely to circulate in the forthcoming 'influenza
season'. Annual vaccination using the current vaccine is
therefore necessary b e c a u s e of potential changes in the
circulating viruses, but also d u e to immunity from previous
vaccinations b e i n g relatively short-lived. At present, the
vaccine consists of two type A viruses, H 1 N 1 and H 3 N 2 , and
one type B virus. T h e exact strains of these viruses to b e used
are identified b y an international n e t w o r k of laboratories that
maintain surveillance for n e w influenza virus variants
throughout the world.
One d o s e of inactivated vaccine is generally sufficient to
induce a protective i m m u n e response, although children
occasionally require a b o o s t e r (37). Unless the antigen u s e d is
entirely n e w , m o s t adults will m o u n t a b o o s t e r antibody
response, e v e n to strains w h o s e antigens are marginally
different. Vaccine antibodies react against the HA and NA and
the level of s e r u m antibody h a s b e e n s h o w n to correlate
inversely with the occurrence of established infection with
influenza viruses (75). T h e c o m p o s i t i o n of the vaccine rarely
causes systemic or febrile reactions.
T h e effectiveness of influenza vaccine in preventing or
attenuating illness varies, d e p e n d i n g primarily o n the age and
i m m u n o c o m p e t e n c e of the vaccine recipient a n d the degree
of antigenic similarity b e t w e e n the virus strains included in
the vaccine and those circulating during the influenza season.
W h e n the m a t c h b e t w e e n vaccine and circulating viruses is
close, influenza vaccine h a s b e e n s h o w n to prevent illness in
approximately 7 0 % of healthy children and y o u n g adults,
whilst preventing hospitalisation for p n e u m o n i a and
influenza a m o n g elderly p e o p l e living in the community.
Chemoprophylaxis
Antiviral drugs, such as amantadine and rimantadine are
effective against all type A influenza viruses (including H5N1
and H 9 N 2 ) . If e m p l o y e d at the beginning of an outbreak,
these drugs can prevent disease and spread of the virus. Of
those receiving these antivirals within 2 4 to 4 8 h o u r s of the
onset of the disease, 7 0 % to 9 0 % h a v e greatly reduced
symptoms. However, effectiveness is limited b y the rapid
e m e r g e n c e of resistant viral strains a n d b y the adverse effects
of the drugs themselves (66, 166). T h e recent addition of
neuraminidase inhibitors (GG167 and GS4104) to the
portfolio of antiviral drugs is an important development.
Zanamivir (GG167) is efficacious against influenza virus
(including H 5 N 1 ) infection in h u m a n s and animals without
apparent side-effects or the induction of resistant influenza A
viruses under clinical trial conditions (58, 67). Antivirals offer
the advantage of not interfering with antibody production
since infection is not c o m p l e t e l y prevented.
Perspectives
The next pandemic
Prediction of the next p a n d e m i c strain of influenza is
important to allow greater preparedness and successful
vaccine prophylaxis. However, implicit in predicting the next
virus subtype, or at least the next HA subtype, is a k n o w l e d g e
of h o w p a n d e m i c viruses emerge. As discussed earlier, several
theories exist regarding h o w antigenic shifts occur and
p a n d e m i c s arise. T h e p r o b l e m in correctly assessing the actual
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Rev. sci. tech. Off. int. Epiz., 19 (1)
m e c h a n i s m s is that relatively little data is available, as viruses
from only three p a n d e m i c s exist, although recently at least
s o m e molecular data o n the 1918 p a n d e m i c virus h a s b e c o m e
available (130, 170). As this important w o r k progresses,
further understanding of p a n d e m i c s in general, and of the
singular virulence of the 1 9 1 8 virus in particular, is
anticipated. T h e current consensus o p i n i o n appears to b e that
p a n d e m i c viruses arise as a result of transfer to h u m a n s of all
or part of the g e n o m e of viruses a d a p t e d to another animal
species, w h e r e the virus is c a p a b l e or b e c o m e s c a p a b l e of
spread in a fully susceptible population, or at least in that
proportion of the population that is susceptible. However,
this d o e s not explain w h y this transference appears to h a p p e n
so rarely and w h y s o f e w subtypes appear to circulate.
Equally, if pigs are important as a n intermediate host for the
emergence of p a n d e m i c virus as a result of reassortment
b e t w e e n avian and h u m a n viruses, an explanation is required
to describe w h y the n u m b e r of subtypes of avian influenza
virus isolated naturally from pigs is limited to H1 and H 3 . T h e
H5 and H9 viruses isolated from h u m a n s in Hong K o n g
indicate that direct avian influenza virus infection a n d
reassortment with a h u m a n virus c o u l d also result in the
emergence of p a n d e m i c virus. This in turn raises the question
of w h y this appears to occur so rarely and w h y three natural
transmissions w e r e r e c o r d e d b e t w e e n 1996 a n d 1999, but
none a p p e a r e d to b e r e c o r d e d before that date. Hosts of
influenza A viruses and the potential for r e c o m b i n a t i o n
and/or spread to h u m a n s are summarised in Table TV.
Data is insufficient to confirm any of the current popular
theories o n the e m e r g e n c e of p a n d e m i c influenza viruses or to
dismiss any of the less likely theories. In addition, n o n e of the
theories are necessarily mutually exclusive, and it c o u l d b e
that these viruses arise b y several different m e c h a n i s m s .
Virulence
As discussed earlier in regard to susceptible birds, a very clear
distinction exists b e t w e e n influenza viruses of l o w virulence
w h i c h p r o d u c e disease similar to that of m a m m a l s , and the
viruses w h i c h cause HPAI w h i c h m a y result in mortality rates
approaching 100% in infected flocks. T h e primary reason for
this difference is the p r e s e n c e of multiple basic a m i n o acids at
the cleavage site of the haemagglutinin precursor, w h i c h in
turn allows the virulent viruses to replicate systemically, while
those l o w virulence viruses are restricted to replication in the
respiratory and intestinal tracts (134). This finding in birds
does have s o m e implications for h u m a n infections. W h i l e it is
clear that trypsin-like e n z y m e s m a y bring about cleavage of
the viruses with a single arginine, furin is the prime candidate
for a ubiquitous protease responsible for cleaving HAO with
multiple basic a m i n o acids, allowing systemic replication and
ultimately death. Mammals, including h u m a n s , also have
furin-like proteases c a p a b l e of cleaving at multiple basic
amino acid motifs. However, the question as to w h e t h e r or
not viruses with multiple basic a m i n o acids at the HAO
cleavage site c o u l d cause systemic infections and highly
pathogenic disease in h u m a n s remains unanswered, b e c a u s e
the viruses k n o w n to h a v e infected h u m a n s ( H l , H 2 and H3
subtypes) all h a v e motifs at the cleavage site indicating they
w o u l d only b e cleaved b y trypsin-like e n z y m e s . T h e h i g h
mortality associated with the 1 9 1 8 p a n d e m i c virus was
thought to h a v e b e e n the result of a multiple basic a m i n o acid
HAO cleavage site, but this p r o v e d not to b e the case (130).
T h e o b v i o u s inference was that the very h i g h mortality (6/18),
amongst the p e o p l e infected with the H 5 N 1 virus in H o n g
K o n g occurred b e c a u s e the virus w a s c a p a b l e of systemic
infection d u e to the k n o w n p r e s e n c e of multiple basic amino
acids at the HAO cleavage site, allowing cleavage to b e
m e d i a t e d b y o n e or m o r e furin-like proteases. However,
evidence that this w a s the case is lacking. Generally, the
eighteen patients presented with severe respiratory s y m p t o m s
and p n e u m o n i a a p p e a r e d to b e the principal cause of death,
not unlike infections with ' c o m m o n ' influenza viruses.
Similarly, infections of other m a m m a l s with avian influenza
viruses give few clues to the significance of multiple basic
a m i n o acids at the HAO cleavage site. An infection of h a r b o u r
seals from 1978 to 1980 off the north-east coast of the USA,
with H7N7 avian influenza, resulted in the d e a t h of an
estimated 2 0 % of the population. W h i l e this mortality rate is
c o m p a r a b l e to that w h i c h occurred in h u m a n s in H o n g Kong,
the HAO cleavage site of the H7N7 virus did not h a v e a motif
containing multiple basic amino acids (189). Conversely,
H7N7 viruses responsible for equine influenza type 1, for
w h i c h A/equine/Prague/56 (H7N7) is the type strain, d o have
multiple basic amino acids at the HAO cleavage site, and yet in
infections of h o r s e s with this strain, virus replication is
invariably restricted to the respiratory tract (50).
