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Journal of Applied Microbiology 2003, 94, 47S–58S
Migratory birds and West Nile virus
J.H. Rappole1 and Z. Hubálek2
1
Smithsonian Conservation & Research Center, Front Royal, VA 22630, USA, and 2Laboratory of Medical
Zoology, Institute of Vertebrate Biology, Academy of Sciences, Klášternı´ 2, CZ69142 Valtice, Czech Republic
1.
2.
3.
4.
Summary, 47S
Introduction, 47S
West Nile Virus in the Old World, 48S
Arrival and Movement of West Nile Virus in the New
World: 1999–2002, 49S
5. Mode of Entry of West Nile Virus into the Western
Hemisphere, 52S
6. The ‘Migrant Bird as Introductory Host’ Hypothesis, 52S
1. SUMMARY
West Nile virus was first recorded in the New World during
August 1999 in New York City. Aetiology of the disease in
the Old World indicated birds as the likely introductory and
amplifying hosts with ornithophilous mosquitoes, e.g. Culex
pipiens, as the principal vectors. Speculation regarding likely
agents for movement of the virus in its new environment
focused on migratory birds, but evidence to date is
equivocal. While spread of the disease has been fairly rapid,
at a rate of roughly 70 km a month, it has not shown the
kind of long-distance, leap frog movements one might
expect if transient birds were the principal introductory
hosts. Furthermore, movement of the disease has not been
focused southward, but shows a radiating pattern with
detection sites located in all directions from New York
where terrestrial habitat was available. In addition, tests
among potential New World, avian hosts have revealed
prolonged viraemia (up to 5 days) only in the relatively nonmigratory House Sparrow (Passer domesticus). Dispersal
movements by this species could account for the observed
pattern of West Nile virus spread in the Western Hemisphere to date. Regardless of whether avian migration,
dispersal, or some other agent is responsible, West Nile
virus should reach the New World tropics in another
1–2 years, at which time a vast number of new potential
introductory and amplifying avian hosts would be exposed
to the disease and mosquito vectors would be available
Correspondence to: John H. Rappole, 1500 Remount Road, Smithsonian
Conservation & Research Center, Front Royal, VA 22630, USA
(e-mail: [email protected]).
ª 2003 The Society for Applied Microbiology
7.
8.
9.
10.
6.1 Factors supporting hypothesis, 53S
6.2 Factors not supporting the hypothesis, 53S
Alternative Hypotheses for West Nile Movement in the
New World, 53S
The Future for West Nile Virus in the New World, 56S
Acknowledgements, 56S
References, 57S
throughout most of the year, likely causing serious, longterm threats to human health and vulnerable avian populations in the region.
2. INTRODUCTION
On 23 September 1999, West Nile virus was identified using
polymerase chain reaction and DNA sequencing from
materials isolated from the tissues of dead birds collected
in early September 1999 at the Bronx Zoo in New York City
(Centers for Disease Control and Prevention 1999). This
virus had first been identified from the blood of a woman
from the West Nile region of Uganda in 1937 (Smithburn
et al. 1940), but has since been found to be a rather common
pathogen throughout much of the Old World, particularly in
the African tropics, Middle East and temperate Eurasia
(Karabatsos 1985; Peiris and Amerasinghe 1994). An
estimated 40% of the human population of Egypt’s Nile
delta was seropositive for the virus in the 1950s (Smithburn
et al. 1954). Outbreaks in human populations of increasing
frequency and severity in Europe, western Asia and the
Middle East since 1990 indicate possible changes in its
epidemiology (Gariépy et al. 2001). However, until the
outbreak in New York, the occurrence of West Nile virus
had never previously been documented in the Western
Hemisphere.
Mosquitoes of the genus Culex serve as the most common
vector for West Nile virus in both the Old and New World
(Hubálek and Halouzka 1999; Andreadis et al. 2001), and
birds, especially those occurring in large flocks in areas
frequented by these mosquitoes, e.g. wetlands and urban
sites, are the most common amplifying hosts (Hubálek and
48S J . H . R A P P O L E A N D Z . H U B Á L E K
Halouzka 1999). Epidemics appear to be caused by a high
rate of viral infection among avian hosts, which is then
passed by mosquitoes to humans (Hubálek and Halouzka
1999). Most vertebrates are susceptible to West Nile virus
infection, and some, e.g. humans and horses, can suffer
mortality rates of up to 10% of those clinically diagnosed
with the infection (Garmendia et al. 2001). Nevertheless,
there is little evidence to date to indicate that groups other
than birds can serve as significant amplifying hosts. For
instance, viraemia does not appear to be sufficient in
humans, or indeed most vertebrates other than birds, to
allow them to function as sources for transmission of the
virus to other organisms by mosquito vectors or any other
mode (Komar 2000). However, the virus can be passed from
bird to bird in the laboratory without an obvious intermediary vector (McLean et al. 2001).
The epidemiology of West Nile virus thus seems well
understood, with infection originating in a large, dense
population of birds, where mosquitoes serve to transfer the
virus from bird to bird, and also from bird to human. Such
episodes are seasonal in temperate areas, with both avian and
human infection rates dropping to near zero as winter
approaches and mosquitoes become dormant (Hubálek and
Halouzka 1999). What has not been clear is how the virus
originates at a particular site. Arboviral persistence at a site
through periods of mosquito dormancy has been documented for other viruses (Reeves 1974), and new data from both
the field (Miller et al. 2000) and laboratory (Nasci et al.
2001) now indicate that such vertical transmission (viral
infection passed from female mosquito to offspring) could
account for spring reappearance of West Nile at temperate
sites like New York City where high infection rates occurred
in previous years. However, this phenomenon does not
explain how the virus can move from one temperate region
to another, infecting birds, mosquitoes, people and other
organisms at places where no previous infections were
known. Based on information in the Old World literature
(summarized by Hubálek and Halouzka 1999), Rappole et al.
(2000) speculated that migratory birds might serve as the
principal introductory hosts for the virus in the New World.
In the current paper, we examine new data on movement of
West Nile virus in the Western Hemisphere, use this
information to examine the validity of the ‘Migrant Bird as
Introductory Host’ hypothesis, and propose alternative
explanations.
