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Rev. sci. tech. Off. int. Epiz., 1 9 9 8 , 1 7 (1), 256-268
Myxoma virus in rabbits
P.J. Kerr
S.M. B e s t
(1) Vertebrate Biacontrol Cooperative Research Centre, Commonwealth Scientific and Industrial Research
Organisation, Wildlife and Ecology, P.O. Box 84, Lyneham, ACT 2602, Australia
(2) Division of Biochemistry and Molecular Biology, Australian National University, Canberra, ACT 0200,
M y x o m a virus in E u r o p e a n rabbits (Oryctolagus
is o n e of t h e best
d o c u m e n t e d e x a m p l e s of host-virus c o - e v o l u t i o n . In t h e n a t u r a l hosts
or S. bachmani
rabbits in t h e A m e r i c a s ) , m y x o m a virus c a u s e s a
b e n i g n c u t a n e o u s f i b r o m a . In E u r o p e a n r a b b i t s , h o w e v e r , m y x o m a virus c a u s e s
t h e f u l m i n a n t d i s e a s e , m y x o m a t o s i s . W h e n i n t r o d u c e d into w i l d E u r o p e a n rabbit
populations in A u s t r a l i a , E u r o p e and G r e a t B r i t a i n , t h e virus w a s initially highly
l e t h a l , killing in e x c e s s of 9 9 % of i n f e c t e d rabbits. D e v e l o p m e n t of r e s i s t a n c e w a s
e n c o u r a g e d by t h e e m e r g e n c e of a t t e n u a t e d virus strains w h i c h a l l o w e d the
survival o f m o d e r a t e l y r e s i s t a n t rabbits. T h i s m a y h a v e o c c u r r e d m o r e rapidly in
hot c l i m a t e s , as h i g h a m b i e n t t e m p e r a t u r e s i n c r e a s e t h e survival r a t e of infected
rabbits. R e s i s t a n t rabbits are less e f f e c t i v e t r a n s m i t t e r s of t h e virus a n d this may
e n c o u r a g e t h e e m e r g e n c e of m o r e v i r u l e n t virus s t r a i n s . Little is k n o w n of the
m e c h a n i s m of r e s i s t a n c e . T h e r e h a v e b e e n s u g g e s t i o n s of n o n - g e n e t i c
r e s i s t a n c e . H o w e v e r , t h e s e a r e y e t to be c o n f i r m e d e x p e r i m e n t a l l y .
Disease resistance - Genetics - M y x o m a virus - M y x o m a t o s i s - Rabbits - Virulence.
Myxoma virus and myxomatosis
The co-evolution of infectious and parasitic diseases with their
of genetic
polymorphisms and hence drivers of evolution in animal
populations (3). However, there are very few opportunities to
study this form of evolution as it occurs. The release of
virulent myxoma virus, which had evolved in
rabbits in South America, into completely naive
populations of European rabbits (Oryctolagus
Myxoma virus is a member of the poxvirus family and is
classified in the genus Leporipoxvirus (43). The myxoma vims
is a large virus with a double-stranded DNA genome of
163 kilobases (kb) which replicates in the cytoplasm of
infected cells. There are two major geographic types of the
virus: South American (natural host: S. brasiliensis) and
Californian (natural host: S. bachmani). In the natural hosts,
the native virus causes a benign cutaneous fibroma (21).
Australia and Europe, and the subsequent natural selection of
attenuated strains of virus and resistant rabbits, provided one
of the best documented natural studies of host-pathogen
co-evolution in mammals ( 1 4 , 2 1 , 2 2 ) . In this paper, the
authors describe the development of resistance to myxoma
virus in populations of European rabbits and suggest that this
was dependent on the emergence of attenuated virus strains.
pathogenesis of myxoma virus is then discussed, in
relation to viral virulence genes and possible mechanisms for
resistance, and the implications of resistance for co-evolution
of the virus and the rabbit.
Natural isolates of both types of myxoma virus are lethal for
European rabbits. However, it was the South American
viruses that were released into European rabbit populations
and on which this review focuses. These viruses cause a
disease which is characterised by swelling of the face and
head, together with mucoid cutaneous tumours, and which is
termed 'myxomatosis' for the mucoid nature of the cut surface
of the lesions. The virus is spread by blood-feeding arthropod
vectors, such as mosquitoes or fleas. Transmission is passive:
the virus adheres to the mouthparts of the vector but does not
replicate within the vector. Epidemics occur annually or less
Rev. sci. tech. Off. int. Epiz., 17 (1)
frequently, depending on the emergence of large numbers of
susceptible kittens in the spring and summer and the
availability of vectors (21, 2 2 ) .
The use of myxoma virus as a
biological control for the
European rabbit
intensive release campaigns ( 1 5 , 5 4 ) . Attenuated strains of
myxoma virus emerged during these early epidemics ( 3 6 , 4 7 ) .
Some of these viruses allowed survival of a few infected
rabbits in laboratory studies and this was confirmed by
serological observations in the field ( 1 5 , 3 6 , 45). The fact that
there would be an opportunity for selection of genetically
resistant rabbits in the field became apparent, and studies
were set u p ' for long-term monitoring of both the
development of genetic resistance and the virulence of field
strains of virus (54).
