West Nile virus was first isolated in the West Nile district of Uganda in 1937 and was subsequently isolated from patients, birds, and mosquitoes in Egypt in the early 1950s. The virus was soon recognized as the most widespread of the flaviviruses, with geographic distribution including Africa and Eurasia. Until now, it has not been recorded in North America. Mosquitoes, largely bird-feeding species, are the principal vectors of West Nile virus. The virus has been isolated from 43 mosquito species, predominantly of the genus Culex .
Wild birds are the principal hosts of West Nile virus.
The virus has been isolated from a number of wetland and terrestrial avian species in diverse areas. High, long-term viremia, sufficient to infect vector mosquitoes, has been observed in infected birds, and the virus persists in the organs of inoculated ducks and pigeons for 20 to 100 days. Migratory birds are therefore hypothesized to be instrumental in the introduction of the virus to temperate areas of Eurasia during spring migrations. Mammals are less important than birds in maintaining transmission cycles of the virus in ecosystems. Only horses and lemurs seem to support West Nile virus circulation locally.