3. WEST NILE VIRUS IN THE OLD WORLD
West Nile virus is enzootic in the African tropics, and has
been for at least 70 years (Garmendia et al. 2001). However,
until recently, outbreaks in Old World temperate regions
appeared to be epizootic and often isolated in both space and
time. For instance, human cases of West Nile viral infection
have been recorded in southern France in 1962, southern
Russia in 1963, Belarus in 1977, western Ukraine in 1985,
Romania in 1996, Czechland in 1997 and southern Russia in
1999, as well as several other sites in southern Europe
(Hubálek and Halouzka 1999). Occurrence of the virus
among humans in Europe has shown other distinctive
characteristics as well: (i) Outbreaks generally occur from
July–September at or near wetlands or urban sites. (ii) The
most common vectors are mosquitoes of the genus Culex,
females of which feed mostly on birds and mammals. (iii)
Birds are the primary vertebrate hosts, several species of
which can produce levels of viraemia sufficient for transmission by vectors to other hosts, including humans
(Hubálek 2000). These characteristics have led researchers
to propose that migratory birds, infected with West Nile
virus on their African wintering grounds, carry the virus
northward on spring migration to stopover sites in Europe
where they can serve as introductory hosts under certain
conditions, i.e. at sites with numerous potential vectors
(mosquitoes) and amplifying hosts (large flocks of birds –
not necessarily the same species as the introductory host)
(Hannoun et al. 1972; Hubálek and Halouzka 1999). This
hypothesis would explain why outbreaks often occur in or
near wetlands and urban areas, where introductory host,
vector, amplifying host and human victim co-occur. It
would provide an explanation for the ability of the virus to
move from site to site, and explain the timing of outbreaks as
well, with migrants carrying the virus northward in April
and May during spring migration and infecting local bird
populations which serve as amplifying hosts, eventually
infecting large portions of the vector population within
2–3 months, and subsequently passing the virus to humans
in the area by July or August. Additional support for the
hypothesis is provided by the fact that individuals of many
species of migratory birds have been found to be carrying
West Nile virus when captured during migration (Nir et al.
1967; Watson et al. 1972; Ernek et al. 1977). Also, a few
laboratory studies have been performed that document
viraemia in some species of migrants of sufficient intensity
and duration to allow an infected bird, in theory at least, to
move the virus in infectious form from one locality to
another (Work et al. 1955; Taylor et al. 1956; Fedorova and
Stavskiy 1972; Chunikhin 1973; Semenov et al. 1973).
The most direct evidence in support of this idea comes
from a recent study in which 13 dead and dying White
Storks (Ciconia ciconia) were taken from a group of ca. 1200
birds that had landed at a site (Eilat) in southern Israel
2 days previously, on 26 August 1998. Laboratory tests
documented high levels of infection with West Nile virus in
this sample. Presumably, these birds had contracted and
transported the virus from stopover sites on their southward migration route across southeastern Europe, an idea
supported by the genetic similarity of the Eilat viral
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
MIGRATORY BIRDS AND WEST NILE VIRUS
samples with samples from Romania and Volgograd, Russia
(Malkinson et al. 2002).
An interesting aspect of the recent outbreaks of West Nile
virus in Israel is the fact that a number of individuals of
several species (White Stork, Domestic Pigeon Columba
livia, Domestic Goose Anser anser, White-eyed Gull Larus
leucophthalmus) sickened and died as an apparent result of
viral infection (Malkinson et al. 2002). In previous Old
World outbreaks, actual observations of sickness or death
among avian hosts has been rare (Hubálek and Halouzka
1999; Komar 2000), leading Malkinson et al. (2002) to
suggest that the infectious agent might represent a new form
of West Nile virus.
4. ARRIVAL AND MOVEMENT OF
WEST NILE VIRUS IN THE NEW WORLD –
19 9 9 –2 0 0 2
Significant numbers of sick and dead birds in and around
the Bronx Zoo in New York City, as well as several human
patients suffering from encephalitis of unknown origin from
the borough of Queens in southern New York City during
August 1999, were the first indication of a new epizootic
incident (Centers for Disease Control and Prevention 1999).
The first human testing seropositive for West Nile virus in
the Western Hemisphere was on 5 August 1999 while the
first seropositive bird was found dead on 8 August 1999,
although the pathogen responsible for either case was not
positively identified until 23 September 1999 (Centers for
Disease Control and Prevention 1999; US Department of
Agriculture 2002). Ultimately, 62 human cases were laboratory-confirmed during this initial outbreak (August–
October 1999), all from the New York City area, seven of
whom died (Centers for Disease Control and Prevention
2000a). Twenty-five equine cases also were documented in
the region, in addition to large numbers of infected birds
(Steele et al. 2000; Garmendia et al. 2001; US Department
of Agriculture 2002). The date for the last known human
onset for the disease in 1999 was 22 September, while the
last bird to test seropositive for West Nile virus in 1999 was
found dead in New York City on 5 November. A Red-tailed
Hawk, found dead in Bronxville, Westchester County, New
York, 20 km north of Queens, was found on 6 February
2000. Autopsy indicated death resulting from encephalitic
lesions typical of acute infection. Mid-winter is an unlikely
period for a mosquito vector to cause infection, and pathologists speculated that the hawk may have eaten a bird that
had died in late fall from the virus and contracted the
infection orally (Garmendia et al. 2001), a mode of transmission documented experimentally in laboratory mice (Odelola
and Oduye 1977) and, possibly, birds (Komar 2000).
Genetic analysis demonstrated that the form of the virus
responsible for the New York outbreak was virtually iden-
49S
tical to that previously recorded from Israel (Lanciotti et al.
1999).
Thousands of birds of as many as 18 species died during
the 1999 outbreak, including an estimated 3000 American
Crows (Corvus brachyrhynchos) from the New York City area
alone (Steele et al. 2000). As noted above, significant avian
mortality has not been characteristic of West Nile virus
outbreaks in the Old World (Hubálek and Halouzka 1999;
Komar 2000). As the epidemic progressed during the late
summer and fall of 1999, reports of dead birds seropositive
for West Nile virus expanded outward from New York City,
eventually reaching many of the counties of New York, New
Jersey and Connecticut located within 250 km of the city
(Fig. 1). The most northerly record came from Columbia
County, New York, about 170 km north from the epicentre
for the outbreak in the New York City Borough of Queens;
the most easterly record was from Suffolk County, New
York on Long Island about 230 km east of Queens; and the
most distant record for a bird testing seropositive for West
Nile virus in 1999 was an American Crow found in
Baltimore, Maryland, on 14 October, roughly 300 km
southwest from Queens (Fig. 1).
MA
NY
RI
CT
PA
NJ
MD
DE
VA
150 km
Fig. 1 Counties by state from which dead birds testing positive for
West Nile virus were reported in 1999. Counties from which positive
specimens have been documented are shaded grey
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
50S J . H . R A P P O L E A N D Z . H U B Á L E K
principal mosquito vectors and the absence of evidence of
the virus in New World tropical and subtropical regions,
makes it appear likely that the virus did not depend on
migratory birds to survive in the New World, but rather on
its mosquito vectors.