Although the question of the virulence of the 1 9 1 8 p a n d e m i c
virus c o u l d not b e answered b y the HAO cleavage site amino
acid motif, another m e c h a n i s m for cleaving viral HAO that
confers increased virulence of the virus, p r o p o s e d b y G o t o
and K a w a o k a , c o u l d have s o m e relevance (54). T h e authors
investigated a variant h u m a n H1N1 virus w h i c h is
neurotropic for m i c e and discovered that the NA b i n d s and
sequesters the ubiquitous protease plasminogen, leading to
the production of h i g h levels of plasmin at the surface of the
virus. This in turn ensures efficient cleavage of the HA,
enabling the virus to invade a w i d e variety of tissues.
Plasminogen sequestration has not b e e n found in any other
h u m a n influenza virus, but it will b e interesting to see if future
genetic analyses reveal that the 1918 virus neuraminidase h a d
the genetic markers for this property, namely: a
carboxyl-terminal lysine and lack of a glycosylation site at
position 146.
Surveillance
Clearly, careful surveillance c o u l d lead to the early
identification of viruses representing candidates for a n e w
p a n d e m i c virus. However, s o m e p r o b l e m s arise in achieving
the benefits of unrestricted surveillance in all species, and
targeted surveillance therefore s e e m s m o r e appropriate.
Nevertheless, n o distinct markers for identifying a panzootic
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Rev. sci. tech. Off. int. Epiz., 19 (1)
virus or a potential progenitor p a n d e m i c virus are currently
k n o w n . Consequently, identifying a specific, narrow target for
surveillance is difficult.
T h e n u m b e r s a n d variety of influenza viruses isolated from
surveillance of wild birds h a v e not b e e n particularly helpful in
the prediction of h u m a n influenza patterns, although this
surveillance m a y b e of importance from the point of v i e w of
disease in d o m e s t i c poultry. Infections and trends of infection
in d o m e s t i c poultry m a y b e of greater significance and
importance to h u m a n s . T h e current w i d e s p r e a d infections of
c o m m e r c i a l poultry flocks in m a n y countries of Asia with
H 9 N 2 (see the subsection entided 'Domestic poultry' in
'Ecology') m a y b e extremely significant in v i e w of the k n o w n
infections of h u m a n s with virus of this subtype. Surveillance
studies in other domestic animals m a y also b e particularly
pertinent, especially in pigs, in v i e w of their k n o w n
susceptibility to b o t h avian and h u m a n viruses.
Given the apparent origin of three of the four p a n d e m i c s
w h i c h occurred in the 20th Century, and possibly s o m e of the
earlier p a n d e m i c s , in the south of the People's Republic of
China, Shortridge and Stuart-Harris p r o p o s e d the c o n c e p t of
an influenza epicentre, i.e., a geographical area w h e r e birds,
other animals and h u m a n s live closely together in conditions
w h e r e viruses have the greatest opportunity to pass from o n e
species to another (153). Surveillance and monitoring of
influenza viruses in various species w o u l d s e e m prudent in
that geographical region, although not to the exclusion of
other countries and geographical areas.
Contingency plans
Pandemics of influenza exert extreme pressures o n all aspects
of society in terms of b o t h public health and e c o n o m y . Even
the relatively mild p a n d e m i c of 1968 h a d e n o r m o u s impacts
in d e v e l o p e d countries through absenteeism, loss of earnings,
pressures o n hospital services and costs of palliative
medication. It is essential that governments draw up
practicable contingency plans in advance of the next
p a n d e m i c to deal with o b v i o u s essential planning s u c h as the
production and stockpiling of vaccine and antiviral chemicals,
prioritisation of treatment and arrangements for the
functioning of hospitals, e m e r g e n c y services and even
government in the face of mass incapacitation of the working
force. In addition, s h o u l d mortality b e c o m p a r a b l e to the
1918 p a n d e m i c , the potential for hysteria and social upheaval
amongst the population must not b e o v e r l o o k e d . It is essential
that any contingency plan for p a n d e m i c influenza should b e
reviewed frequently and regularly to assimilate n e w scientific
k n o w l e d g e in this fast-moving field.
Zoonoses récentes dues aux virus influenza A
D.J. Alexander & I.H. Brown
Résumé
La grippe est une maladie aiguë très contagieuse qui frappe les hommes ainsi que
les animaux depuis des t e m p s reculés. Les virus influenza, qui f o n t partie de la
famille des Orthomyxoviridae,
sont répartis en trois types : A , B et C, selon les
caractéristiques antigéniques de la nucléoprotéine. Les virus influenza de type A
infectent une grande variété d'espèces animales (porcins, équidés, mammifères
marins, o i s e a u x ) ; chez l'homme, ils sont à l'origine de pandémies parfois
dévastatrices, c o m m e celle qui fit plus de vingt millions de victimes dans le monde
en 1918. Les deux antigènes de surface du virus, l'hémagglutinine et la
neuraminidase, sont les plus importants par les propriétés immunologiques qu'ils
confèrent à l'hôte et sont donc soumis aux plus fortes variations. S'agissant des
virus influenza A, quinze sous-types de l'hémagglutinine et neuf sous-types de la
neuraminidase, antigéniquement distincts, sont actuellement r é p e r t o r i é s ; un
virus possède un sous-type de l'hémagglutinine et un sous-type de la
neuraminidase, combinés de manière a p p a r e m m e n t aléatoire. Les virus isolés
chez les m a m m i f è r e s présentent relativement p e u de combinaisons de
sous-types, tandis que chez les oiseaux, tous les sous-types, dans la plupart des
combinaisons, ont été isolés. A u X X siècle, des souches antigéniquement
e
217
Rev. sci. tech. Off. int. Epiz., 19 (1)
différentes sont apparues soudainement chez l'homme, phénomène désigné
comme cassure antigénique (shift : en 1918 (H1N1), en 1957 (H2N2), en 1968
(H3N2) et en 1977 ( H 1 N 1 ) ; ces mutations ont c h a c u n e été à l'origine d'une
pandémie. Des épidémies surviennent f r é q u e m m e n t entre deux pandémies, sous
l'effet d'une dérive antigénique (drift du virus prévalent. Les épidémies qui
sévissent actuellement dans le monde sont dues aux sous-types H1N1 et H3N2 du
virus influenza A ou au virus influenza B. Pour mesurer l'impact de ces épidémies
la meilleure méthode reste le suivi épidémiologique des épisodes meurtriers de
pneumonie et de grippe. Des études phylogénétiques montrent que des oiseaux
aquatiques pourraient être la source de tous les virus influenza A affectant les
autres espèces. On pense que l'apparition des souches responsables de
pandémies chez l'homme est le résultat de l'un des trois mécanismes suivants :
- le réassortiment génétique (dû au fait que le génome du virus c o m p o r t e
plusieurs segments) de virus influenza A aviaires et humains ayant infecté le
même hôte ;
- le transfert direct de l'ensemble du virus d'une autre espèce ;
- la réémergence d'un virus ayant déjà provoqué une épidémie dans le passé.
Depuis 1996, les virus H7N7, H5N1 et H9N2 se sont transmis des oiseaux à
l'homme, a p p a r e m m e n t sans se propager dans la population humaine. De rares
cas de transmission de l'homme aux animaux ont également été établis. Ces
observations ont conduit à l'hypothèse que le réassortiment éventuel des virus
humains et aviaires intervient chez une espèce intermédiaire, avec transfert
ultérieur à l'homme. Les porcins jouent probablement ce rôle d'intermédiaire : en
effet, ces animaux peuvent servir d'hôtes à des virus pathogènes aviaires et
h u m a i n s ; de plus, il a été prouvé que les porcins sont impliqués dans la
transmission entre espèces des virus influenza, n o t a m m e n t la transmission du
virus H1N1 à l'homme. La surveillance mondiale de la grippe est assurée par un
réseau de laboratoires placé sous l'égide de l'Organisation mondiale de la santé.
La principale mesure prophylactique de la grippe chez l'homme est la v a c c i n a t i o n ,
qui a pour objectif d'éviter l'apparition d'épidémies entre deux pandémies.
Mots-clés
Écologie - Grippe - Influenza - Mammifères - Oiseaux - Pandémie - Prophylaxie Réassortiment - Santé publique - Transmission entre espèces - Zoonoses.