The lethal nature of myxomatosis for European rabbits
suggested that the myxoma virus could be used as a biological
control agent for rabbits in countries such as Australia, where
the introduced European rabbit had caused environmental
devastation and major agricultural losses. Several trials were
conducted in the 1 9 3 0 s in both Australia and Europe using
the Moses' strain of the Brazilian myxoma virus (later referred
to as the 'standard laboratory strain' or 'SLS'), to evaluate the
effectiveness of myxoma virus for this purpose ( 8 , 3 9 ) .
However, the virus failed to become established, possibly
because there were insufficient arthropod vectors to spread it
beyond the sites of introduction. Further trials were
conducted in Australia after the Second World War, and in
the summer of 1 9 5 0 - 1 9 5 1 , myxoma virus spread explosively
from an experimental site in the Murray Valley in
south-eastern Australia. This spread was driven by
mosquitoes, predominantly Culex annulirostris
but also
which were present in plague
proportions that summer ( 4 4 ) . Myxomatosis has been
endemic in Australia since this release (21).
In the virulence studies, myxoma virus isolates were grouped
into five levels of virulence (I-V), based on the survival rates
and survival times of small groups of laboratory and hence
unselected rabbits (Table I) ( 1 8 ) . Grade I viruses were the
most virulent, killing essentially 100% of infected rabbits with
an average survival time of less than 13 days; grade V viruses
were the least virulent, with mortality rates of less than 5 0 % .
A separate strain of myxoma virus which originated in Brazil
was obtained from a laboratory in Lausanne in Switzerland
and hence is referred to as the 'Lausanne strain'. It was
released in France at Mallebois in the summer of 1952. From
this initial single release site, myxoma virus spread across the
entire rabbit range in Europe. In the autumn of 1953, virus
from France was illegally released into Great Britain, where
myxoma virus is now also endemic (22).
Virulence and transmission of the virus were strongly linked
because transmission depends on arthropod vectors probing
through infected epidermis and virus adhering to the
mouthparts of the vector. This is most likely to occur when
virus titres are over 1 0 infectious units per gram of skin (17).
As shown in Figure 1, highly lethal viruses reached this
infectivity threshold for only a few days before the rabbit died,
whereas moderately attenuated viruses, such as grade 111 or
grade IV strains, allowed infected rabbits to survive longer and
increased the period for which they were infectious for
mosquitoes (17). In contrast, very attenuated viruses, such as
grade V strains, were only infectious for a short period before
viral replication was controlled by the host. Thus, the
moderately attenuated viruses were more likely to be
transmitted and these rapidly became the predominant strains
in the field in Australia (Fig. 2 ) (18).
Myxomatosis in rabbits in
Australia: development of
resistance associated with
the attenuation of virus strains
In the initial epidemic in Australia in the summer of
1950-1951, myxoma virus was estimated to have killed as
many as 9 9 . 8 % of infected wild rabbits, and many
populations were reduced by more than 9 0 % . Although
predominantly transmitted by mosquitoes, the virus
overwintered and reappeared the following summer in
epidemics of myxomatosis which were augmented by
Table I
Virulence grades of myxoma virus
Virus grade
Average survival time
Mortality (%)
< 13 days
13-16 days
17-28 days
29-50 days
Not relevant
a) Table adapted from Fennerand Marshall (18)
This evolution of attenuated virus strains was probably critical
for the establishment of myxoma virus in the new host as,
unlike South America, where S. brasiliensis was available as a
reservoir host, there was no reservoir host in Australia. Thus,
highly lethal viruses which left no survivors were likely to
reduce the density of rabbit populations to such an extent that
the virus became extinct locally; this appears to have been the
case with earlier attempts at introduction.
Rev. sci. tech. Off. int. Epiz., 17 (
summer of 1 9 5 1 - 1 9 5 2 , is used here (37, 3 8 ) . Following the
first epidemic at this site, rabbit kittens were captured each
spring prior to the annual epidemic and taken to a central
laboratory. Seronegative animals were held until at least four
months of age and then were challenged with the mildly
attenuated KM 13 (the prototype grade III) strain of myxoma
Days post inoculation
Fig. 1
Mosquito infectivity of myxoma virus strains
Virus titres in the skin at the inoculation site of rabbits infected with strains
of myxoma virus of different virulence grades are shown over time post
inoculation of a small dose of virus intradermally. The threshold for mosquito
infectivity is shown at 10 plaque-forming units (PFU)/g. Data for the grade I,
III and IV strains are taken from Fenner et al. (17). Data for the grade V strain
are taken from S.M. Best and P.J. Kerr (unpublished data)
The mortality rates in unselected wild and domestic rabbits
challenged with this virus were 8 8 % and 8 9 % , respectively,
and similar mortality rates were seen in rabbits from Lake
Urana challenged with KM 13 following the first two
epidemics. After the fourth epidemic, however, the mortality
rate had dropped to approximately 5 0 % and after the seventh
epidemic, mortality due to challenge was reduced to 26%
(Fig. 3 ) . A similar change was observed in the proportion of
rabbits which showed severe clinical signs. In the trials
following the first two epidemics, only 0 % to 2 % of rabbits
showed mild clinical signs; however, after seven epidemics,
3 0 % of rabbits showed mild clinical signs on challenge with
KM13 (21, 3 7 , 38).