After reappearing in May 2000 in the New York region,
records confirmed reoccurrence of the virus in many of its
previous localities documented from 1999, but also showed
evidence of continued outward expansion, although not in a
strictly north–south direction, as might have been expected
based on a ‘Migratory Bird as Introductory Host’ scenario,
but rather evenly in all directions (Fig. 2), with the majority
of reports centred in the area where the 1999 epidemic
occurred. The locality furthest from the original New York
epicentre documented in 2000 was an American Crow found
in Chatham County, North Carolina, on 27 September,
about 700 km south of New York City. Assuming a New
World arrival date for West Nile virus of 1 June 1999 and an
infection season lasting from 15 April to 15 November in
Mid-Atlantic states, the virus moved at a rate of roughly
67 km a month (June–October, 1999; 15 April–27 September 2000). The most westerly record in 2000 came from a
mosquito collected in Erie County, Pennsylvania, about
570 km from the New York epicentre for the virus; the most
northerly record came from an Ovenbird (Seiurus aurocapillus) collected in August in Clinton County, New York,
about 480 km north of the epicentre; the most easterly
record came from birds collected in Barnstable County,
Speculation regarding the future of West Nile virus in the
New World was intense after the end of the 1999 infection
season. The history of Old World temperate-zone epidemics
of West Nile virus indicated that in order for the virus to
persist in the New World, birds might have to transport the
virus to the New World tropics or subtropics in autumn,
introduce it to a new set of avian, amplifying hosts, and then
bring the virus north from these areas in the spring (Rappole
et al. 2000). Following this scenario, the virus might die out
in the New World unless a suitable migrant bird host carried
the virus southward and introduced it into a tropical or
subtropical environment where it could establish a yearround base. Given this possibility, efforts were made in
several southern states to find evidence of the virus in dead
birds. Such efforts were futile. No dead birds positive for
West Nile virus were reported from southern states during
the winter of 1999–2000.
However, the virus did reappear. On 22 May, an
American Crow was found dead in Rockland County, New
York, about 50 km north of the 1999 epicentre, and later
confirmed seropositive for West Nile virus to be followed by
many more bird, horse, mosquito and human reports
(Centers for Disease control and Prevention 2000b; Bernard
et al. 2001; Garmendia et al. 2001; Marfin et al. 2001).
Interestingly, all reports were from the northeastern United
States, and in fact, centred on the region of the original 1999
outbreak (Fig. 2). This pattern, plus information documenting the fact that the virus can survive the winter in its
ME
Canada
VT
NH
MA
NY
CT
RI
MI
NJ
PA
MD
OH
DE
IN
WV
VA
KY
300 km
TN
NC
Fig. 2 Counties by state from which dead
birds testing positive for West Nile virus were
reported in 2000. Counties from which positive specimens have been documented are
shaded grey
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
MIGRATORY BIRDS AND WEST NILE VIRUS
Massachusetts, about 370 km east of the epicentre. No
records for birds testing positive for West Nile virus were
reported from Canada during 2000 despite examination of
2288 birds, 185 of which were submitted for laboratory tests,
mostly from regions bordering the northeastern United
States (Canadian Cooperative Wildlife Health Centre 2002).
The pattern of West Nile recurrence in 2001 was similar
to that seen in 2000. The first record for the year was from
an American Crow found dead on 30 April in Upper Saddle
River, Bergen County, New Jersey, about 40 km from the
1999 epicentre (Groves 2001). Subsequently, the majority of
human, avian, equine and mosquito cases came from within
500 km of the 1999 epicentre (Fig. 3). Nevertheless, the
virus continued to expand its distribution outward, reaching
a maximum distance of about 2100 km from the original
1999 New York detection point with report of a horse
positive for antibodies for West Nile virus in Calcasieu
Fig. 3 Counties by state from which dead
birds testing positive for West Nile virus were
reported in 2001. Concentric circles centred
on the New York City borough of Queens are
shown in radius increments of 500 km
51S
Parish, Louisiana in August (Arbonet 2002). Both human
and avian cases were reported from the Florida Keys in
August, 1900 km from New York, and a possible human
case was reported from even further south on 24 August
from Cayman Brac in the Caribbean, although no avian or
mosquito records have been documented from that site.
Records from El Dorado, Arkansas (1850 km from Queens,
New York), St Louis, Missouri (1450 km), Walcott, Iowa
(1430 km) and Milwaukee,Wisconsin (1200 km), all represent the western limits of expansion for 2001; while a record
from Sabbatus, Maine (720 km), represents the furthest
northern and eastern expansion of the virus during 2001
(Cornell University Center for the Environment 2002a).
Confirmed records of infected birds also were reported from
Ontario in southern Canada, marking the first time the virus
has been documented outside the United States in the
Western Hemisphere. In moving from its southmost point
500 km
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
52S J . H . R A P P O L E A N D Z . H U B Á L E K
of distribution in August 2000 (Chatham County, North
Carolina) to Calcasieu Parish, Lousiana in August 2001, a
distance of about 1400 km, the virus appears to have
increased its rate of range expansion significantly in the New
World from about 70 km a month to as much as 170 km a
month, although the increased rate could be due, at least in
part, to an increase in number of months of vector activity in
the southern United States and/or failure to detect evidence
of further southward range expansion during 2000.
Data for 2002 has provided additional evidence of
continued expansion of the range for the disease, with dead
Blue Jays (Cyanocitta cristata) testing positive for West Nile
virus antibodies reported from Houston, Texas (Brewer and
Hopper 2002), 2700 km from the original 1999 outbreak in
New York City. However, one significant change in the
behaviour of the virus became obvious in 2002, namely that
the quiescent period for vector activity, which appears to
run from November to April in the northeastern states, is
short or non-existent in the Gulf states. Cases of West Nile
were reported from four Florida counties on 18 February
2002 (three horses, two wild birds and a sentinel chicken)
(Florida Department of Health 2002).
5 . M O D E O F E N T R Y FO R W E S T N I L E
VIRUS INTO THE WESTERN HEMISPHERE
Determination of how West Nile virus first crossed the
Atlantic to invade the New World is entirely conjectural.
Hypotheses have ranged from bioterrorism or infected
mosquitoes hitch-hiking in airplanes (Preston 1999) to
imported infected frogs (Zwerdling 2001). We suggested
that the most likely mode of entry was the one apparently
used by the virus in the Old World, namely via an avian
introductory host (Rappole et al. 2000). We proposed
three possible scenarios in which birds could serve as
the introductory host for West Nile virus into the New
World: (i) normal migration, (ii) storm-driven birds or
Infected bird migrates
northward in viremic
state to spring
stopover site with
high concentrations of
potential vectors and
amplifying hosts,
3
serving as an
introductory host
at a new, temperate
site.