•
Zoonosis recientes causadas por virus de la influenza A
D.J. Alexander &I.H. Brown
Resumen
La influenza (o gripe) es una enfermedad aguda y extremadamente contagiosa
que viene afectando a hombres y animales desde tiempos remotos. Los virus
causantes de esa enfermedad, pertenecientes a la familia Orthomyxoviridae,
se
clasifican, según las propiedades antigénicas de sus proteínas de nucleocápside,
en tres tipos distintos: A, B y C. Los virus de la influenza de tipo A infectan a un
amplio abanico de especies animales, que comprenden el cerdo, el caballo,
218
Rev. sci. tech. Off. int. Epiz., 19 (1)
mamíferos marinos y aves, y provocan también ocasionalmente en el hombre
pandemias de consecuencias devastadoras, como la de 1918 que causó la
muerte de más de veinte millones de personas en todo el mundo. Las dos
glucoproteínas de superficie del virus, la hemaglutinina y la neuraminidasa, son
los principales antígenos que inducen inmunidad protectora en el huésped, y
exhiben por consiguiente el mayor nivel de variación. Para los virus de la
influenza A se c o n o c e n actualmente quince subtipos de la hemaglutinina y nueve
subtipos de la neuraminidasa antigénicamente distintos. Un virus posee un solo
subtipo de cada una de las proteínas, que parecen presentarse en cualquier
combinación. Pese a que en especies de mamíferos se han aislado sólo unas
pocas combinaciones de subtipos, en las aves se han obtenido todos los subtipos,
asociados en casi todas las combinaciones posibles. El siglo XX ha presenciado
en cuatro ocasiones la aparición repentina de cepas antigénicamente distintas
en el ser humano, un fenómeno llamado "salto a n t i g é n i c o " (shift) que en cada
ocasión ha dado lugar a una pandemia: en 1918 (H1N1), 1957 (H2N2), 1968 (H3N2) y
1977 (H1N1). En los intervalos entre esas grandes pandemias también se han
declarado frecuentes epidemias, producto de cambios antigénicos graduales en
el virus prevalente, proceso conocido como "deriva antigénica" (drift). En la
actualidad, las poblaciones humanas de todo el mundo sufren epidemias
causadas por los subtipos H1N1 y H3N2 del virus de la influenza A y por el virus de
la influenza B. Uno de los procedimientos más eficaces para medir los efectos de
esas epidemias consiste en registrar el exceso de mortalidad debida a neumonías
o gripes. Ciertos estudios filogenéticos sugieren que tal vez las aves acuáticas
sean la fuente de todos los virus de la influenza A que afectan a las demás
especies. Se piensa que las cepas causantes de pandemias en el hombre han
surgido por uno de los tres mecanismos siguientes:
- reordenamiento genético (proceso resultante de la segmentación del genoma
vírico) entre virus aviares y humanos de la influenza A presentes en un mismo
hospedador;
- transferencia directa del virus entero desde otra especie al hombre;
- resurgimiento de un virus que pudo haber causado una epidemia muchos años
antes.
Desde 1996, los virus H7N7, H5N1 y H9N2 se han transmitido de las aves al
hombre, aunque en apariencia no han conseguido difundirse entre las
poblaciones humanas. Aunque está demostrado que el virus puede transmitirse
de animales a humanos, los episodios de este tipo son poco frecuentes. Ello ha
llevado a pensar que el hipotético reordenamiento entre virus aviares y humanos
tiene lugar en un animal intermediario, con posterior transmisión del virus
resultante a la población humana. M u c h o s ven en el cerdo el principal candidato
a esa función intermediaria, pues no sólo puede ejercer de hospedador de
infecciones productivas de virus tanto humanos como aviares sino que, además,
hay sólidas pruebas de su intervención en episodios de transmisión
interespecifica de virus de la influenza, y particularmente en la transmisión de
virus H1N1 al hombre. La labor de vigilancia de la enfermedad a escala mundial
recae en una red de laboratorios patrocinados por la Organización Mundial de la
Salud. La principal medida de lucha contra la gripe en el ser humano es la
inmunoprofilaxis, destinada sobre todo a combatir la aparición de epidemias
entre pandemias.
Palabras clave
Aves - Ecología - Influenza - Mamíferos - Pandemia - Profilaxis - Reordenamiento Salud pública-Transmisión interespecífica - Zoonosis.
Rev. sci. tech. Off. int. Epiz., 19 (1)
219
References
1. Acland H.M., Silverman-Bachin L.A. & Eckroade R.J.
(1984). - Lesions in broiler and layer chickens in an
outbreak of highly pathogenic avian influenza virus
infection. Vet. Pathol, 2 1 , 564-569.
2. Alexander D.J. (1982). - Ecological aspects of influenza A
viruses in animals and their relationship to human influenza:
a review. J. roy. Soc. Med., 75, 799-811.
3. Alexander D.J. (1993). - Orthomyxovirus infections. In Viral
infections of vertebrates, Vol. III. Viral infections of birds
(J.B. McFerran & M.S. McNulty, eds). Elsevier, Amsterdam,
287-316.
4. Alexander D.J. (1998). - Avian influenza viruses and
pandemic influenza in humans. State vet.]., 8, 8-10.
5. Alexander D.J. (2000). - A review of avian influenza. In Proc.
of the European Society for Veterinary Virology (ESVV)
Symposium on influenza viruses of wild and domestic
animals, 16-18 May 1999, Ghent. Vet. Microbiol (in press).
6. Alexander D.J., Allan W.H., Parsons D. & Parsons G. (1978).
- The pathogenicity of four avian influenza viruses for fowls,
turkeys and ducks. Res. vet. Sci., 24, 242-247.
7. Alexander D.J. & Spackman D. (1981). - Characterisation of
influenza A viruses isolated from turkeys in England during
March-May 1979. Avian Pathol, 10, 281-283.
8. Alexander D.J., Parsons G. & Manvell R.J. (1986). Experimental assessment of eight avian influenza A viruses of
H5 subtype for chickens, turkeys, ducks and quail. Avian
Pathol, 15,647-662.
9. Alexander D.J. & Gough R.E. (1997). - Virus diseases of the
respiratory organs: world situation and recent developments.
In Proc. 11th International Congress of the World Veterinary
Poultry Association. Acta vet. hung., 45 (Suppl), 1-22.
10. Anon. (1997). - United Kingdom Health Departments'
multiphase contingency plan for pandemic influenza.
Department of Health, London, 72 pp.
11. Aymard M., Brigaud M., Chastel C., Fontaine M., Tillon J.-P.
& Vannier P. (1980). - Comparaison de l'immunité sérique
anti-influenza A de diverses populations humaines et des
porcs. Comp. Immunol. Microbiol. infect. Dis., 3, 111-119.
12. Banks J., Speidel E. & Alexander DJ. (1998). Characterisation of an avian influenza A virus isolated from a
human - is an intermediate host necessary for the emergence
of pandemic influenza viruses? Arch. Virol, 143, 781-787.
13. Beare A.S. & Webster R.G. (1991). - Replication of avian
influenza viruses in humans. Arch. Virol, 119, 37-42.
14. Becker W.B. (1966). - The isolation and classification of
Tern virus: influenza A-Tern South Africa - 1961. J. Hyg.
(London), 64, 309-320.
15. Bender C , Hall H., Huang J., Klimov A., Cox N., Hay A.,
Gregory V., Cameron K., Lim W. & Subbarao K. (1999). Characterisation of the surface proteins of influenza A
(H5N1) viruses isolated from humans in 1997-1998.
Virology, 254, 115-123.
16. Berg M., Englund L., Abusugra I.A., Klingebom A. &
Linne T. (1990). - Close relationship between mink
influenza (H10N4) and concomitantly circulating avian
influenza viruses. Arch. Virol, 113, 61-71.
17. Beveridge W.I.B. (ed.) (1977). - Influenza. The last great
plague. Heinemann, London, 124 pp.
18. Binns M.M., Daly J.M., Chirnside E.D., Mumford J.A.,
W o o d J.M., Richards CM. & Daniels R.S. (1993). - Genetic
and antigenic analysis of an equine influenza H3 isolate from
the 1989 epidemic. Arch. Virol, 130, 33-43.
19. Blaskovic D., Jamrichova O., Rathova V., Skoda R.,
Kociskova D. & Kaplan M.M. (1970). - Experimental
infection of weaning pigs with A/swine influenza virus. 2.
The shedding of virus by infected animals. Bull. WHO, 4 2 ,
767-770.
