Abstract
During the summer and fall of 1999 an outbreak of human viral encephalitis occurred in New York
City, with the earliest case onset 5 August 1999.2 This was later determined to result from infection with a
strain of West Nile virus (WNV), a mosquito transmitted flavivirus, that was identical to a strain that circulated in
Israel in 1998 and was the first documented occurrence of WNV in the Western Hemisphere.4 Human cases were
preceded by a WNV epornitic in free ranging birds [especially common crows (Corvus brachyrhynchos) and fish crows
(Corvus ossifragus)] in New York, with later involvement of birds in Connecticut and New Jersey.1,4,5
Later, nondomestic captive birds in the collections of the Wildlife Conservation Society/Bronx Zoo (WCS) were infected
with WNV with resulting morbidity and mortality.4,5 Illnesses and deaths also occurred in horses (Equus
caballus) in New York.3,4
A WNV serologic survey of the WCS collection was performed to confirm infection of clinical cases,
assess the extent of WNV exposure, and to investigate when the virus was introduced to the collection. Heparinized plasma
or serum samples were tested by a plaque reduction neutralization assay. There were no differences in test results
between serum or plasma samples.
Avian serologic testing was conducted on 315 samples obtained between 6 June 1999 and 8 February 2000
from 277 individual birds (74 species in 16 orders). WNV specific antibodies were not detected in any of the 36 birds
housed indoors, while 39% (94/241) of birds (34 species in 13 orders) housed in enclosures with a potential for mosquito
exposure had WNV specific antibodies (Table 1). Based on these serologic results, avian WNV infections were most often
asymptomatic (78.7%; 74/94). Clinical illness was recognized in 21.3% (20/94) of infected birds, with a high case
fatality rate (nine of these 20 birds recovered and 11 died). Most of the birds that recovered only had mild clinical
signs. This serologic survey underestimates the actual avian collection mortality, since some birds died peracutely
without serologic testing. None of the archived samples from 39 individual seropositive birds (obtained between 17
February 1995 and 27 May 1999) were confirmed to have WNV specific antibodies. The earliest serologic confirmation of WNV
infection at WCS was in a bird on 9 August 1999, after the outbreak had already been recognized in free ranging crows and
in humans. All six of the birds imported by the WCS in 1999 had completed the federally mandated 30-day quarantine at a
USDA quarantine facility remote from the outbreak origin. When sampled in October 1999 (3-4 mo after arrival at WCS),
none had WNV specific antibodies.
The clinical signs of WNV infection in WCS collection birds were usually nonspecific. Some were found
dead with no premonitory signs. Many exhibited depression, anorexia, weakness, weight loss, and recumbency. However,
several birds had neurologic abnormalities including abnormal head or neck posture, ataxia, tremors, circling,
disorientation, unilateral or bilateral posterior paresis, and impaired vision. The course of clinical illness was
usually less than 1 wk, but ranged from 1-24 days before recovery or death. Hematologic and biochemical changes were
variable and nonspecific.
Mammalian serologic testing was conducted on 116 samples obtained between 5 August 1999 and 31
January 2000 from 104 individual mammals (33 species in seven orders). None of the three mammals housed indoors, but 5.9
% (6/101) of mammals housed outdoors, had WNV specific antibody. These included five Asian species in four Orders [two
Indian elephants (Elephas maximus indicus), one greater Indian rhinoceros (Rhinoceros unicornis), one
babirusa (Babyrousa babyrussa), one lesser panda (Ailurus fulgens fulgens), and one snow leopard
(Panthera uncia)]. Retrospective analysis of banked samples demonstrated seroconversion of one elephant between 12
August 1999 and 24 September 1999. An archived sample from the rhinoceros obtained in September 1994 was negative. None
of the eight equids of three species [donkey (Equus asinus asinus), domestic miniature horse, or Przewlaski's
horse (Equus przewalskii)] tested had WNV specific antibody.
Most mammal WNV infections were asymptomatic. A greater Indian rhinoceros developed depression,
anorexia, and a lip droop and spontaneously recovered. A second greater Indian rhinoceros developed similar clinical
signs and also recovered, but a blood sample was not obtained from that animal.
