West Nile virus (WNV) was first isolated from an infected woman in 1937 in the West Nile district of Uganda, and has since been found to be a fairly common human pathogen in Africa, the Middle East, Asia, Australia, and temperate areas of Europe (893,894). The first documented cases of WNV encephalitis were in New York City in 1952 when patients with advanced malignancies were experimentally inoculated with an Egyptian strain of this virus for its possible oncolytic effect (895). Naturally acquired WNV was initially recognized in North America in August 1999 when an unusual cluster of cases of meningoencephalitis and muscle weakness in New York City was carefully investigated (896). The virus has since spread throughout much of the United States, and cases in Canada and the Caribbean region have been diagnosed (895,897,898). The number of human WNV infections reported to the CDC peaked at 9,122 (with 223 deaths) in 2003; 2,313 cases (with 79 deaths) were reported for the following year through November 16, 2004 (895,898). Outbreaks of WNV have been described for many countries, including Romania, Russia, Israel, South Africa, and the United States (898).

WNV is a single-stranded RNA virus of the family Flaviviridae, genus Flavirus. It is classified within the Japanese encephalitis virus antigenic complex, which also includes other human pathogens such Japanese encephalitis, Murray Valley encephalitis, Kunjin, and St. Louis encephalitis viruses (899,900). Mosquitoes (primarily Culex species) serve as the vector for WNV although birds are the most common amplifying hosts (893,899). Infected birds usually survive WNV infection and become immune, but the infections can be virulent in crows and jays. The virus can be recovered from the oral secretions and feces of birds, and bird-to-bird transmission can occur in the laboratory in the absence of mosquitoes (900). Most vertebrates are susceptible to WNV, and natural infection has been demonstrated in humans, horses, skunks, cats, rabbits, squirrels, chipmunks, and a few bat species (899). The WNV strains isolated in New York in 1999 are most closely related to an isolate obtained from the brain of a dead Chilean flamingo in Israel in 1998, but how this virus was imported into the United States in not known (901).