1
2
(iii) importation. Given the levels of viraemia required to
pass the virus from introductory host to vector to amplifying
host, and the generally short duration of such viraemic
states, we believe that importation of an infected bird is the
most likely mode of entry. Also, given that the form of the
virus appears to be quite similar to that found in the Middle
East (Lanciotti et al. 1999), an import from that region
would be the most likely culprit, perhaps for a zoo or private
collection. Although such imports are required to pass
lengthy quarantine periods, it would seem at least possible
for them to be exposed to local, New York mosquitoes at
Kennedy Airport during the period of transport from the
plane to their quarantine site, at which time a vector
mosquito could become infected and transfer the virus to a
local avian host. In addition, it is possible that imported,
West Nile virus infected birds could pass quarantine without
revealing significant clinical symptoms.
6. THE ‘MIGRANT BIRD AS INTRODUCTORY
HOST’ HYPOTHESIS
In contrast to mode of entry to the hemisphere for the virus,
there seems to be a consensus regarding how the virus has
been able to move from its apparent arrival point in New York
City across much of eastern North America in the 36 months
since its initial appearance. Migratory birds are considered to
be the most likely introductory host, presumably transporting
the virus to new vectors and hosts at sites hundreds of
kilometres distant from New York (US Geological Survey
1999; Centers for Disease Control and Prevention 2000a;
University of Georgia 2001; American Museum of Natural
History 2002). Old World information on this point, while
mostly circumstantial and correlative, is nevertheless supportive of this hypothesis, as discussed above. Indeed, in a
previous paper (Rappole et al. 2000), we speculated on how
the ‘Migrant Bird as Introductory Host’ hypothesis might be
expected to work in the New World (Fig. 4).
Bird on southward
migration is infected
with West Nile virus
at a stopover site in
the New York area in
September, 1999.
Bird migrates in viremic state
to subtropical wintering site,
serving as introductory host
to large concentrations of
vectors (mosquitoes) and
amplifying hosts (other birds).
Fig. 4 ‘Migrant bird as introductory host’
hypothesis for movement of West Nile virus
from region to region
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
MIGRATORY BIRDS AND WEST NILE VIRUS
6.1 Factors supporting hypothesis
Two factors lend support to the hypothesis. First, the virus
obviously has moved far and fast in a manner that is not
inconsistent with migratory bird transport; and secondly,
migratory birds have been well documented to be the most
susceptible to, and commonest sufferers from, West Nile
virus infection among all vertebrate groups (Table 1).
6.2 Factors not supporting hypothesis
Nevertheless, there are factors that do not support the
hypothesis:
1) Rate of movement – members of most migratory species
fly at night. Flight speeds range from 30 to 70 km h)1
depending significantly on winds; strong tail winds can
double flight speeds. When conditions are favourable, they
generally depart at dusk, or shortly thereafter, and fly
continuously for as much as 10 h until dawn. Thus, a single
night’s flight for an average migrant might cover 200–
400 km, with considerable variation depending on species,
weather and obstacles (Kerlinger 1995). For example, a
Swainson’s Thrush (Catharus ustulatis), radio-tracked during
migration, flew 1530 km over a 6-day period, flying an
average of 7 h a night for an average nightly distance of
201 km (Cochran 1987), and other radio-tracked thrushes
flew up to 450 km in a single night (Cochran et al. 1967).
These speeds and distances are not unusual for the hundreds
of species of migrants that pass through the New York City
region. Thus, if migrants were, in fact, moving West Nile
virus, it would not be unreasonable to expect the virus to
move hundreds of kilometres in a matter of days. However,
this is not the pattern of movement that has been observed.
Instead, the virus moved a maximum of 300 km during the
3 months of known activity in 1999, and another 400 km
during 7 months of known activity in 2000. This rate of travel
is very slow if migratory birds are involved in transport.
2) Direction of movement – although actual direction can
vary depending on winds and exact destination, migratory
birds generally travel on a north–south axis, i.e. fall migrants
usually travel in a southerly direction on migratory flights,
while spring migrants move in a northerly direction.
However, during 1999, the virus moved nearly as far north
(170 km) and east (230 km) as it did southwest (300 km).
Similarly, while reaching a maximum distance from Queens
of 700 km southwest during 2000, it also reached distances
of 480 km north, 570 km west and 370 km east.
3) Pattern of recurrence in subsequent years – if migratory
birds were responsible for movement of the virus, there
would be no reason to expect an outbreak of West Nile virus
the following year to occur in or near the site of the original
outbreak. Migrants of most bird species in the New World
seldom use the same stopover sites on northward, spring
53S
migration as they do on southward, fall migration because
migration routes are determined by complex interactions of
factors such as direction of prevailing winds, age and sex of
the bird, weather patterns, location of available food
resources and geographical barriers (e.g. large bodies of
water or mountains). These factors seldom combine to
favour the same route in different seasons (Rappole 1994).
Yet the obvious pattern for recurrence of West Nile in 2000,
2001 and 2002 was for it to recur in greatest concentration at
sites where it had occurred previously. This pattern suggests
that the virus probably over-wintered in the vector population, and was passed subsequently to local, avian amplifying hosts, rather than being moved south and north by
migrants in transit between breeding and wintering areas.
4) Host competence – the species suggested as the most
likely candidate as ‘Introductory Host’ for West Nile virus
(American Crow) is also the one known to suffer the highest
mortality rate from the virus (Eidson et al. 2001). Obviously,
crows are highly susceptible to infection, in fact apparently
shedding sufficient levels of the virus in faeces to allow birdto-bird transmission orally in laboratory situations (McLean
et al. 2001). Nevertheless, it remains unknown whether they
are physically able to move long distances, e.g. undertaking a
migratory flight of 200–300 km, once they have been infected.
5) Duration of viraemia – studies have documented that
levels of West Nile viraemia in several species of New World
birds (including those with migratory populations, e.g.
American Crow and Common Grackle as well as species that
are mostly resident, like the Blue Jay and House Sparrow)
are sufficiently high (105Æ4–1012Æ6 PFU/ml) to allow them to
serve as competent hosts for the virus (Komar et al. 2000;
Bernard and Kramer 2001). However, duration of such high
levels of viraemia has been found to be limited in time for
most species tested (usually <24 h). Interestingly, the House
Sparrow, a resident species, has demonstrated viraemia of
sufficient duration to indicate its ability to serve as a
competent host for West Nile virus (Komar et al. 2000).
