Materials and Methods

A DNA vaccine for West Nile virus (WNV) was evaluat-ed to determine whether its use could protect fish crows (Corvus ossifragus) from fatal WNV infection. Captured adult crows were given 0.5 mg of the DNA vaccine either orally or by intramuscular (IM) inoculation; control crows were inoculated or orally exposed to a placebo. After 6 weeks, crows were challenged subcutaneously with 105 plaque-forming units of WNV (New York 1999 strain). None of the placebo inoculated–placebo challenged birds died. While none of the 9 IM vaccine–inoculated birds died, 5 of 10 placebo-inoculated and 4 of 8 orally vaccinated birds died within 15 days after challenge. Peak viremia titers in birds with fatal WNV infection were substantially higher than those in birds that survived infection. Although oral administration of a single DNA vaccine dose failed to elicit an immune response or protect crows from WNV infection, IM administration of a single dose prevented death and was associated with reduced viremia. West Nile virus (WNV), a mosquito-borne flavivirus,was recognized for the first time in the Western Hemisphere during summer 1999 in New York City and was associated with human, equine, and avian deaths (1–4). This virus is transmitted by a variety of mosquito species, mostly in the genus Culex (5–7). The New York 1999 strain of WNV differed genetically from other known strains of WNV except for an Israeli strain isolated from a dead goose in Israel in 1998 (1). With the exception of a laboratory study in Egypt involving hooded crows (Corvus corone) and house sparrows (Passer domesticus) (8), only these two nearly identical strains are known to kill birds (9,10). In 2000, WNV was detected in>4,000 bird carcass-es in the United States (11), and the overall mortality rate was considered much greater. Several deaths attributed to WNV in the United States have occurred in valuable cap-tive birds in zoologic collections (12). Currently, no treat-ment or vaccine is available for susceptible birds. Vaccination may protect birds from lethal WNV infec-tions. Accordingly, we examined a DNA vaccine devel-oped for use in horses (13) for its ability to protect crows, a species known to be highly susceptible to lethal infection with this virus (8,10)