To assess the development of genetic resistance to
myxomatosis in wild rabbit populations, rabbit kittens were
collected from field sites and challenged with myxoma virus
strains of defined virulence. Many such studies were
performed but, as an example, a longitudinal data set from
Lake Urana in New South Wales, where
spring/summer epidemics of myxomatosis occurred from the
Number of epidemics
Fig. 3
Development of innate resistance to myxoma virus at Lake Urana,
Data adapted from Marshall and Fenner (37) and Marshall and Douglas (38)
The percentage mortality rate of rabbits trapped at Lake Urana after two,
three, four, five or seven epidemics of myxomatosis and challenged with the
grade III KM13 strain of myxoma virus is shown as a histogram.
The number of rabbits tested at each timepoint is shown above the bars
Fig. 2
Virulence of myxoma virus in Australia
Data adapted from Fenner (14)
The percentage of isolates of myxoma virus from Australia assigned to each
virulence grade from 1952 to 1981. The number of isolates tested for each
period is shown above the grade III bar. Virulence grades are shown
in Table I
Using the data from Lake Urana as an example (37), Table II
demonstrates how selection for innate resistance and
attenuation of myxoma virus occurred concurrendy.
Following the initial epidemic at Lake Urana, which reduced
the population in one area from an estimated 5 , 0 0 0 rabbits to
50 rabbits (15), a large proportion of kittens would have been
b o m to the 7 8 % of parents which were seronegative.
Antibodies are detectable for at least two to three years
following infection, so these animals must have avoided
infection with myxoma virus during the first epidemic (15).
Hev. sci. tech. Off. int. Epiz., 17 (1)
Thus, there was relatively little selection pressure for
resistance. However, in the following years there was a
predominance of attenuated strains of myxoma virus which
allowed survival of more infected rabbits. This may have been
aided by high ambient temperatures (see below). Table II
shows that from the second epidemic onwards, virtually all
kittens were the offspring of parents which had survived
myxomatosis. Thus, there was a very high selection pressure
at the population level, and animals which had avoided
myxomatosis were almost unrepresented in the breeding
population. In addition, in the Mediterranean climate of Lake
Urana, most rabbits only bred for one season, although four to
five litters might be produced in this time, so that the breeding
population was essentially being replaced annually (46).
Table II
Proportion of breeding animals which survived myxomatosis at Lake
Data adapted from Marshall and Fenner (37)
Number of immune
Immune adults
(and number)
of virus strains
III (5)
IV (5)
a) The number of immune animals Is indicated as a proportion of the number of animals tested
b) In this epizootic, the highly virulent Lausanne strain of myxoma virus was extensively
released at Lake Urana (19) and therefore the results of virus isolations have been disregarded
Initially, resistance to myxoma virus in rabbit populations
seems to have developed fairly uniformly and quite rapidly in
different geographic areas, although the degree of resistance in
different populations may have varied somewhat. As an
example, the survival rates of rabbits collected in 1 9 5 9 from
Maryvale station in the Victorian Wimmera region, Ouyen in
the Victorian Mallee and Penshurst in south-western Victoria
and challenged with the K M 1 3 2a strain ( 6 7 % lethal in
unselected rabbits) are shown in Table III. Approximately
60% to 8 0 % of rabbits survived challenge under laboratory
conditions, with a higher proportion surviving from the
Mallee (21, 3 8 ) .
Survival rates under laboratory conditions may have led to an
underestimation of the effect of resistance on survival in the
field. This was emphasised when the results of animal-house
challenge were compared with challenge in outdoor pens
located in the collecting areas. Over 9 0 % of rabbits from both
Table III
Testing rabbits from different geographic areas for resistance to
myxoma virus
Data taken from Fenner and Ratcliffe (21 ) and Marshall and Douglas (38)
Rabbits recovered
from challenge
Climate and epidemic history
with KM 13 2a (%)
Laboratory Outdoor pens
Annual epidemics from 1951
Not tested
Hot summers
Annual epidemics from 1951
tow-level epidemics from 1952
Maryvale and Ouyen survived challenge with KM13 2a when
housed in outdoor pens during the summer (Table III). The
critical difference appears to have been the high ambient
temperatures during summer in the field pens. Temperatures
in the animal house ranged from 17°C to 23°C while
temperatures in the field ranged from 15°C to 38°C (38). This
sparing effect of high environmental temperatures on the
clinical disease caused by attenuated strains of myxoma virus
was experimentally demonstrated (35). Unselected laboratory
rabbits were housed in hot (26°C to 39°C), temperate (20°C
to 22°C) or cold (-3°C to 27°C) environments and infected
with the attenuated KM13 2a strain of myxoma virus. In the
hot environment, 1 0 % to 3 0 % of rabbits died from virus
infection, compared to 9 2 % of those in the cold and 6 3 % in
the temperate environment. This temperature effect may have
been important in allowing rabbits in Australia to survive
infection with moderately attenuated virus strains during the
early evolution of resistance, and may have aided selection of
resistant rabbits in hotter climates.
Resistance in a population is a graduated effect, the
manifestation of which depends on the individual rabbit, the
population from which the rabbit is drawn and the virus
strain. For example, on challenging rabbits from Maryvale,
Ouyen and Penshurst with SLS, 9 0 % , 8 6 % and 8 5 % to 9 0 %
of rabbits were killed, i.e., a 1 0 % to 1 5 % survival rate ( 2 1 ,
38). However, a longer survival time was noted compared to
that of unselected rabbits. Testing of rabbits from these
populations with the more virulent 'Glenfield' strain of virus
showed that all the rabbits tested were completely susceptible
to this virus (21). More recent data suggest that resistance has
not continued to develop uniformly and, for example, may be
more advanced in the hot Mallee region than in the cooler
Gippsland region of Victoria. For rabbits from the Mallee
tested with three grade I strains of myxoma virus, SLS was
6 0 % lethal, Glenfield 9 1 % lethal and Lausanne 9 8 % lethal,
whereas the corresponding figures for Gippsland rabbits
were: SLS, 7 9 % lethal; Glenfield, 9 5 % lethal and Lausanne,
100% lethal (22).