7. ALTERNATIVE HYPOTHESES FOR WEST
NILE MOVEMENT IN THE NEW WORLD
The considerations listed above raise questions concerning
the validity of the ‘Migrant Bird as Introductory Host’
hypothesis. Until these can be reconciled by field and
laboratory studies, it would seem appropriate to consider
alternative hypotheses, as measures for defence against the
virus are designed based, in part, on our understanding of
how it moves from one point to another. Accordingly, we
suggest below alternatives for West Nile virus movement in
the Western Hemisphere:
1) Sick migrant bird as introductory host – as noted
above, several migrant bird species have been documented
with high levels of viraemia. However, high levels of
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
54S J . H . R A P P O L E A N D Z . H U B Á L E K
Common name
Scientific name
Residency status*
Double-crested Cormorant
Least Bittern
Great Blue Heron
Green Heron
Black-crowned Night-Heron
Black Vulture
Canada Goose
Mute Swan
Mallard
Bald Eagle
Sharp-shinned Hawk
Cooper’s Hawk
Broad-winged Hawk
Red-tailed Hawk
American Kestrel
Merlin
Ring-necked Pheasant
Ruffed Grouse
Wild Turkey
Virginia Rail
Sandhill Crane
Killdeer
Ruddy Turnstone
Sanderling
Laughing Gull
Ring-billed Gull
Herring Gull
Great Black-backed Gull
Black Skimmer
Rock Dove
Mourning Dove
Great Horned Owl
Snowy Owl
Common Nighthawk
Ruby-throated Hummingbird
Belted Kingfisher
Northern Flicker
Eastern Phoebe
Blue Jay
American Magpie
American Crow
Fish Crow
Common Raven
Black-capped Chickadee
Tufted Titmouse
Eastern Bluebird
Phalacrocorax auritus
Ixobrychus exilis
Ardea herodias
Butorides virescens
Nycticorax nycticorax
Coragyps atratus
Branta canadensis
Cygnus olor
Anas platyrhynchos
Haliaeetus lecocephalus
Accipiter striatus
Accipiter cooperii
Buteo platypterus
Buteo jamaicensis
Falco sparverius
Falco columbarius
Phasianus colchicus
Bonasa umbellus
Meleagris gallopavo
Rallus limicola
Grus canadensis
Charadrius vociferus
Arenaria interpres
Calidris alba
Larus atricilla
Larus delawarensis
Larus argentatus
Larus marinus
Rynchops niger
Columba livia
Zenaida macroura
Bubo virginianus
Nyctea scandica
Chordeiles minor
Archilochus colubris
Ceryle alcyon
Colaptes auratus
Sayornis phoebe
Cyanocitta cristata
Pica hudsonia
Corvus brachyrhynchos
Corvus ossifragus
Corvus corax
Poecile atricapillus
Baeolophus bicolor
Sialia sialis
Veery
Hermit Thrush
Wood Thrush
American Robin
Grey Catbird
Northern Mockingbird
European Starling
Cedar Waxwing
Catharus fuscescens
Catharus guttatus
Hylocichla mustelina
Turdus migratorius
Dumetella carolinensis
Mimus polyglottos
Sturnus vulgaris
Bombycilla cedrorum
Temperate/subtropical
Subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Resident/temperate/subtropical
Mostly Resident
Temperate/subtropical
Temperate
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Resident
Resident
Resident
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical
Temperate/subtropical/tropical
Resident
Temperate/subtropical/tropical
Resident
Temperate
Tropical
Tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Resident
Resident
Resident/temperate/subtropical
Resident/temperate/subtropical
Resident
Resident
Resident
Resident/temperate/subtropical
Tropical
Tropical
Temperate/subtropical/tropical
Tropical
Temperate/subtropical/tropical
Temperate/subtropical/tropical
Resident
Resident/temperate/subtropical
Temperate/subtropical/tropical
Table 1 Residency status for New World
bird species that have tested positive for West
Nile virus. Taxonomy follows the American
Ornithologists’ Union Check-List (1998,
2000)
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
MIGRATORY BIRDS AND WEST NILE VIRUS
Table 1 (Contd.)
55S
Common name
Scientific name
Residency status*
Black-throated Blue Warbler
Yellow-rumped Warbler
Blackpoll Warbler
Ovenbird
Canada Warbler
Song Sparrow
Northern Cardinal
Red-winged Blackbird
Common Grackle
Brown-headed Cowbird
House Finch
American Goldfinch
House Sparrow
Dendroica caerulescens
Dendroica coronata
Dendroica striata
Seiurus aurocapillus
Wilsonia canadensis
Melospiza melodia
Cardinalis cardinalis
Agelaius phoeniceus
Quiscalus quiscula
Molothrus ater
Carpodacus mexicanus
Carduelis tristis
Passer domesticus
Tropical
Temperate/subtropical/tropical
Tropical
Tropical
Tropical
Resident/temperate/subtropical
Resident
Temperate/subtropical/tropical
Resident/temperate/subtropical
Temperate/subtropical/tropical
Resident
Temperate/subtropical/tropical
Resident
*Residency status categories: resident – a significant portion (>10%) remain on or near the
breeding region throughout the annual cycle. However, even for populations that appear largely
resident, some birds disperse and some young individuals may undergo short (<500 km)
migratory movements in fall; temperate – a significant portion (>10%) of the breeding population
migrates southward in fall to temperate regions for the winter period; subtropical – a significant
portion (>10%) of the breeding population migrates southward in fall to subtropical regions for
the winter period; tropical – a significant portion (>10%) of the breeding population migrates
southward in fall to tropical regions for the winter period.
viraemia are of short duration in most species that have
been tested and accompanied or followed by illness and
decreasing motor ability in many species. On the other
hand, apathic, less mobile birds (e.g. American Crows) can
attract more vector mosquitoes to feed on them successfully. It seems unlikely that such birds could move long
distances in an infectious state, but they might be able to
move 50–100 km before viraemia drops below infectious
levels or they succumb to the illness. However, if sick
migrants were the agents, direction of movement would
more likely be along the typical north–south migration axis,
which would not explain the broad lateral spread of the
virus observed to date.
2) Dispersing sedentary bird as introductory host – Komar
et al. (2001) state, ‘Thus, of the species we evaluated for
seroprevalence, the House Sparrow was an important host
because of its abundance, high seroprevalence, and biological
competence.’ These factors make the House Sparrow a likely
candidate as an important amplifying host for the virus, in
addition to American Crows. The fact that House Sparrows
can move significant distances during dispersal episodes
makes them a plausible candidate as introductory host as well.