20. Brown I.H. (2000). - The epidemiology and evolution of
influenza viruses in pigs. In Proc. of the European Society for
Veterinary Virology (ESVV) Symposium on influenza viruses
of wild and domestic animals, 16-18 May 1999, Ghent. Vet.
Microbiol. (in press).
21. Brown I.H., Done S.H., Spencer Y.I., Cooley W.A.,
Harris P.A. & Alexander D.J. (1993). - Pathogenicity of a
swine influenza H1N1 virus antigenically distinguishable
from classical and European strains. Vet. Rec., 132, 598-602.
22. Brown I.H., Alexander D.J., Chakraverty P., Harris P.A. &
Manvell R.J. (1994). - Isolation of an influenza A virus of
unusual subtype (H1N7) from pigs in England, and the
subsequent experimental transmission from pig to pig. Vet.
Microbiol, 39, 125-134.
23. Brown I.H., Chakraverty P., Harris P.A. & Alexander DJ.
(1995). - Disease outbreaks in pigs in Great Britain due to an
influenza A virus of H1N2 subtype. Vet. Rec., 136, 328-329.
24. Brown I.H., Harris P.A. & Alexander D.J. (1995). Serological studies of influenza viruses in pigs in Great
Britain 1991-1992. Epidemiol Infect., 114, 511-520.
25. Brown I.H., Crawshaw T.R., Harris P.A. & Alexander D.J.
(1998). - Detection of antibodies to influenza A virus in
cattle in association with respiratory disease and reduced
milk yield. Vet. Ree, 143, 637-638.
26. Brown I.H., Harris P.A., McCauley J.W. & Alexander D.J.
(1998). - Multiple genetic reassortment of avian and human
influenza A viruses in European pigs, resulting in the
emergence of an H l N 2 virus of novel genotype. J. gen. Virol,
79, 2947-2955.
220
27. Burnet F.M. & Stone J.D. (1947). - The receptor destroying
enzyme of Vibrio cholera. Aust. J. experim. Biol. med. Sci., 25,
227-233.
28. Callan R.J., Early G., Kida H. & Hinshaw V.S. (1995). - The
appearance of H3 influenza viruses in seals. J. gen. Virol., 76,
199-203.
29. Campbell C.H., Webster R.G. & Bresse S.S. (1970). - Fowl
plague virus from man. J. infect. Dis., 122, 513-516.
30. Campitelli L., Donatelli I., Foni E., Castrucci M.R.,
Fabiani C , Kawaoka Y., Krauss S. & Webster R.G. (1997). Continued evolution of H1N1 and H3N2 influenza viruses
in pigs in Italy. Virology, 2 3 2 , 310-318.
31. Campos-Lopez H., Rivera-Cruz E. & Irastorza-Enrich M.
(1996). - Situación y perspectivas del programa de
erradicación de la influenza aviar en México. In Proc. 45th
Western Poultry Disease Conference, 1-5 May, Cancun,
Mexico, 13-16.
Rev. sci. tech. Off. int. Epiz., 19 (1)
42. Easterday B.C. (1978). - The enigma of zoonotic influenza.
In Proc. 3rd Symposium on Microbiology. World Health
Organization Collaborating Centre for Collection and
Evaluation of Data on Comparative Virology, Munich,
102-107.
43. Easterday B.C. (1980). - Animals in the influenza world.
Philos. Trans. roy. Soc. Lond., B, 288, 433-437.
44. Easterday B.C. (1980). - The epidemiology and ecology of
swine influenza as a zoonotic disease. Comp. Immunol.
Microbiol. infect. Dis., 3, 105-109.
45. Easterday B.C. (1981). - Swine influenza. In Diseases of
swine, 5th Ed. (A.D. Leman, ed.). Iowa State University
Press, Iowa, 181-194.
46. Easterday B.C. & Beard C.W. (1984). - Avian influenza. In
Diseases of poultry, 8th Ed. (M.S. Hofstad, H.J. Barnes,
B.W. Calnek, W.M. Reid & H.W. Voder, eds). Iowa State
University Press, Iowa, 482-496.
32. Castrucci M.R., Donatelli I., Sidoli L., Barigazzi G.,
Kawaoka Y. & Webster R.G. (1993). - Genetic reassortment
between avian and human influenza A viruses in Italian pigs.
Virology, 193, 503-506.
47. Fang R., Min J o u W., Huylebroeck D., Devos R. & Fiers W.
(1981). - Complete structure of A/duck/Ukraine/63
influenza haemagglutinin gene: animal virus as progenitor of
human H3 Hong Kong 1968 influenza haemagglutinin. Cell,
25, 315-323.
33. Chambers T.M., Yamnikova S., Kawaoka Y., Lvov D.K. &
Webster R.G. (1989). - Antigenic and molecular
characterisation of subtype H13 haemagglutinin of influenza
virus. Virology, 172, 180-188.
48. Fatkhuddinova M.F., Kir'ianova A.I., Isachenko V.A. &
Zakstel'skaya L.Y. (1973). - Isolation and identification of
the A-Hong Kong (H3N2) virus in respiratory diseases of
cattle. [In Russian.] Vopr. Virusol, 18 (4), 464-478.
34. Claas E.C.J., Kawaoka Y., De Jong J.C., Masurel N. &
Webster R.G. (1994). - Infection of children with
avian-human reassortant influenza virus from pigs in
Europe. Virology, 204, 453-457.
49. Gething M.J., Bye J., Skehel J. & Waterfield M. (1980). Cloning and DNA sequence of double-stranded copies of
haemagglutinin genes from H2 and H3 strains elucidates
antigenic shift and drift in human influenza virus. Nature,
287, 301-306.
35. Claas E.C.J., Osterhaus A.D.M.E., Van Beek R., De Jong J.C.,
Rimmelzwaan G.F., Senne D.A., Krauss S., Shortridge K.F. &
Webster R.G. (1998). - Human influenza A H5N1 virus
related to a highly pathogenic avian influenza virus. Lancet,
351, 472-477.
36. Couch R.B., Douglas R.G., Giggs S., Knight V. & Kasel J.A.
(1969). - Production of the influenza syndrome in man with
equine influenza. Nature, 224, 512-514.
37. Couch R.B. & Kasel J.A. (1983). - Immunity to influenza in
man. Annu. Rev. Microbiol, 37, 529-549.
38. Dasco C.C., Couch R.B., Six H.R., Young J.F., Quarles J.M.
& Kasel J.A. (1984). - Sporadic occurrence of zoonotic
swine influenza virus infections. J. clin. Microbiol, 20,
833-835.
39. Dorset M., McBryde C.N. & Niles W.B. (1922). - Remarks
on 'Hog Flu'. J. Am. vet. med. Assoc., 62, 162-171.
50. Gibson C.A., Daniels R.D., McCauley J.W. & Schild G.C.
(1988). - Hemagglutinin gene sequencing studies of equine
1 influenza A viruses. In Proc. 5th International Conference
on equine infectious diseases, 7-10 October 1987,
Lexington, Kentucky. University Press of Kentucky,
Lexington, 51-59.
51. Girard C , Morin M. & Elazhary Y. (1992). - Experimentally
induced porcine proliferative and necrotising pneumonia
with an influenza A virus. Vet. Rec., 130, 206-207.
52. Gorman O.T., Bean W.J., Kawaoka Y., Donatelli I., Guo Y.J.
& Webster R.G. (1991). - Evolution of influenza A virus
nucleoprotein genes: implications for the origins of H1N1
human and classical swine viruses. J. Virol, 65, 3704-3714.
53. Goto H., Takai M., Iguchi H., Benkele W., Ohta C,
Yamamoto Y., Kida H., Noro S. & Sakurada N. (1992). Antibody responses of swine to type A influenza viruses in
the most recent several years. J. vet. med. Sci., 54, 235-241.
40. Douglas R.G. (1975). - Influenza in man. In The influenza
viruses and influenza (E.D. Kilboume, ed.). Academic Press,
Inc., New York, 395-443.
54. Goto H. & Kawaoka Y. (1998). - A novel mechanism for the
acquisition of virulence by human influenza A virus. Proc.
natl Acad. Sci. USA, 95, 10224-10228.
41. Dowdle W.R. (1976). - Influenza: epidemic patterns and
antigenic variation. In Influenza: virus, vaccines, and strategy
(P. Selby, ed.). Academic Press, Inc, London, 17-21.