Development of resistance to
myxoma virus in Europe and
Great Britain
As in Australia, the initial epidemics of myxomatosis in France
and Great Britain were highly lethal and rabbit numbers were
greatly reduced. As shown above, the Lausanne strain of
myxoma virus released in France and subsequently in Great
Britain is inherently more virulent than the SLS released in
Australia - although this could only be demonstrated in
genetically resistant rabbits. The virus spread more slowly
across Great Britain than Australia and France, but by the end
of 1 9 5 5 , most parts of Great Britain were affected ( 2 2 ) .
Similarly to Australia, strains which allowed some infected
rabbits to survive emerged in Great Britain within a year or
two of the initial release ( 2 5 ) . By 1 9 6 2 , grade III viruses
predominated in the field, although virulent grade I and II
viruses were much more prevalent than in Australia and grade
II virus prevalence has increased with time (Fig. 4 ) ( 2 0 , 5 7 ) .
This increase has occurred despite the fact that further releases
of virulent virus have not been made in any organised sense in
Great Britain (or in France), whereas widespread releases of
grade I viruses were made in Australia for many years and
some release of the Lausanne strain still occurs.
Rev. sci. tech. Off. int. Epiz., 17 (1|
rates was observed (from over 8 0 % to approximately 20%)
(Fig. 5 ) . Among the 5 0 survivors of challenge with Brecon in
1976, 10 showed only a primary lesion at the inoculation site
with no signs of generalised disease (55). W h e n rabbits from
this area were challenged with the grade I Cornwall strain in
1974 and 1 9 7 5 , there were no recoveries, although survival
times were prolonged (11 to 2 6 days) compared to those of
domestic rabbits (11 to 14 days) (55). These observations on
resistance in Norfolk were extended by testing rabbits from
other areas of Great Britain. This showed that in four
widely separated populations between 1 9 7 8 and 1980,
approximately 5 0 % of rabbits survived challenge with
myxoma virus, including grade I and grade II virus strains
Fig. 5
Innate resistance to myxoma virus in Great Britain
Data adapted from Ross and Sanders (55)
Rabbit kittens were captured from a site in Norfolk which had experienced
annual epidemics of myxomatosis since 1960. The kittens were held in
captivity until over three months old and then challenged with the grade III
Brecon strain of myxoma virus. The percentage mortality rate following
challenge for each year and the number of rabbits tested are shown
Fig. 4
Virulence of myxoma virus in Great Britain
Data adapted from Fenner and Chappie (20) and Ross and Sanders (57)
The percentage of isolates of myxoma virus in Great Britain assigned to each
virulence grade for the years 1962,1975 and 1981 is shown, together with
the number of isolates tested at each time
Resistance to myxoma virus was slower to develop in Great
Britain than in Australia. Testing of wild rabbits from one
location in Norfolk, where annual epidemics of myxomatosis
occurred from at least 1960, revealed very little resistance to
myxomatosis between 1 9 6 6 and 1 9 6 9 ( 5 5 ) . These rabbits
were challenged with the mildly attenuated Brecon strain
(grade III) which killed essentially 1 0 0 % of unselected
rabbits. However, Vaughan and Vaughan pointed to extended
survival times in challenged wild rabbits as an indicator of the
early development of resistance in populations of rabbits (69).
Between 1 9 7 0 and 1 9 7 6 , a dramatic decrease in mortality
The reasons for the slower development of resistance in Great
Britain compared with Australia are unclear. Perhaps
resistance development was linked with the more lethal virus
introduced into Great Britain and the slower selection of
attenuated virus strains, although, as shown in Figure 4 , there
were large numbers of attenuated virus strains in the field
which could have enhanced selection: this may also reflect the
ability of mosquitoes to disseminate virus strains widely in
Australia compared with the more local and less seasonal
dissemination which occurs with European rabbit fleas
(Spilopsj/llus cuniculi), the predominant vector in Great
Britain. Another possible reason is the generally milder
summer temperatures in Great Britain compared with those in
Australia, which may have decreased survival rates and
slowed down selection. Similarly, in the winter, very
attenuated viruses could have proved lethal due to the very
cold conditions.
In France, where both mosquitoes and rabbit fleas were
available as vectors, attenuated strains of myxoma virus were
recovered from the field within two years of the initial virus
Rev. sci. tech. Off. int. Epiz., 17 (1)
release (27). However, ten years after release, 3 0 % of strains
were still grade I or II (compared with < 1 5 % in Australia)
(Fig. 2 ) . By 1 9 6 8 , this figure had dropped to 6%. There are
few data available on resistance to myxomatosis in Europe.
Fenner and Ross quote a study conducted by Galaup in 1 9 8 8 ,
who reported that 1/7, 1/4 and 7/7 rabbits from three sites in
France survived challenge with the Lausanne strain of
myxoma .virus ( 2 2 ) . This indicated that resistance was
present, but the numbers of animals from each area were too
small to make conclusions about geographic variability.