Fig. 5 shows distances at which House Sparrows have been
recaptured from original capture point for the 1117 birds
recaptured in the United States during the past 50 years. The
data show that large numbers of this supposedly sedentary
species move significant distances. For instance, 193 of 755
(25Æ6%) recaptured birds banded at <1 year of age moved
>15 km from their original point of capture.
600
562
560
Adult
Young
Unknown
500
400
300
200
202
179
133
100
56
4
7
2
2
7 3
0
0–14 km
15–45 km
46–90 km
>90 km
Recapture distance
Fig. 5 Dispersal patterns for the House Sparrow in eastern North
America based on recapture data from the US National Bird Banding
Laboratory
3) Arthropods other than mosquitoes could serve as
vectors – if such arthropods as ticks could serve as vectors,
they could be both infected and transported by migrants,
and serve to introduce the virus to distant sites.
4) Displaced mosquitoes – mosquitoes have been known
to be blown tens of kilometres from sites of origin by strong
prevailing winds or vehicles. Thus mosquitoes themselves
could serve as introductory agents for the virus to new sites
under certain circumstances.
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
56S J . H . R A P P O L E A N D Z . H U B Á L E K
8. THE FUTURE FOR WEST NILE VIRUS IN
THE NEW WORLD
The best support for the ‘Migrant Bird as Introductory
Host’ hypothesis is the fact that the virus has reached the
Caribbean island of Cayman Brac, 560 km south of the
Florida Keys, where a single human case was diagnosed in
August 2001 unaccompanied by mosquito or avian evidence
of infection elsewhere on the island (Cornell University
Center for the Environment 2002b). The victim had not
travelled outside the island during 2001.
If migrants can serve as introductory hosts, transporting
the virus across ocean and desert barriers, then it should not
take long before the mainland Neotropics become enzootic
for the virus. In fact, that event already should have
occurred. At present, however, there is no evidence of this.
Nevertheless, it should be remembered that, with the
possible exception of Argentina (Cornell University Center
for the Environment 2001, and the Yucatan Peninsula in
Mexico (National Institute of Allergy and Infectious Diseases 2000), surveillance programmes are scarce and underfunded in the region.
If House Sparrows or similar ‘sedentary’ species are the
principal agents moving the virus by normal dispersal, then
the virus should continue at its present rate (70 km a
month for 6 or 7 months a year in north temperate regions;
70 km a month for 8–12 months a year in south temperate
and subtropical regions) across the United States and
Canada south perhaps to the latitude of San Antonio and
Corpus Christi in Texas, and the US border in the
southwestern US However, it may take some months or
even years longer for West Nile to invade the mainland
Neotropics because of the existence of significant barriers
to dispersal in the form of the Gulf of Mexico and the
broad belt of arid and semi-arid habitats that separate most
of the New World temperate regions from the tropics
(Dinerstein et al. 1995) (Fig. 6). In fact, 25 avian species,
including Blue Jay, American Crow, Common Grackle
(Quiscalus quiscula) and Fish Crow (Corvus ossifragus), find
the southernmost extreme of their ranges just north of the
latitude of Corpus Christi, Texas (Rappole and Blacklock
1985, 1994). Nevertheless, dispersing House Sparrows or
short-distance migrants (e.g. those cited as ‘subtropical’
migrants in Table 1) eventually will find their way across
these barriers, just as they have in the past. The House
Sparrow, first introduced into North America in New York
City in 1851, is now found almost throughout the
continent from central Canada south to southern Nicaragua
(Lowther and Cink 1995). It spread from its original
release point in New York to south Texas in about
35 years (Barrows 1889).
Whether by natural means, e.g. House Sparrow dispersal,
or artificial means, e.g. importation to zoos or other animal
facilities, West Nile virus eventually will enter the mainland
Neotropics, where it is very likely that it will spread rapidly
throughout the region, given the year-round abundance of
both competent vectors (ornithophilous mosquitoes) and
avian hosts. Human populations will more likely suffer from
this invasion, as will horses, and other domestic and wild
mammals and birds, although the likely results of such an
epidemic are unknown. The prevalence of vast, exposed
garbage disposal sites where high populations of both birds
and mosquitoes occur in proximity to large human populations pose a very serious threat to public health. Nevertheless, damage may be moderated among indigenous
vertebrates, including humans, by the conferral of some
level of immunity to West Nile virus from prior exposure to
related indigenous flaviviruses, e.g. St Louis Encephalitis
virus or Yellow Fever virus, and/or generations of host
experience with these or other heterologous pathogens
(Tesh et al. 2002).
9. ACKNOWLEDGEMENTS
Fig. 6 Habitats of Mexico and Central America (light grey ¼ arid and
semi-arid areas) (Dinerstein et al. 1995)
We thank Stephen C. Guptill, Geography and Spatial Data
Systems, National Mapping Division, US Geological
Survey, for assistance with maps depicting US locations
for birds found seropositive for West Nile virus. Ms
Kathleen Klimkiewicz of the National Bird Banding
Laboratory, Patuxent Environmental Research Center,
US Geological Survey, provided House Sparrow banding-recapture data. Data on avian, human, and mosquito
seropositive records for West Nile virus are derived from
the National Atlas of the United States of America, the
Centers for Disease Control and Prevention, and the US
Geological Survey.
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
MIGRATORY BIRDS AND WEST NILE VIRUS
10 . R E F E R E N C E S
American Museum of Natural History (2002) West Nile fever: a medical
detective story. http://sciencebulletins.amnh.org/biobulletin/biobulletin/
story1380.html.
American Ornithologists’ Union (1998) Check-List of North American
birds. Seventh Edition. American Ornithologists’ Union, Lawrence,
Kansas: Allen Press.
American Ornithologists’ Union (2000) Forty-second supplement to
the American Ornithologists’ Union Check-List of North American
birds. Auk 117, 847–858.
Andreadis, T.G., Anderson, J.F. and Vossbrinck, C.R. (2001) Mosquito surveillance for West Nile virus in Connecticut, 2000: isolation
from Culex pipiens, Cx. restuans, Cx. salinarius, and Culiseta melanura.
Emerging Infectious Diseases 7, 670–674.
Arbonet (2002) Louisiana arbovirus transmission summary – 2001.
http://arbonet.caeph.tulane.edu/2001TransSum.htm.
Bernard, K.A. and Kramer, L.D. (2001) West Nile virus activity in the
United States, 2001. Viral Immunology 14, 319–338.
