Infection with this virus does not always result in clinical disease. Studies have shown that normally only a small percentage of humans infected with the virus will show symptoms of disease. The general symptoms of West Nile fever resulting from infection with WNV range from fever, rash and headache to meningitis, encephalitis, coma and death. It has been isolated from at least 43 species of mosquitoes throughout the world. It has been isolated from a variety of avian species, including crows, ducks and pigeons.
Birds are able to maintain a long-term infection.
Consequently, migratory birds are considered to be instrumental in transporting the virus to new areas. Mammals are less important in maintaining WNV infections. However, WNV has been isolated from several mammalian species. These species include cattle, horses, dogs, and lemurs. Of these animals only horses and lemurs are thought to be capable of passing WNV to uninfected mosquitoes during the blood meal.