West Nile and other encephalitides are most often characterized by mild fever, head and body aches and, in some cases, a rash and swollen lymph nodes. In rare cases the disease may progressively worsen and result in death. There are no vaccines available at present to immunize people and horses against this disease. Mosquitoes are the only proven vectors of the West Nile virus. The cycle of transmission for West Nile virus is similar to that of St. Louis and Eastern Equine Encephalitis viruses.
Wild birds serve as the reservoir for the virus.
Mosquitoes feed on infected birds and transmit the virus to other birds after a short incubation period. Infected birds may become ill and recover or may exhibit no noticeable symptoms. Wild birds may also die of the infection, however, sometimes in relatively large numbers. In the case of West Nile Encephalitis, crow mortality has been high. The virus becomes widespread in the wild bird population by midsummer when mosquitoes are abundant.