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. Circulation of West Nile fever in Europe is similar to that of St. Louis encephalitis in North America, where the rural cycle of wild birds alternates with the urban cycle of domestic birds. I first received calls about oddly sick crows in the New York City area the last week of August 1999. Several people called me about finding sick and unusually approachable crows, as well as an unusual number of dead crows. Finding one dead crow doesn't mean much, but finding several crows within a short period of time is highly unusual and indicates that something odd is going on.