55. Gourreau J.M., Kaiser C , Valette M., Douglas A.R., Labie J.
& Aymard M. (1994). - Isolation of two H1N2 influenza
viruses from swine in France. Arch. Virol, 135, 365-382.
Rev. sci. tech. Off. int. Epiz., 19 (1)
56. Guan Y., Shortridge K.F., Krauss S., Li P.H., Kawaoka Y. &
Webster R.G. (1996). - Emergence of avian H1N1 influenza
viruses in pigs in China. J. Virol, 70, 8041-8046.
57. Guan Y., Shortridge K.F., Krauss S. & Webster R.G. (1999).
- Molecular characterization of H9N2 influenza viruses:
were they the donors of the 'internal' genes of H5N1 viruses
in Hong Kong? Proc. natl Acad. Sci. USA, 96, 9363-9367.
58. Gubareva L.V., McCullers J.A., Bethell R.C. & Webster R.G.
(1998). Characterization
of influenza
A/Hong
Kong/156/97 (H5N1) virus in a mouse model and protective
effect of zanamivir on H5N1 infection in mice. J. infect. Dis.,
178, 1592-1596.
59. Guo Y., Min W., Fenigen J., Ping W. & Jiming Z. (1983). Influenza ecology in China. In The origin of pandemic
influenza viruses (W.G. Laver, ed.). Elsevier Science
Publishing, Inc., New York, 211-220.
60. Guo Y., Wang M., Kawaoka Y., Gorman O., Ito T. & Saito T.
(1992). - Characterisation of a new avian-like influenza A
virus from horses in China. Virology, 188, 245-255.
61. Haesebrouck F., Biront B., Pensaert M.B. & Leunen J.
(1985). - Epizootics of respiratory tract disease in swine in
Belgium due to H3N2 influenza virus and experimental
reproduction of disease. Am.J. vet. Res., 46, 1926-1928.
62. Halvorson D.A., Kodihalli S., Laudert E., Newman J.A.,
Pomeroy B.S., Shaw D. & Sivanandan V. (1992). - Influenza
in turkeys in the USA, 1987-1991. In Proc. 3rd International
Symposium on avian influenza, 27-29 May, Wisconsin.
University of Wisconsin, Madison, 33-42.
63. Halvorson D.A., Frame D.D., Friendshuh A.J. & Shaw D.P.
(1998). - Outbreaks of low pathogenicity avian influenza in
USA. In Proc. 4th International Symposium on avian
influenza, 28-31 May 1997, Athens, Georgia. American
Association of Avian Pathologists, Pennsylvania, 36-46.
64. Hampson A.W. (1997). - Surveillance for pandemic
influenza. J. infect. Dis., 176 (Suppl. 1), 8-13.
65. Hannant D. & Mumford J.A. (1996). - Equine influenza. In
Virus infections of equines (E. Studdert, ed.). Elsevier,
Amsterdam, 285-293.
66. Hayden F.G., Belshe R.B., Clover R.D., Hay A.J., Oakes M.G.
& Soo W. (1989). - Emergence and apparent transmission
of rimantadine-resistant influenza A virus in families. New
Engl. J. Med., 3 2 1 , 1696-1702.
67. Hayden F.G., Osterhaus A.D.M.E. & Treanor J.J. (1997). Efficacy and safety of neuraminidase inhibitor zanamivir in
the treatment of influenza virus infections. New Engl. J. Med.,
337, 874-880.
68. Heilman C. & La Montagne J.R. (1990). - Influenza: status
and prospects for its prevention, therapy and control.
Pediatr. Clin. N. Am., 37, 669-688.
69. Heller L., Espmark A. & Virden P. (1956). - Immunological
relationship between infectious cough in horses and human
influenza A. Arch. ges. Virusforsch., 7, 120-124.
221
70. Hinshaw V.S., Bean W.J., Webster R.G. & Easterday B.C.
(1978). - The prevalence of influenza viruses in swine and
the antigenic and genetic relatedness of influenza viruses
from man and swine. Virology, 84, 51-62.
71. Hinshaw V.S., Webster R.G. & Turner B. (1980). - The
perpetuation of orthomyxoviruses and paramyxoviruses in
Canadian waterfowl. Can.J. Microbiol, 26, 622-629.
72. Hinshaw V.S., Webster R.G. & Rodriguez J. (1981). Influenza A viruses: combinations of haemagglutinin and
neuraminidase subtypes isolated from animals and other
sources. Arch. Virol, 67, 191-206.
73. Hinshaw V.S., Bean W.J., Geraci J.R., Fiorelli P., Early G. &
Webster R.G. (1986). - Characterisation of two influenza A
viruses from a pilot whale. J. Virol., 58, 655-656.
74. Hirst G.K. & Pons M.W. (1973). - Mechanism of influenza
recombination. II. Virus aggregation and its effect on plaque
formation by so-called noninfective virus. Virology, 56,
620-631.
75. Hobson D., Curry R.L., Beare A.S. & Ward-Gardner A.
(1972). - The role of serum haemagglutination-inhibition
antibody protection against challenge with influenza A2 and
B viruses. J. Hyg. (London), 70, 767-777.
76. Hodder R.A., Gaydos J.C., Allen R.G., Top F.H.,
Nowosiwsky T. & Russell P.K. (1977). - Swine influenza A
at Fort Dix, New Jersey. Extent of spread and duration of the
outbreak. J. infect. Dis., 136, 369-375.
77. Homme P.J. & Easterday B.C. (1970). - Avian influenza
virus infections. IV. Response of pheasants, ducks, and geese
to influenza A/turkey/Wisconsin/1966 virus. Avian Dis., 14,
285-290.
78. International Committee on Taxonomy of Viruses (1995). Family Orthomyxoviridae. In Virus taxonomy: classification
and nomenclature of viruses. Sixth Report of the
International Committee on Taxonomy of Viruses
(F.A. Murphy, C.M. Fauquet, D.H.L. Bishop, S.A. Ghabrial,
M.A.W. Jarvis, G.P. Martelli, M.A. Mayo & M.D. Summers,
eds). Springer-Verlag, Vienna and New York, 293-299.
79. Ito T., Nelson J., Couceiro S.S., Kelm S., Baum L.G.,
Krauss S., Castrucci M.R., Donatelli L, Kida H., Paulson J.C.,
Webster R.G. & Kawaoka Y. (1998). - Molecular basis for
the generation in pigs of influenza A viruses with pandemic
potential. J. Virol., 72, 7367-7373.
80. Johnson D.C, Maxfield B.C. & Moulthrop J.I. (1977). Epidemiologic studies of the 1975 avian influenza outbreak
in chickens in Alabama. Avian Dis., 2 1 , 167-177.
81. Jones T.C. & Maurer F.D. (1943). - The pathology of equine
influenza. Am.J. vet. Res., 1, 15-31.
82. Jones-Lang K., Ernst-Larson M., Lee B., Goyal S.M. & Bey R.
(1998). - Prevalence of influenza A virus (H1N1) antibodies
in bovine sera. Microbiologica, 2 1 , 153-160.
83. Kanegae Y., Sugita S., Shortridge K.F., Yoshioka Y. &
Nerome K. (1994). - Origin and evolutionary pathways of
the H1 haemagglutinin gene of avian, swine and human
influenza viruses: cocirculation of two distinct lineages of
swine virus. Arch. Virol, 134, 17-28.
222
Rev. sci. tech. Off. int. Epiz.. 19(1)
84. Kaplan M.M. (1980). - The role of the World Health
Organization in the study of influenza. Philos. Trans. roy. Soc.
Lond., B, 2 8 8 , 4 1 7 - 4 2 1 .
101. Louria D.B., Blumenfeld H.L. & Ellis J.T. (1959). - Studies
on influenza in the pandemic of 1957-1958. II. Pulmonary
complications of influenza. J. clin. Invest., 38, 213-265.
85. Kaplan M.M. & Webster R.G. (1977). - The epidemiology of
influenza. Sci. Am., 237, 88-106.
102. Ludwig S., Haustein A., Kaleta E.F. & Scholtissek C. (1994).
- Recent influenza A (H1N1) infections of pigs and turkeys
in Northern Europe. Virology, 202, 281-286.
86. Kasel J.A. & Couch R.B. (1969). - Experimental infection in
man and horses with influenza A viruses. Bull. WHO, 4 1 ,
447-452.
87. Katsuda K., Sato S., Shirahata T., Lindstrom S., Nerome R.,
Ishida M., Nerome K. & Goto H. (1995). - Antigenic and
genetic characteristics of H1N1 human influenza virus
isolated from pigs in Japan. J. gen. Virol., 76, 1247-1249.