Selection of domestic rabbits for
resistance to myxomatosis
An obvious experimental approach to studying the evolution'
of genetic resistance to myxoma virus was to attempt to
replicate the selection for genetic resistance in laboratory
rabbits. Clearly this selection could not be performed on a
scale comparable to that which occurred in field populations
on a continent-wide basis. However, from 1 9 5 4 to 1 9 7 6 ,
domestic rabbits were selected for resistance by challenging
males with strains of myxoma virus of different virulences and
breeding from the survivors; the survival rate of challenged
rabbits was too low to use selection on the female line. An
apparent response to selection for resistance to myxoma virus
was achieved with an estimated heritability of 3 5 % to 4 0 %
(61). This peaked after about six generations of selection and
after 1 9 6 8 no further response to selection was achieved. The
trends in resistance were similar to those observed in the field
but comparable levels of resistance were not reached. For
example, after approximately four generations of selection,
the grade III KM13 virus still killed around 8 0 % of the
selected rabbits compared with 9 0 % to 9 5 % of unselected
rabbits (61). This survival rate is similar to that observed in
rabbits from Lake Urana challenged with KM 13 after three
epidemics (Fig. 3 ) . After approximately six generations of
selection, around 2 0 % of selected rabbits also survived
challenge with SLS (61).
Genetic resistance versus
acquired resistance (sire effect)
Reanalysis of these selection experiments suggested that the
resistance observed could largely be explained by some factor,
transmitted from the sire to the dam, which conferred a
temporary resistance to myxomatosis on the offspring which
was not associated with maternal antibody ( 6 3 ) . This
non-genetic, 'acquired' resistance enhanced survival of kittens
to myxoma virus challenge if the kittens were born to does
which had mated with males that had survived myxomatosis.
Even more complicated was the fact that the kittens from
subsequent mating of that female with males which had not
been exposed to myxoma virus were also protected. This
enhanced survival of kittens was termed the 'sire effect'.
In this analysis, and on retrospective analysis of other data
from Australia (50, 71), the risk of death of progeny following
challenge with myxoma virus was reduced by 2 0 % to 2 5 % if
b o m within seven months of infection of the male, and was
enhanced when the strain of virus used to challenge the buck
was the same as that used to challenge the progeny. The effect,
however, was temporary, with domestic kittens infected prior
to 18 weeks of age having a higher recovery rate than those
infected later. The protection was claimed to last up to
60 weeks of age in wild rabbits (71).
The suggestion has been made that in areas with annual
epidemics of myxomatosis, as most kittens are born to females
which have mated with males which had survived
myxomatosis during the previous epidemic, the sire effect
could explain much of the apparent resistance observed in the
field ( 6 3 , 7 1 ) . However, no experimental test of this
hypothesis has been performed and a mechanism by which it
might operate remains difficult to imagine. If the hypothesis is
correct, there would be fundamental implications for
understanding the infectious disease biology of mammals,
especially in species with a short generation time. Meanwhile,
there is no doubt that a significant component (and perhaps
the most important component) of the resistance to
myxomatosis observed in wild rabbits in Australia is
genetically acquired. The authors have shown this using
animal-house-bred wild rabbits as part of the mechanistic
studies of genetic resistance. Neither sires nor dams of these
rabbits have had any exposure to myxomatosis and the rabbits
have a very high level of resistance to the grade I SLS
(S.M. Best and P.J. Kerr, in preparation).
The phenotype of resistance and
possible genotypes
The clinical effect on the rabbit of challenge with myxoma
virus depends on a series of variables. These include the
immune status, age, nutritional status, intercurrent disease
status and ambient temperature, as well as the innate
resistance of the animal. If effects other than innate resistance
are controlled within a population of wild rabbits in which
selection for resistance has been operating, there can be a
range of clinical outcomes following challenge with myxoma
virus. Some rabbits will develop generalised myxomatosis
with death occurring within two to three weeks of inoculation
(low resistance but prolonged survival compared to domestic
rabbits). Some may have moderate to severe clinical signs with
eventual recovery (moderate resistance). The third group
show very mild clinical signs; sometimes only a primary lesion
at the site of inoculation or, in some cases, no evidence of
infection apart from seroconversion (strong resistance).
Rev. sci. tech. Off. int. Epiz., 17 (1)
The proportion of animals in each group will vary according
to the virulence of the virus used for challenge and the overall
level of resistance in the population. Clearly, development of
resistance in a population is not an 'all or nothing' effect but
involves a series of steps. If the virus is virulent enough, it
appears that resistance may be overcome in many cases.
Within a population, there may be individuals with any of
these phenotypes: in fact, two or three rabbits per thousand
survived infection during the initial epidemics in Australia
Myxoma virus also modulates the host immune response by
downregulating major histocompatibility complex (MHC)-I
molecules on the surface of infected cells and thus potentially
inhibiting recognition by cytotoxic T lymphocytes ( 5 ) , and
downregulating CD-4 (cluster of differentiation antigen)
molecules on the surface of infected T lymphocytes (4).
Myxoma virus may also encode proteins which inhibit the
action of type II interferons and complements, as these are
encoded by other poxviruses which have homologues of some
or most of the myxoma virus virulence genes ( 1 ) .
Development of resistance must have required the existence
of polymorphisms in the population at one or more critical
genetic loci as resistance is unlikely to have depended on
novel mutations occurring over such a short time period.