Bernard, K.A., Maffei, J.G., Jones, S.A., Kauffman, E.B., Ebel, G.D.,
Dupuis II, A.P., Ngo, K.A., Nicholas, D.C., Young, D.M., Shi,
P.-Y., Kulasekera, V.L., Eidson, M.E., White, D.J., Stone, W.B.,
NY State Surveillance Team and Kramer, L. (2001) West Nile virus
infection in birds and mosquitoes, New York State, 2000. Emerging
Infectious Diseases 7, 679–685.
Brewer, S. and Hopper, L. (2002) West Nile virus confirmed in
western Harris County. http://www.chron.com/cs/CDA/story.hts/
topstory2/1460633.
Canadian Cooperative Wildlife Health Centre (2002) Canada and West
Nile virus: surveillance and preparedness. http://wildlife.usask.ca/
english/frameWestNile.htm.
Centers for Disease Control and Prevention (1999) West Nile-like viral
encephalitis – New York, 1999. Morbidity and Mortality Weekly
Report 48, 890–892.
Centers for Disease Control and Prevention (2000a) Guidelines for
surveillance, prevention, and control of West Nile virus infection –
United States. Morbidity and Mortality Weekly Report 49, 25–28.
Centers for Disease Control and Prevention (2000b) West Nile virus
activity – New York and New Jersey, 2000. Morbidity and Mortality
Weekly Report 49, 640–642.
Chunikhin, S.P. (1973) Introduction to arbovirus ecology (in Russian).
Med Virusosl (Moskva) 21, 87–88.
Cochran, W.W. (1987) Orientation and other migratory behaviours
of a Swainson’s Thrush followed for 1500 km. Animal Behaviour 35,
927–929.
Cochran, W.W., Montgomery, G.G. and Graber, R.R. (1967) Migratory flights of Hylocichla thrushes in spring: a radiotelemetry study.
Living Bird 6, 213–225.
Cornell University Center for the Environment (2001) Workshop: WNV in South America. http://www.cfe.cornell.edu/erap/
WNV-LArchive/10-09-01.html.
Cornell University Center for the Environment (2002a) West Nile
virus update: by country and state by state. wysiwyg://23/http://
www.cfe.cornell.edu/erap/WNV/WNVUpdate.cfm
Cornell University Center for the Environment (2002b) West Nile
virus update: by country and state by state. http://www.cfe.
cornell.edu/erap/WNV/WNVUpdate.cfm.
57S
Dinerstein, E., Olson, D.M., Graham, D.J., Webster, A.L., Primm,
S.A., Bookbinder, M.P. and Leded, G. (1995) A conservation
assessment of the terrestrial ecoregions of Latin America and the
Caribbean. Washington, DC: The World Bank.
Eidson, M., Komar, N., Sorhage, F., Nelson, R., Talbot, T.,
Mostashari, F. and McLean, R. (2001) Crow deaths as a sentinel
surveillance system for West Nile virus in the northeastern United
States, 1999. Emerging Infectious Diseases 7, 615–620.
Ernek, E., Kozuch, O, Nosek, J, Teplan, J. and Folk, C. (1977)
Arboviruses in birds captured in Slovakia. Journal of Hygiene,
Epidemiology and Immunology (Prague) 21, 353–359.
Fedorova, T.N. and Stavskiy, A.V. (1972) Latent infection of wild ducks
with Omsk hemorrhagic fever and West Nile viruses (in Russian).
In Aktualnye prolemy virusologii i profilaktiki virusnykh zabolevaniy.
ed. Chumakov, M.P. pp. 226–233. Moskva: Inst. Poliomiel Virus Enc.
Florida Department of Health (2002) First animal cases of West Nile
virus for 2002 have been reported. http://www9.myflorida. com/
disease_ctrl/epi/htopics/arbo/alerts/2002/021802.pdf.
Gariépy, C., Lambert, L., Macrisopoulos, P., Massicotte, J. and
Picard, J. (2001) Infections causées par le virus du Nil occidental
profil épidémiologique. Bulletin D’Information en Sante´ Environnementale 12, 1–5.
Garmendia, A.E., VanKruiningen, H.J. and French, R.A. (2001) The
West Nile virus: its recent emergence in North America. Microbes
and Infection 3, 223–229.
Groves, R. (2001) West Nile shows up in Bergen, Middlesex. http://
199.173.2.7/healthw/nilebg200105082.htm.
Hannoun, C., Corniou, B. and Mouchet, J. (1972) Role of migrating
birds in arbovirus transfer between Africa and Europe. In Transcontinental Connections of Migratory Birds and their Role in the
Distribution of Arboviruses ed. Cherepanov, A.I. pp. 167–172.
Novosibirsk: Nauka.
Hubálek, Z. (2000) European experience with the West Nile virus
ecology and epidemiology: could it be relevant for the New World?
Viral Immunology 13, 415–426.
Hubálek, Z. and Halouzka, J. (1999) West Nile fever – a reemerging
mosquito-borne viral disease in Europe. Emerging Infectious Diseases
5, 643–650.
Karabatsos, N. ed. (1985) International Catalogue of Arboviruses,
Including Certain Other Viruses of Vertebrates. 3rd ed., and Supplements 1986–98. San Antonio, Texas, USA: American Society of
Tropical Medicine and Hygiene.
Kerlinger, P. (1995) How birds migrate. Stackpole Books, Mechanicsburg, Pennsylvania.
Komar, N. (2000) West Nile viral encephalitis. Revue du Science et
Technologie Office International des Epizooties 19, 166–176.
Komar, N., Davis, B., Bunning, M. and Hettler, D. (2000) Experimental infection of wild birds with West Nile virus (New York 1999
strain). American Journal of Tropical Medicine and Hygiene 62(suppl.
3), 229–230.
Lanciotti, R.S., Roehrig, J.T., Deubel, V., Smith, J., Parker, M., Steele,
K., Crise, B., Volpe, K.E., Crabtree, M.B., Scherret, J.H., Hall, R.A.,
Mackenzie, J.S., Cropp, C.B., Panigraphy, B., Ostlund, E., Schmitt,
B., Malkinson, M., Banet, C., Weissman, J., Kome-Savage, H.M.,
Stone, W., McNamara, T. and Gubler, D.J. (1999) Origin of the West
Nile virus responsible for an outbreak of encephalitis in the
northeastern United States. Science 286, 2333–2337.
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S
58S J . H . R A P P O L E A N D Z . H U B Á L E K
Lowther, P.E. and Cink, C.L. (1995) House Sparrow. In The birds of
North America, No. 12 ed. Poole, A., Stettenheim, P. and Gill, F.