88. Kawaoka Y., Krauss S. & Webster R.G. (1989). - Avian to
human transmission of the PB1 gene of influenza A virus in
the 1957 and 1968 pandemics. J. Virol, 63, 4603-4608.
89. Kay R.M., Done S.H. & Paton D.J. (1994). - Effect of
sequential porcine reproductive and respiratory disease
syndrome and swine influenza on the growth and
performance of finishing pigs. Vet. Rec., 135, 199-204.
90. Kida H., Yanagawa R. & Matsuoka Y. (1980). - Duck
influenza lacking evidence of disease signs and immune
response. Infect. Immun., 30, 547-553.
91. Kida H., Shortridge K.F. & Webster R.G. (1988). - Origin of
the haemagglutinin gene of H3N2 influenza viruses from
pigs in China. Virology, 162, 160-166.
92. Kida H., Ito T., Yasuda J., Shimizu Y., Itakura C. &
Shortridge K.F. (1994). - Potential for transmission of avian
influenza viruses to pigs. J. gen. Virol, 75, 2183-2188.
93. Klenk H.D., Rott R , Orlich M. & B l o d o m J. (1975). Activation of influenza A viruses by trypsin treatment.
Virology, 68, 426-439.
94. Klingeborn B.L., Englund L., Rott R., Juntii N. &
Rockborn G. (1985). - An avian influenza A virus killing a
mammalian species - the mink. Arch. Virol, 86, 347-351.
95. Kluska V., Macku M. & Mensik J. (1961). - Evidence for pig
influenza virus antibodies in humans. Ceskoslov. Ped., 16,
408-411.
96. Kurtz J., Manvell R.J. & Banks J. (1996). - Avian influenza
virus isolated from a woman with conjunctivitis. Lancet, 348,
901-902.
103. Ludwig S., Stets L., Planz O., Van H., Fitch W.M. &
Scholtissek C. (1995). - European swine virus as a possible
source for the next influenza pandemic? Virology, 212,
555-561.
104. Luoh S.M., McGregor M.W. & Hinshaw V.S. (1992). Hemagglutinin mutations related to antigenic variation in H1
swine influenza viruses. J. Virol, 66, 1066-1073.
105. McCauley J.W. & Mahy B.W.J. (1983). - Structure and
function of the influenza virus genome. Biochem. J., 211,
281-294.
106. Madie J., Martinovic S., Naglic T., Hajsig D. & Cvetnic S.
(1996). - Serological evidence for the presence of A/equine-1
influenza virus in unvaccinated horses in Croatia. Vet. Rec.,
138, 68.
107. Matrosovich M., Zhou N.N., Kawaoka Y. & Webster R.G.
(1999). - The surface glycoproteins of H5 influenza viruses
isolated from humans, chickens and wild aquatic birds have
distinguishable properties. J. Virol., 73, 1146-1155.
108. Miller D.L., Read D., Diamond J., Pereira M.S. &
Chakraverty P. (1973). - Hong Kong influenza in the Royal
Air Force. J. Hyg. (London), 71, 535-547.
109. Mohan R., Saif Y.M., Erickson G.A., Gustafson G.A. &
Easterday B.C. (1981). - Serologic and epidemiologic
evidence of infection in turkeys with an agent related to
swine influenza virus. Avian Dis., 25, 11-26.
110. Molibog E.V., Smetanin M.A. & Yakhno M.A. (1975). Study of type A influenza virus isolated from cattle in
Udmurtiya. Ekol Virus., 3, 63-67.
111. Mumford J. & W o o d J. (1993). - WHO/OIE meeting:
consultation on newly emerging strains of equine influenza.
Report of a meeting held at the Animal Health Trust, 18-19
May 1992, Newmarket, United Kingdom. Vaccine, 11 (11),
1172-1175.
98. Lange W., Vagt M. & Masihi K.M. (1984). - Influenza bei
Schweinen - Verbreitung und Bedeutung für den Menschen.
Bundesgesundheitsblatt, 27, 265-271.
112. Mumford J.A., Chambers T. & W o o d J. (1995). Consultation of OIE and WHO experts on progress in
surveillance of equine influenza and application to vaccine
strain selection. Report of a meeting held at the Animal
Health Trust, 18-19 September. Animal Health Trust,
Newmarket, United Kingdom, 19 pp.
99. Lasley F.A. (1987). - Economics of avian influenza: control
versus non-control. In Proc. 2nd International Symposium
on avian influenza, 3-5 September 1986, Athens, Georgia.
United States Animal Health Association, University of
Wisconsin, 390-399.
113. Naeem K. (1998). - The avian influenza H7N3 outbreak in
South Central Asia. In Proc 4th International Symposium on
avian influenza, 28-31 May 1997, Athens, Georgia.
American Association of Avian Pathologists, Pennsylvania,
31-35.
100. Loosli C.G., Hamre D. & Berlin B.S. (1953). - Airborne
influenza virus A infections in immunised animals. Trans.
Assoc. Am. Physic, 66, 222-230.
114. Nakajima K , Desselburger U. & Palese P. (1978). - Recent
human influenza A (H1N1) viruses are closely related
genetically to strains isolated in 1950. Nature, 274, 334-339.
97. Lang G., Gagnon A. & Geraci J.R. (1981). - Isolation of an
influenza A virus from seals. Arch. Virol, 68, 189-195.
Rev. sci. tech. Off. int Epiz, 19(1)
115. Nakamura R.M., Easterday B.C., Pawlisch R. & Walker G.L
(1972). - Swine influenza: epizootiological and serological
studies. Bull. WHO, 47, 481-487.
116. Nardellì L., Pascucci S., Gualandi G.L. & Loda P. (1978). Outbreaks of classical swine influenza in Italy in 1976.
Zentralbl. Veterinärmed., B, 25, 853-857.
117. Nerome K., Ishida M., Nakayama M., Oya A., Kanai C. &
Suwicha K. (1981). - Antigenic and genetic analysis of
A/Hong Kong (H3N2) influenza viruses isolated from swine
and man. J. gen. Virol, 56, 441-445.
118. Nerome K , Ishida M., Oya A., Kanai C. & Suwicha K.
(1982). - Isolation of an influenza H1N1 virus from a pig.
Virology, 117, 485-489.
119. Okazaki K., Yanagawa R. & Kida H. (1983). - Contact
infection of mink with five subtypes of avian influenza virus.
Arch. Virol, 77, 265-269.
120. Olsen C.W., McGregor M.W., Cooley J., Schantz B., Hotze B.
& Hinshaw V.S. (1993). - Antigenic and genetic analysis of a
recently isolated H1N1 swine influenza virus. Am.J. vet. Res.,
54, 1630-1635.
121. Ouchi A., Nerome K., Kanegae Y., Ishida M., Nerome R.,
Hayashi K., Hashimoto T., Kaji M., Kaji Y. & Inaba Y.
(1996). - Large outbreak of swine influenza in southern
Japan caused by reassortant (H1N2) influenza viruses: its
epizootic background and characterisation of the causative
viruses. J. gen. Virol, 77, 1751-1759.
122. Oxburgh L., Karlsson A. & Klingeborn B. (2000). - Studies
of the circulation of individual lineages of H3N8 equine
influenza virus in Sweden. In Proc. of the European Society
for Veterinary Virology (ESVV) Symposium on influenza
viruses of wild and domestic animals, 16-18 May 1999,
Ghent. Ghent University, 48.
123. Palese P., Schulman J.L., Bodd G. & Meindl P. (1974). Inhibition of influenza and para-influenza virus replication
in tissue culture by 2-deoxy-2,3-dehydro-N-trifluoroacetylneuraminic acid (FANA). Virology, 59, 490.
124. Peiris M., Yam Y.C., Chan K.H., Ghose P. & Shortridge K.F.
(1999). - Influenza A H9N2: aspects of laboratory diagnosis.
J. clin. Microbiol, 37, 3426-3427.
125. Pensaert M., Ottis K., Vandeputte J., Kaplan M.M. &
Bachmann P.A. (1981). - Evidence of natural transmission of
influenza A virus from wild ducks to swine and its potential
importance for man. Bull. WHO, 59, 75-78.
126. Polasa H., Yojana-Devi B. & Sharma G. (1984). - Replication
of human influenza viruses in mice. In Proc. 6th
International Congress of Virology, 1-7 September, Sendai,
Japan. International Congress of Virology, Sendai, 302 pp.