However, the genetic loci involved in resistance to myxoma
virus have not been identified. The resistance phenotypes
generalised above could possibly be explained by
polymorphisms at a single locus with multiple alleles which
encode for different grades of resistance. Alternatively, and
perhaps more likely, the phenotypes could represent multiple
unlinked loci, such as those which occur in genetic resistance
to mousepox virus ( 7 , 1 0 ) . In mousepox resistance, protection
is encoded by a major dominant autosomal gene and one or
more other genes provide some protection, depending on the
virulence of the virus and perhaps the route of inoculation.
The roles of the proteins encoded by virulence genes can
provide some clues to important host factors in resistance,
although the fact that these viruses evolved in S. brasiliensis
(and thus the genes were not selected specifically for virulence
in European rabbits) must be remembered.
The survival of rabbits within a challenged group will
represent gene frequencies within the population from which
these rabbits were drawn. At each resistance locus, a rabbit
could be either homozygous or heterozygous. If a particular
polymorphism was in some way disadvantageous, there may
be advantages to heterozygotes and thus homozygotesusceptible rabbits would also be maintained in the
population. Unlike mousepox and some other murine models
(7, 5 1 , 5 3 , 7 0 ) , the sex of the animal appears to have no
influence on survival from myxomatosis ( 1 5 , 6 2 ) .
Pathogenesis of myxomatosis
and understanding resistance
The pathogenesis of myxoma virus in laboratory rabbits is
shown in Figure 6. Following intradermal inoculation of a
small dose of virulent virus, initial replication of virus occurs
at the inoculation site. Within two days of inoculation, virus is
found in the draining lymph node and high titres of virus are
found in the node three to four days after infection. Systemic
spread occurs in leucocytes (virus is not found free in the
serum), while replication can be detected in distal lymph
nodes, spleen, lung, testis and other organs and at
mucocutaneous junctions, such as the conjunctiva, three to
five days after infection. At this stage, the main sign of disease
is a red, raised swelling at the inoculation site and slightly
swollen eyelids. Secondary lesions begin to appear in the skin
Virus interactions with the host
Like other poxviruses, myxoma virus encodes proteins which
bind host antiviral and proinflammatory factors such as
interferon-Y (IFN-y) ( 6 8 ) , interleukin-1 (IL-1) ( 3 3 ) , tumour
necrosis factor (TNF) ( 5 8 ) and chemokines ( 2 4 , 3 0 ) . These
proteins significantly modulate the host immune and
inflammatory responses (Table IV) ( 3 3 , 3 4 ) . Other proteins,
such as M 1 1 L ( 2 3 ) and Serp 1 ( 3 1 ) , are strongly
anti-inflammatory but the cellular targets of these proteins
have not been identified. By inhibiting apoptosis (cell death),
M11L, T5 and T 2 (32, 4 1 ) are critical for virus replication in
lymphocytes and thus in dissemination of the virus within the
host (see below). Serp 2, which inhibits IL-1, converting
enzyme ( 5 2 ) , is also likely to be important in inhibiting
apoptosis (29). Disruption of any of the genes encoding these
proteins may dramatically attenuate the virus for European
rabbits, leading these genes to be termed 'virulence genes'.
Fig. 6
Pathogenesis of myxoma virus
Diagrammatic representation of the spread of myxoma virus in the rabbit,
showing the main tissues in which the virus replicates and the critical cells
for dissemination
Rev. sci. tech. Off. int. Epiz., 17 (1)
Table IV
Identified virulence genes in myxoma virus
Function identified
Secreted 35 kDa
Chemokine binding
Secreted 55-60 kDa glycoprotein
TNFa/B binding
Inhibition of apoptosis in lymphocytes
(58, 67)
55 kDa poxvirus host-range superfamily
Secreted 37 kDa
Inhibition of apoptosis in lymphocytes
Secreted serine protease inhibitor; 55 kDa glycoprotein
IFN-y binding
Chemokine binding
' Inhibition of inflammation
Type II membrane protein, 166 amino acids
Secreted 85 amino acid peptide
Inhibition of inflammation, inhibition of apoptosis in lymphocytes
EGF/TGF-a homologue
Serp 2*
34 kDa; cytoplasmic
IL-1 p-converting enzyme inhibition
(42, 68)
(23,32, 48|
(49, 65)
* Role in virulence not yet determined
kDa : kiloDaltons
TNF : tumour necrosis factor
IFN : interferon
EGF : epidermal growth factor
TGF : transforming growth factor
: interleukin
of the ears and margins of the eyelids and subsequently over
much of the body, if the animal lives long enough (16, 2 1 ) .
The major sites of pathology are in the lymphoid tissues, skin
and mucocutaneous junctions and in the testes in the male
(26). The cause of death is obscure; Mims could not attribute
death to damage of any specific organ (40), while others have
suggested that death is due to superinfection with Gramnegative bacteria complemented by immunosuppression of
the host immune system (64). However, following infection
with highly virulent viruses, death frequently occurs with
minimal evidence of secondary bacterial infection,
particularly with Californian strains of virus (21).
Possible mechanisms of
Conceptually, resistance can be viewed as the control of virus
replication and spread within the host. Comparison with the
natural hosts of myxoma virus suggests that the virus has
evolved virulence genes to survive within the dermis/
epidermis long enough to ensure transmission, but the host is
able to prevent the systemic spread of the virus. These viral
factors are presumed to overwhelm 0. cuniculus, but the
ability to replicate and spread in host leucocytes is also likely
to be critical. This is supported by observations on rabbit
fibroma virus, a Leporipoxvirus closely related to myxoma
virus but the natural host of which is S. floridanus. In
European rabbits, rabbit fibroma virus causes only a
cutaneous fibroma. The virus replicates to high titres in the
skin at the inoculation site, but is unable to replicate in rabbit
lymphocytes in vitro. Instead, infected lymphocytes undergo
apoptosis and this is believed to be a key event in controlling
the spread of the virus and thus the development of systemic
disease ( 3 2 , 4 1 , 6 4 ) .