WCS halted shipments of birds to other institutions at the onset of the collection bird epornitic,
before the cause of the outbreak had been identified. When shipments were resumed, only indoor birds without mosquito
exposure or outdoor birds seronegative after the mosquito exposure season ended were approved for shipment. The source of
WNV responsible for the New York outbreak is unknown but it is speculated that it may have entered the United States by
way of an infected person, an illegally imported bird or domestic pet, or an unintentionally introduced virus-infected
tick or mosquito.4 WNV is documented to have persisted throughout the winter in both a bird and mosquitoes. It
is therefore widely feared that there might be future, recurrent outbreaks of WNV infection in people, horses, and both
captive and free-ranging birds in the Northeastern United States. In addition, there is a potential for dissemination of
WNV to other parts of the country through the movements of infected people, captive or domestic mammals or birds,
free-ranging migratory birds, or virus infected mosquitoes.
Table 1. Results of West Nile virus infection in seropositive avian species at the Wildlife
Conservation Society.
Order / Common name |
Latin name |
Result of avian West Nile virus infection |
Asymptomatic |
Morbidity |
Seroconversion
and recovery |
Mortality |
Anseriformes |
Abbysinian blue-winged goose |
Cyanochen cyanopterus |
|
X |
|
|
Roseybill duck |
Netta peposcaca |
X |
|
|
|
Domestic goose |
Anser anser |
|
X |
X |
|
Trumpeter swan |
Cygnus cygnus buccinator |
|
X |
|
|
Charadriiformes |
Grey gull |
Larus modesta |
|
X |
|
|
Laughing gull |
Larus atricilla |
|
X |
X |
X |
Inca tern |
Larosterna inca |
|
X |
|
|
Ciconiiformes |
Waldrapp ibis |
Geronticus eremita |
|
X |
|
|
Black crowned night heron |
Nycticorax nycticorax |
|
X |
|
|
Lesser adjutant stork |
Leptoptilos javanicus |
|
X |
|
|
Columbiformes |
Bleeding heart pigeon |
Gallicolumba luzonica |
|
X |
|
|
Mauritius pink pigeon |
Columba mayeri |
|
|
X |
|
Cuculiformes |
Lady Ross' plantain eater |
Musophaga rossae |
|
X |
|
|
Falconiformes |
King vulture |
Sarcorhamphus papa |
|
X |
|
|
Galliformes |
Blythes Tragopan |
Tragopan blythi |
X |
|
|
|
Domestic chicken |
Gallus gallus |
|
X |
|
|
Green junglefowl |
Gallus varius |
|
X |
|
|
Kenya crested guineafowl |
Guttera pucherani |
|
X |
X |
|
Bulwar's wattled pheasant |
Lophura bulweri |
|
X |
|
|
Himalayan impeyan pheasant |
Lophophorus impeyanus |
|
X |
|
|
Mountain peacock pheasant |
Polyplectron inopinatum |
|
X |
|
|
Green crested wood partridge |
Rollulus roulroul |
X |
|
|
|
Gruiformes |
Manchurian crane |
Grus japonensis |
|
X |
|
|
Hooded crane |
Grus monacha |
|
X |
|
|
White naped crane |
Grus vipio |
|
X |
|
|
Passeriformes |
Brown sicklebill bird of paradise |
Epimachus meyeri |
|
X |
|
|
Pelecaniformes |
Guanay cormorant |
Phalacrocorax bougainvillei |
|
X |
X |
|
American white pelican |
Pelecanus erythrorhynchos |
|
X |
|
|
Brown pelican |
Pelecanus occidentalis |
|
X |
|
|
Phoenicopteriformes |
Chilean flamingo Phoenicopterus chilensis |
|
X |
X |
X |
|
Sphenisciformes |
Megellanic penguin |
Spheniscus magellanicus |
|
X |
|
|
Strigiformes |
Barred Owl |
Strix varia |
|
X |
|
|
Milky eagle owl |
Bubo lacteus |
|
|
X |
|
Snowy owl |
Nyctea scandiaca |
|
|
X |
|
Editor's Note: From the original pdf it is impossible to tell where the last 2 x's belong.
Does Morbidity belong with seroconversion, i.e., only three x columns.
Acknowledgments
The authors acknowledge and thank the WCS veterinary technicians; the WCS Departments of Education,
Mammalogy, and Ornithology; and the technical staff of the Research Serology Branch and Pathology Division, USAMRIID,
whose assistance made this project possible.
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