127. Pomeroy B.S. (1982). - Avian influenza in the United States
(1964-1980). In Proc. 1st International Symposium on avian
influenza, 22-24 April 1981, Beltsville, Maryland. Carter
Composition Corporation, Richmond, 13-17.
128. Ravenholt R.T. & Foege W.H. (1982). - 1918 influenza,
encephalitis lethargica, Parkinsonism. Lancet, II, 860-864.
223
129. Reichelderfer P.S., Kendal A.P., Shortridge K.F. &
Hampson A. (1989). - Influenza surveillance in the Pacific
Basin. Seasonality of virus occurrence: a preliminary report.
In Current topics in medical virology (S. Doraisingham,
A.E. Ling & Y.G. Chan, eds). World Scientific, Singapore,
412-438.
130. Reid A.H., Fanning T.G., Hultin J.V. & Taubenberger J.K.
(1999). - Origin and evolution of the 1918 'Spanish'
influenza virus hemagglutinin gene. Proc. natl Acad. Sci. USA,
96, 1651-1656.
131. Rekik M.R., Arora D.J.S. & Dea S. (1994). - Genetic
variation in swine influenza virus A isolate associated with
proliferative and necrotising pneumonia in pigs. J. clin.
Microbiol, 32, 515-518.
132. Roberts D.H., Cartwright S.F. & Wibberley G. (1987). Outbreaks of classical swine influenza in pigs in England in
1986, Vet. Rec., 121, 53-55.
133. Rota P.A., Rocha E.P., Harmon M.W., Hinshaw V.S.,
Sheerar M.G., Kawaoka Y., Cox N.J. & Smith T.F. (1989). Laboratory characterization of a swine influenza virus
isolated from a fatal case of human influenza. J. clin.
Microbiol, 27, 1413-1416.
134. Rott R. (1992). - The pathogenic determinant of influenza
virus. Vet. Microbiol, 33, 303-310.
135. Rowan M.K. (1962). - Mass mortality among European
common terns in South Africa in April-May 1961. Br. Birds,
55, 103-114.
136. Schafer J.R., Kawaoka Y., Bean W.J., Suss J., Senne D. &
Webster R.G. (1993). - Origin of the pandemic 1957 H2
influenza A virus and the persistence of its possible
progenitors in the avian reservoir. Virology, 194, 781-788.
137. Schafer W. (1955). - Sero-immunologic studies on
incomplete forms of the virus of classical fowl plague. [In
German.] Arch. experim. VetMed., 9, 218-230.
138. Schild G.C. (1972). - Evidence for a new type specificstructural antigen of the influenza virus particle. J. gen. Virol,
15,99-103.
139. Schnurrenberger P.R., W o o d s G.T. & Martin R.J. (1970). Serologic evidence of human infection with swine influenza
virus. Am. Rev. respir. Dis., 102, 356-361.
140. Schoenbaum S.C. (1996). - Impact of influenza in persons
and populations. In Options for the control of influenza, III
(L.E. Brown, A.W. Hampson & R.G. Webster, eds). Elsevier,
Amsterdam, 17-25.
141. Scholtissek C , Rohde W., von Hoyningen V. & Rott R.
(1978). - On the origin of the human influenza virus
subtypes H2N2 and H3N2. Virology, 87, 13-20.
142. Scholtissek C , Burger H., Bachmann P.A. & Hannoun C.
(1983). - Genetic relatedness of haemagglutinins of the H1
subtype of influenza A viruses isolated from swine and birds.
Virology, 129, 521-523.
224
143. Scholtissek C , Burger H., Kistner O. & Shortridge K.F.
(1985). - The nucleoprotein as a possible major factor in
determining host specificity of influenza H3N2 viruses.
Virology, 147, 287-294.
144. Scholtissek C , Ludwig S. & Fitch W.M. (1993). - Analysis
of influenza A virus nucleoproteins for the assessment of
molecular genetic mechanisms leading to new phylogenetic
virus lineages. Arch. Virol., 131, 237-250.
145. Senne D.A., Panigraphy B., Kawaoka Y., Pearson J.E., Suss J.,
Lipkind M., Kida H. & Webster R.G. (1996). - Survey of the
hemagglutinin (HA) cleavage site sequence of H5 and H7
avian influenza viruses: amino acid sequence at the HA
cleavage site as a marker of pathogenicity potential. Avian
Dis., 40, 425-437.
146. Sheerar M.G., Easterday B.C. & Hinshaw V.S. (1989). Antigenic conservation of H1N1 swine influenza viruses.
J. gen. Virol, 70, 3297-3303.
Rev. sci. tech. Off. int. Epiz., 13(1)
158. Simonsen L., Schonberger L.B., Stroup D.F., Arden N.H. &
Cox N.J. (1996). - The impact of influenza on mortality in
the USA. In Options for the control of influenza, III
(L.E. Brown, A.W. Hampson & R.G. Webster, eds). Elsevier,
Amsterdam, 26-33.
159. Slemons R.D. & Easterday B.C. (1978). - Virus replication in
the digestive tract of ducks exposed by aerosol to type A
influenza. Avian Dis., 22, 367-377.
160. Smith W., Andrewes C.H. & Laidlaw P.P. (1933). - A virus
obtained from influenza patients. Lancet, 2, 66-68.
161. Snyder M.H.,
Buckler-White A.J., London W.T.,
Tierney E.L. & Murphy B.R. (1987). - The avian influenza
virus nucleoprotein gene and a specific constellation of avian
and human virus polymerase genes each specify attenuation
of avian/human influenza A/Pintail/79 reassortant viruses for
monkeys. J. Virol, 6 1 , 2857-2863.
147. Shoham D. (1993). - Biotic-abiotic mechanisms for longterm preservation and reemergence of influenza type A virus
genes. Prog. med. Virol, 40, 178-192.
162. Sovinova O., Tumova B., Poutska F. & Nemec J. (1958). Isolation of a virus causing respiratory disease in horses. Acta
virol, 2, 52-61.
148. Shope R.E. (1931). - Swine influenza. III. Filtration
experiments and etiology. J. experim. Med., 54, 373-385.
163. Stallknecht D.E. & Shane S.M. (1988). - Host range of avian
influenza virus in free-living birds. Vet. Res. Commun., 12,
125-141.
149. Shortridge K.F. (1992). - Pandemic influenza: a zoonosis.
Semin. respir. Infect., 7, 11-25.
150. Shortridge K.F. (1999). - The 1918 'Spanish' flu: pearls from
swine. Nature Med., 5, 384-385.
151. Shortridge K.F. (2000). - A review of interspecies
transmission of influenza viruses: the Hong Kong
perspective. In Proc. of the European Society for Veterinary
Virology (ESVV) Symposium on influenza viruses of wild
and domestic animals, 16-18 May 1999, Ghent. Vet.
Microbiol. (in press).
152. Shortridge K.F., Webster R.G., Butterfield W.K. &
Campbell C.H. (1977). - Persistence of Hong Kong influenza
virus variants in pigs. Science, 196, 1454-1455.
153. Shortridge K.F. & Stuart-Harris C.H. (1982). - An influenza
epicenter? Lancet, 2, 812-813.
154. Shortridge K.F., Chan W.H. & Guan Y. (1995). Epidemiology of the equine influenza outbreak in China,
1993-1994. Vet. Rec., 1 3 6 , 1 6 0 - 1 6 1 .
155. Shortridge K.F., Zhou N.N., Guan Y., Gao P., Ito T.,
Kawaoka Y., Kodihalli S., Krauss S., Markwell D.,
Murti K.G., Norwood M., Senne D.A., Sims L., Takada A. &
Webster R.G. (1998). - Characterization of avian H5N1
influenza viruses from poultry in Hong Kong. Virology, 2 5 2 ,
331-342.
156. Shu L.L., Lin Y.P., Wright S.M., Shortridge K.F. &
Webster R.G. (1994). - Evidence for interspecies
transmission and reassortment of influenza A viruses in pigs
in Southern China. Virology, 2 0 2 , 825-833.
157. Shu L.L., Zhou N.N., Sharp G.B., He S.Q., Zhang T.J.,
Zou W.W. & Webster R.G. (1996). - An epidemiological
study of influenza viruses among Chinese farm families with
household ducks and pigs. Epidemiol Infect., 117, 179-188.
164. Stallknecht D.E., Shane S.M., Kearney M.T. & Zwank P.J.
(1990). - Persistence of avian influenza viruses in water.