As shown in Figure 6, resistance to myxoma virus could be
mediated at the level of replication in the initial inoculation
site or replication in distal tissues. Replication could be
controlled at the level of cell permissivity, as occurs in
flavivirus-resistant mice ( 6 ) , i.e., the ability of the virus to
attach to, enter, replicate in and spread from a cell.
Alternatively, effectors of the innate or acquired immune
systems could intervene to destroy infected cells by using, for
example, natural killer cells (NK cells) and cytotoxic
T lymphocytes or to make cells non-permissive for replication
by the action of interferon. Myxoma virus actively suppresses
apoptosis in infected lymphocytes and the early inflammatory
response to infection (Fig. 6 ) . In particular, critical antiviral
cytokines, such as TNF and IFN-y, are targeted by specific
viral proteins. Resistant rabbits may produce higher levels of
antiviral effector cells and molecules, perhaps through an NK
cell response such as that controlled by the rmp (resistance to
mousepox) 1 gene in mousepox-virus-resistant mice ( 1 0 ) .
Ultimately, as in any acute virus infection, the outcome will
depend on whether the host can slow replication and
dissemination of the virus enough to enable the immune
response to control and clear the infection.
When genetically resistant wild rabbits were compared with
non-resistant domestic rabbits infected with either a grade I
(SLS) or a grade V strain of myxoma virus, the authors found
that virus titres in the draining lymph node were significantly
reduced in the wild rabbits compared to domestic rabbits, but
that skin titres were similar in both groups. Similarly, titres in
distal lymph nodes were much reduced in wild rabbits
compared to domestic rabbits, indicating a reduced
dissemination within the body (S.M. Best and P.J. Kerr,
unpublished data). Thus, the draining lymph node or spread
from the skin to the lymph node appears to be the key
controlling the progress of infection. Limitation of virus
spread within the body also appeared to be the key
Rev. sci. tech. Off. int. Epiz., 17 (1)
mechanism by which high ambient temperatures exerted an
attenuating effect ( 3 5 ) , but the molecular and cellular basis for
this is unclear.
Conclusion: host-virus
co-evolution now and in the
The association of myxoma virus with the original host,
S. brasiliensis, is a good example of a successful parasite which
causes minimal harm to the host. European rabbits and
myxoma virus might be expected to evolve to a climax
association, with attenuation of the virus such that a
transmissible fibroma similar to that seen in S. brasiliensis is
induced. However, theoretical modelling studies indicate that
for a general host-parasite interaction, this is likely to occur
only if transmission is not positively linked to virulence (2).
For this to happen in myxoma virus infections of European
rabbits, skin titres of virus would have to be independent of
dissemination within the host.
Using the data on myxoma virus and rabbits to develop
models, it has been predicted that, as rabbits are selected for
resistance, field strains of virus will be selected for enhanced
virulence ( 2 , 1 2 , 59). There is some evidence of this occurring
in Great Britain (Fig. 4 ) , where increasing numbers of grade II
strains of virus are emerging, and perhaps also in Australia
(Fig. 3 ) . Regions where genetic resistance is highest, such as
the Mallee region of Australia, seem to have predominant
virus strains of higher virulence ( 1 3 , 2 2 ) . No detailed
assessments of virulence have been performed in Australia in
the last ten years but in other studies, 7/11 field strains
isolated in Australia since 1 9 9 0 were of grade I virulence,
indicating that there may well be a shift towards highly
virulent viruses in the field (P.J. Kerr, unpublished findings).
Perhaps the most likely scenario was proposed by Fenner and
Ross (22), who suggested that most of the changes in viral or
host genotypes which can be readily selected for have already
occurred, and that myxomatosis will remain a moderately
severe disease with 'an appreciable mortality', similar to
smallpox in Asia prior to eradication. This assumes that the
absolute resistance occasionally seen in some rabbits will not
become dominant in the field. These rabbits may be selected
against in some situations and the attenuated virus strains may
limit selection pressure for this degree of resistance.
Interestingly, in France, the so-called 'amyxomatous' strains of
myxoma virus appeared (28). These strains had a predilection
for the respiratory tract and appeared to be transmitted by the
respiratory route (as was smallpox). The question of whether
selection. for resistance of rabbits to vector-transmitted
myxoma virus has led to selection for virus strains which are
transmitted by other routes is fascinating, with relevance to
host-pathogen evolution in many situations.
Even over relatively short time-frames, and excluding normal
climatic fluctuations, the environment in which selection of
rabbits and virus occurs has not remained constant. Since
1970, the European rabbit flea has become established in the
temperate areas of Australia, thus providing an additional
vector. More recently, the Spanish rabbit flea has been
extensively released in the arid zones of Australia. The
European rabbit flea altered the epidemiology of myxomatosis
in Australia (9, 1 1 , 6 0 ) , and the Spanish flea is also likely to
affect epidemiology in the arid zones, but the effects of these
new vectors on the evolution of viral virulence and host
resistance are unclear. Recently the calicivirus, viral
haemorrhagic disease of rabbits, has significantly reduced
populations of rabbits in Europe and Australia, and the effects
that this may have on resistance to myxomatosis have not
been studied.