Philadelphia, Pennsylvania: Academy of Natural Sciences.
Malkinson, M., Banet, C., Weisman, Y., Pokamunski, S., King, R.,
Drouet, M.-T. and Deubel, V. (2002) Introduction of West Nile
virus in the Middle East by migrating White Storks. Emerging
Infectious Diseases 8, 392–397.
Marfin, A.A., Peterson, L.R., Eidson, M., Miller, J., Hadler, J.,
Farello, C., Werner, B.B., Campbell, G.L., Layton, M., Smith, P.,
Bresnitz, E., Cartter, M., Scaletta, J., Obiri, G., Bunning, M.,
Craven, R.C., Roehrig, J.T., Julian, K.G., Hinten, S.R., Gubler,
D.J. and the ArboNet Cooperative Surveillance Group (2001)
Widespread West Nile virus activity, eastern United States, 2000.
Emerging Infectious Diseases 7, 730–735.
McLean, R.G., Ubico, S.R., Docherty, D.E., Hansen, W.R., Sileo, L.
and McNamara, T.S. (2001) West Nile virus transmission and
ecology in birds. Annals of the New York Academy of Sciences 951,
54–57.
Miller, B.R., Nasci, R.S., Godsey, M.S., Savage, H.M., Lutwama, J.J.,
Lanciotti, R.S., et al. (2000) First field evidence for natural vertical
transmission of West Nile virus in Culex univittatus complex
mosquitoes from Rift Valley, Kenya. American Journal of Tropical
Medicine and Hygiene 62, 240–246.
Nasci, R.S., Savage, H.M., White, D.J., Miller, J.R., Cropp, B.C.,
Godsey, M.S., Kerst, A.J., Bennett, P., Gottfried, K. and Lanciotti,
R.S. (2001) West Nile virus in overwintering Culex mosquitoes, New
York City, 2000. Emerging Infectious Diseases 7, 742–747.
National Institute of Allergy and Infectious Diseases (2000) NIAID
research on West Nile and related viruses. http://www.niaid.nih.gov/
factsheets/westnile.htm.
Nir, Y., Goldwasser, R., Lasowski, Y. and Avivi, A. (1967) Isolation of
arboviruses from wild birds in Israel. American Journal of Epidemiology 86, 372–378.
Odelola, H.A. and Oduye, O.O. (1977) West Nile virus infection of
adult mice by oral route. Arch. Virol. 54, 251–253.
Peiris, J.S.M. and Amerasinghe, F.P. (1994) West Nile fever. In
Handbook of zoonoses, Section B: Viral, 2nd edn ed. Beran, G.W. and
Steele, J.H. pp. 139–148. Boca Raton, Florida, USA: CRC Press.
Preston, R. (1999) West Nile mystery. New Yorker October 18 & 25,
1999, 90–108.
Rappole, J.H. (1994) The Ecology of Migrant Birds. Washington DC:
Smithsonian Institution Press.
Rappole, J.H. and Blacklock, G.W. (1985) Birds of the Texas Coastal
Bend. Texas: Texas A & M University Press, College Station.
Rappole, J.H. and Blacklock, G.W. (1995) A Field Guide to the Birds of
Texas. College Station, Texas: Texas A & M University Press.
Rappole, J.H., Derrickson, S.D. and Hubálek, Z. (2000) Migratory
birds and spread of West Nile virus in the Western Hemisphere.
Emerging Infectious Diseases 6, 319–328.
Reeves, W.C. (1974) Overwintering of arboviruses. Progress in Medical
Virology 17, 193–220.
Semenov, B.F., Chunikhin, S.P., Karmysheva, V.Y. and Yakovleva,
N.I. (1973) Studies of chronic arbovirus infections in birds. 1.
Experiments with West Nile, Sindbis, Bhanja and SFS viruses (in
Russian). Vestnik Akademii Medicinskikh NaukSSSR (Moskva) 2,
79–83.
Smithburn, K.C., Hughes, T.P., Burke, A.W. and Paul, J.H. (1940) A
neurotopic virus isolated from the blood of a native of Uganda.
American Journal of Tropical Medicine and Hygiene 20, 471–492.
Smithburn, K.C., Taylor, R.M., Rizk, F. and Kader, A. (1954)
Immunity to certain arthropod-borne viruses among indigenous
residents of Egypt. American Journal of Tropical Medicine and
Hygiene 33, 9–18.
Steele, K.E., Linn, M.J., Schoepp, R.J., Komar, N., Geisbert, T.W.,
Manduca, R.M., Calle, P.P., Raphael, B.L., Clippinger, T.L.,
Larsen, T., Smith, J., Lanciotti, R.S., Panella, N.A. and McNamara,
T.S. (2000) Pathology of fatal West Nile virus infections in native
and exotic birds during the 1999 outbreak in New York City.
Veterinary Pathology 30, 208–224.
Taylor, R.M., Work, T.H., Hurlbut, H.S. and Rizk, F. (1956) A study
of the ecology of West Nile virus in Egypt. American Journal of
Tropical Medicine and Hygiene 5, 579–620.
Tesh, R.B., Amelia, P.A., da Rosa, T., Guzman, H., Araujo, T.P. and
Xiao, S.-Y. (2002) Immunization with heterologous flaviviruses
protective against fatal West Nile encephalitis. Emerging Infectious
Diseases 8, 245–251.
University of Georgia (2001) West Nile virus. http://www.ect.uga.edu/
westnilevirus/.
US Department of Agriculture (2002) Summary of West Nile virus in
the United States, 1999. http://www.usgs.gov/public/press/
public_affairs/press_releases/pr1128.html
US Geological Survey (1999) West Nile virus may be new deadly
strain, USGS tells Congress. http://www.usgs.gov/public/press/
public_affairs/press_releases/pr1128m.html.
Watson, G.E., Shope, R.E. and Kaiser, M.N. (1972) An ectoparasite
and virus survey of migratory birds in the eastern Mediterranean. In
Transcontinental connections of migratory birds and their role in the
distribution of arboviruses ed. Cherepanov, I.A. pp. 176–180. Novosibirsk: Nauka.
Work, T.H., Hurlbut, H.S. and Taylor, R.M. (1955) Indigenous wild
birds of the Nile Delta as potential West Nile virus circulating
reservoirs. American Journal of Tropical Medicine and Hygiene 4,
872–878.
Zwerdling, D. (2001) The West Nile virus’ journey to America. http://
www.npr.org/programs/atc/features/2001/mar/010309.disease.html.
ª 2003 The Society for Applied Microbiology, Journal of Applied Microbiology Symposium Supplement, 94, 47S–58S