Avian Dis., 34, 406-411.
165. Stieneke-Grober A., Vey M., Angliker H., Shaw E.,
Thomas G., Roberts C , Klenk H.D. & Garten W. (1992). Influenza virus hemagglutinin with multibasic cleavage site
is activated by furin, a subtilisin-like endoprotease. EMBO J.,
11, 2407-2414.
166. Stoof J . C , Booij J., Drukarch B. & Wolters E.C. (1992). The anti-parkinsonian drug amantadine inhibits the
N-methyl-D-aspartic acid-evoked release of acetylcholine
from rat neostriatum in a non-competitive way. Eur. J.
Pharmacol, 213, 439-443.
167. Stuart-Harris C.H., Schild G.C. & Oxford J.S. (eds) (1985). Influenza in animals and birds. In Influenza: the viruses and
the disease, 2nd Ed. Edward Arnold Ltd, Baltimore,
Maryland, 83-102.
168. Suarez D.L., Perdue M.L., Cox N., Rowe T., Bender C,
Huang J. & Swayne D.E. (1998). - Comparisons of highly
virulent H5N1 influenza A viruses isolated from humans and
chickens in Hong Kong. J. Virol, 72, 6678-6688.
169. Sugimura T., Yonemochi H., Ogawa T., Tanaka Y. &
Kumagai T. (1980). - Isolation of a recombinant influenza
virus (HswlN2) from swine in Japan. Arch. Virol, 66,
271-274.
170. Taubenberger J.K., Reid A.H., Krafft A.E., Bijwaard K.E. &
Fanning T.G. (1997). - Initial genetic characterization of the
1918 'Spanish' influenza virus. Science, 275, 1793-1796.
171. Taylor H.R. & Turner A.J. (1977). - A case report of fowl
plague keratoconjunctivitis. Br. J. Ophthalmol, 6 1 , 86-88.
Rev. sci. tech. Off. int. Epiz., 19 (1)
172. Tian S.F., Buckler-White A.J., London W.J., Recle L.J.,
Channock R.M. & Murphy B.R. (1985). - Nucleoprotein
and membrane protein genes are associated with restriction
of influenza A/mallard/NY/78 virus and its reassortants in
squirrel monkey respiratory tract. J. Virol., 53, 771-775.
173. Top F.H. & Russell P.K. (1977). - Swine influenza A at Fort
Dix, New Jersey (January-February 1976). IV. Summary and
speculation. J. infect Dis., 136 (Suppl), 376-380.
174. Tumova B. (1980). - Equine influenza - a segment in
influenza virus ecology. Comp. Immunol. Microbiol. inject
Dis., 3, 45-59.
175. Tumova B., Stumpa A. & Mensik J. (1980). - Surveillance of
influenza in pig herds in Czechoslovakia in 1974-1979. 2.
Antibodies against influenza A (H3N2), A (HswlNl) and A
(H1N1). Zentralbl. Veterinärmed., B, 27, 601-607.
176. Uppal M.P. & Yadav M.P. (1989). - Isolation of Aequi-2
virus during 1987 equine influenza epidemic in India.
Equine vet. J., 21, 364-366.
177. Urman H.K., Underdahl N.R. & Young G.A. (1958). Comparative histology of experimental swine influenza virus
pneumonia of pigs in disease-free, antibody devoid pigs. Am.
J. vet Res., 19, 913-917.
178. Vey M., Orlich M., Adler S., Klenk H.D., Rott R. &
Garten W. (1992). - Haemagglutinin activation of
pathogenic avian influenza viruses of serotype H7 requires
the recognition motif R-X-R/K-R. Virology, 188, 408-413.
179. Villarreal C.L. & Flores A.O. (1998). - The Mexican avian
influenza H5N2 outbreak. In Proc. 4th International
Symposium on avian influenza, 28-31 May 1997, Athens,
Georgia. American Association of Avian Pathologists,
Pennsylvania, 18-22.
180. Webster R.G. (1993). - Are equine 1 influenza viruses still
present in horses? Equine vet. J., 25, 537-538.
225
186. Webster R.G., Geraci J.R., Petursson G. & Skirnisson K.
(1981). - Conjunctivitis in human beings caused by
influenza A virus of seals. New Engl.]. Med., 304, 911.
187. Webster R.G. & Kawaoka Y. (1988). - Avian influenza. Crit.
Rev. Poult. Biol, 1, 211-246.
188. Webster R.G. & Guo Y.J. (1991). - New influenza virus in
horses. Nature, 3 5 1 , 527.
189. Webster R.G., Bean W.J., Gorman O.T., Chambers T.M. &
Kawaoka Y. (1992). - Evolution and ecology of influenza A
viruses. Microbiol Rev., 56, 152-179.
190. Wentworth D.E., Thompson B.L., Xu X., Regnery H.L.,
Cooley A.J. & McGregor M.W. (1994). - An influenza A
(H1N1) virus, closely related to swine influenza virus,
responsible for a fatal case of human influenza. J. Virol, 68,
2051-2058.
191. W o o d G.W., McCauley J.W., Bashiruddin J.B. &
Alexander D.J. (1993). - Deduced amino acid sequences at
the haemagglutinin cleavage site of avian influenza A viruses
of H5 and H7 subtypes. Arch. Virol, 130, 209-217.
192. W o o d G.W., Banks J., Brown I.H., Strong I. &
Alexander D.J. (1997). - The nucleotide sequence of the
HA1 of the haemagglutinin of an H1 avian influenza virus
isolate from turkeys in Germany provides additional
evidence suggesting recent transmission from pigs. Avian
Pathol, 26, 347-355.
193. World Health Organization (WHO) Expert Committee
(1980). - A revision of the system of nomenclature for
influenza viruses: a WHO memorandum. Bull. WHO, 58,
585-591.
194. Wright P.F., Dolin R. & La Montagne J.R. (1976). - From the
National Institute of Allergy and Infectious Diseases of the
National Institutes of Health, the Center for Disease Control,
and the Bureau of Biologics of the Food and Drug
Administration. Summary of clinical trials of influenza
vaccines - II. J. inject. Dis., 134, 633-638.
181. Webster R.G. (2000). - Characterization of H3N2 influenza
viruses from pigs in the United States. In Proc. of the
European Society for Veterinary Virology (ESVV)
Symposium on influenza viruses of wild and domestic
animals, 16-18 May 1999, Ghent. Vet Microbiol. (in press).
195. Wright S.M., Kawaoka Y., Sharp G.B., Senne D.A. &
Webster R.G. (1992). - Interspecies transmission and
reassortment of influenza A viruses in pigs and turkeys in the
United States. Am.]. Epidemiol, 136, 488-497.
182. Webster R.G., Campbell C.H. & Granoff A. (1973). - The 'in
vivo' production of 'new' influenza A viruses. III. Isolation of
recombinant viruses under simulated conditions of natural
transmission. Virology, 5 1 , 149-162.
196. Yasuda J., Shortridge K.F., Shimizu Y. & Kida H. (1991). Molecular evidence for a role of domestic ducks in the
introduction of avian H3 influenza viruses to pigs in
Southern China, where the A/Hong Kong/68 (H3N2) strain
emerged. J. gen. Virol, 72, 2007-2010.
183. Webster R.G. & Laver W.G. (1975). - Antigenic variation of
influenza viruses. In The influenza viruses and influenza
(E.D. Kilboume, ed.). Academic Press, Inc., New York,
269-314.
184. Webster R.G., Yakhno M.A., Hinshaw V.S., Bean W.J. &
Murti K.G. (1978). — Intestinal influenza: replication
characterisation of influenza viruses in ducks. Virology, 84,
268-278.
185. Webster R.G., Hinshaw V.S., Bean W.J., Van Wyke K.L.,
Geraci J.R. & St Aubin D.J. (1981). - Characterisation of an
influenza A virus from seals. Virology, 113, 712-724.
197. Yuen K.Y., Chan P.K.S., Peiris M., Tsang D.N.C., Que T.L.,
Shortridge K.F., Cheung P.T., To W.K., Ho E.T.F., Sung R.
& Cheng A.F.B. (1998). - Clinical features and rapid viral
diagnosis of human disease associated with avian influenza A
H5N1 virus. Lancet, 3 5 1 , 467-471.
198. Zhou N.N., Shortridge K.F., Claas E.C.J., Krauss S.L. &
Webster R.G. (1999). - Rapid evolution of H5N1 influenza
viruses in chickens in Hong Kong. J. Virol, 73, 3366-3374.