Biology Reference
In-Depth Information
5.3. West Nile Virus Zoological Surveillance Project
5.3.1 infectious agent
West Nile virus (WNV) is a member of the Flaviviridae family of viruses. Like
most members of this family, WNV relies on an arthropod vector, namely,
the mosquito, to transmit infection from host to host. Over 60 separate spe-
cies of mosquitoes have been found to be positive for WNV in the United
States. However, not all are competent vectors. Members of both the Culex
and Aedes genera of mosquitoes have been found to be proficient at transmit-
ting WNV (Kilpatrick et al. 2005; Molai and Andreadis 2006). Birds are the
most common host of WNV, though morbidity and mortality varies greatly
across avian species (O'Donnell and Travis 2007). Mammals infected with
WNV are considered “dead-end hosts” since viremia does not reach high
enough levels to pass on the infection via mosquito bite.
The majority of humans infected with WNV show no observable clinical
symptoms. Approximately 20% develop a mild, self-limiting illness known
as “West Nile fever,” whose symptoms include fever, headache, fatigue, mus-
cle ache, and lymphadenopathy. In less than 1% of human infections, a more
serious neuroinvasive condition develops that is characterized by meningo-
encephalitis with a mortality rate of almost 10%. This severe form of the dis-
ease is most commonly seen in persons over 50 years of age.
WNV was originally isolated from a febrile woman in Uganda in 1937. Over
time, it was found to be a widely distributed across Africa, Asia, Europe, and
the Middle East. Prior to 1999, the virus had never been seen in the Western
Hemisphere. In August of that year, New York City began to observe unusual
cases of encephalitis in humans (Briese et al. 1999). The initial diagnosis was
St. Louis Encephalitis virus (SLE), a member of the Flaviviridae virus fam-
ily commonly found in North America that occasionally causes illness in
humans. However, at the same time, an abnormally high rate of bird mortali-
ties was detected in the same area (Ludwig et al. 2002). While SLE does not
generally cause disease in birds, necropsies performed on deceased animals
showed pathology similar to that produced by SLE (Steele et al. 2000; Hansen
et al. 2001). Dr. Tracey McNamara, the pathologist at the Bronx Zoo, per-
formed these necropsies and sent tissue samples to the National Veterinary
Diagnostic Laboratory (NVSL) in Ames, Iowa, for further testing. The virus
was isolated and forwarded to both the Centers for Disease Control and
Prevention (CDC) and the Armed Forces Institute of Pathology (AFIP). After
sequencing and PCR analyses, the isolate was determined to be West Nile
virus (McNamara 2007). Subsequent testing on isolates from human enceph-
alitis patients demonstrated the presence of the same isolate (Lanciotti et al.
1999; Nash et al. 2001).
As WNV spread across the country, surveillance for the virus in zoologi-
cal institutions became tremendously important. With some zoos having
 
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