This work on genetic resistance to myxoma virus is partially
supported by funding from the Anti-Rabbit Research
Foundation of Australia. Dr R. Seamark and Dr R. Jackson
critically read the manuscript.
Rev. sci. tech. Off. int. Epiz., 17 (1)
Le virus de la myxomatose du lapin
P.J. Kerr & S.M. Best
Le virus de la m y x o m a t o s e du lapin e u r o p é e n (Oryctolagus
est l'un d e s
e x e m p l e s les m i e u x d é c r i t s d'évolution c o n j o i n t e de l'hôte et du virus. C h e z les
h ô t e s n a t u r e l s (lapins Sylvilagus
ou S. bachmani
du c o n t i n e n t
a m é r i c a i n ) , le virus de la m y x o m a t o s e p r o v o q u e un f i b r o m e c u t a n é b é n i n . C h e z le
lapin e u r o p é e n , e n r e v a n c h e , il d é c l e n c h e u n e m y x o m a t o s e f o u d r o y a n t e . A u
d é b u t de l'épizootie, le virus p r o v o q u a i t un t a u x de m o r t a l i t é t r è s é l e v é , s u p é r i e u r
à 9 9 % c h e z le lapin e u r o p é e n en A u s t r a l i e , en E u r o p e c o n t i n e n t a l e et e n
G r a n d e - B r e t a g n e . L'apparition de s o u c h e s v i r a l e s a t t é n u é e s a p e r m i s le
d é v e l o p p e m e n t d'une c e r t a i n e r é s i s t a n c e et, p a r t a n t , la survie de lapins a s s e z
résistants. Cette évolution a été s u r t o u t s e n s i b l e d a n s les c l i m a t s c h a u d s , où la
t e m p é r a t u r e é l e v é e a a u g m e n t é le t a u x de survie d e s a n i m a u x i n f e c t é s . Les lapins
r é s i s t a n t s t r a n s m e t t a n t m o i n s f a c i l e m e n t le v i r u s , l'apparition d e s o u c h e s v i r a l e s
plus v i r u l e n t e s p e u t s'en t r o u v e r f a v o r i s é e . Les m é c a n i s m e s de la r é s i s t a n c e s o n t
e n g r a n d e partie m é c o n n u s . Les h y p o t h è s e s qui ont é t é a v a n c é e s c o n c e r n a n t la
r é s i s t a n c e non g é n é t i q u e r e s t e n t à c o n f i r m e r e x p é r i m e n t a l e m e n t .
Génétique - Lapins - M y x o m a t o s e - Résistance aux maladies - Virulence - Virus de la
El virus del mixoma en el conejo
P.J. Kerr & S.M. Best
La r e l a c i ó n e n t r e el virus del m i x o m a y el c o n e j o e u r o p e o (Oryctolagus
e s uno de los e j e m p l o s de c o e v o l u c i ó n e n t r e h u é s p e d y virus d o c u m e n t a d o s c o n
m á s profusión. En sus h u é s p e d e s n a t u r a l e s (los c o n e j o s a m e r i c a n o s de las
e s p e c i e s Sylvilagus
o S. bachmani),
el virus del m i x o m a c a u s a un
f i b r o m a c u t á n e o de c a r á c t e r b e n i g n o . En el c o n e j o e u r o p e o , sin e m b a r g o , e s e
virus p r o v o c a una e n f e r m e d a d f u l m i n a n t e , la m i x o m a t o s i s . En las f a s e s iniciales
q u e s i g u i e r o n a su i n t r o d u c c i ó n e n las p o b l a c i o n e s s a l v a j e s d e c o n e j o e u r o p e o d e
A u s t r a l i a , Europa c o n t i n e n t a l y G r a n B r e t a ñ a , el virus se r e v e l ó letal en g r a d o
s u m o , c o n una t a s a de m o r t a l i d a d s u p e r i o r al 9 9 % de los c o n e j o s i n f e c t a d o s . La
a p a r i c i ó n de c e p a s v í r i c a s a t e n u a d a s , al f a c i l i t a r la s u p e r v i v e n c i a d e a n i m a l e s
r e s i s t e n t e s , a l e n t ó el d e s a r r o l l o de la r e s i s t e n c i a al virus.
C o n s i d e r a n d o q u e una e l e v a d a t e m p e r a t u r a a m b i e n t e i n c r e m e n t a la t a s a de
s u p e r v i v e n c i a de los c o n e j o s i n f e c t a d o s , es posible q u e a q u e l p r o c e s o se d i e r a
c o n m á s r a p i d e z e n c l i m a s c á l i d o s . Los c o n e j o s r e s i s t e n t e s son t r a n s m i s o r e s
m e n o s e f i c a c e s del virus, h e c h o q u e p u e d e f a v o r e c e r la a p a r i c i ó n d e c e p a s m á s
v i r u l e n t a s . M u y p o c o se s a b e s o b r e el m e c a n i s m o de r e s i s t e n c i a : a u n q u e se ha
s u g e r i d o u n a r e s i s t e n c i a de tipo no g e n é t i c o , falta a ú n c o n f i r m a c i ó n e x p e r i m e n t a l
de tal s u p o s i c i ó n .
Palabras clave
Conejos - Genética - M i x o m a t o s i s - Resistencia a la e n f e r m e d a d - Virulencia - Virus del
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