West Nile Virus has long been present in Africa, the Middle East, southern Europe, India and southern Asia. It was introduced to the western hemisphere in 1999, in New York, USA, and quickly spread through much of North America, attaining the Pacific coast in 2002. The source of the virus in North America is unknown, but several species of Eurasian birds occasionally migrate to North America, especially to the eastern seaboard of North America, and some may have been infected while in Europe. Among the occasional visitors to North America are Eurasian wigeon, Anas penelope ; green - winged teal, Anas crecca ; ruff, Philomachus pugnax ; little gull, Larus minutus ; and black - headed gull, Larus ridibundus . Alternatively, some seabirds are carried by tropical storms annually from Africa to North America, so this is a possible route of introduction. Birds such as cattle egret, Bubulcus ibis ; black - headed gull, Larus ridibundus ; yellow - legged gull, Larus cachinnans ; little egret, Egretta garzetta ; and gray heron, Ardea cinerea are possibilities. Finally, pet, zoo and domestic birds routinely pass through commercial airports such as J.F. Kennedy International Airport in New York, and could be a source of the virus. Epidemics in humans have occurred in Africa and Europe in addition to North America, and infection can cause fever, myalgia, rash, and encephalitis in some victims. In recent years it has become the most important arbovirus in North America. Its importance is likely to diminish as wildlife and humans develop resistance following exposure to the virus.
West Nile Virus is transmitted to birds principally by ornithophilic (bird - feeding) mosquitoes, although it has been found in other animals, and of course in humans. It also can be transmitted by organ transplant, blood transfusion, transplacental, and transmammary. Wild birds are the primary hosts, with ulex spp. being especially important vectors, and with humans and horses being accidental or ‘ dead - end ’ hosts that do not contribute to continued transmission. Domestic birds, except for geese, generally do not develop sufficient viremia to allow transmission. Domestic geese develop the necessary viremia to amplify transmission, and also suffer mortality. Migratory birds are important in the dissemination of West Nile Virus in temperate regions, and in North America the virus can persist during the winter in southern climates from Florida to California and then be reintroduced to northern areas annually.
In North America, West Nile Virus has become a major mortality factor of corvid birds. American crows, Corvus brachyrhynchos , are especially likely to perish when infected. Crows, blue jays, Cyanocitta cristata , black - billed magpie, Pica pica , and other species of Corvidae account for about 80% – 90% of the infected birds in most dead - bird surveys. Other birds that seem to have suffered signifi cant declines in abundance include American robin, Turdus migratorius ; tufted titmouse, Baeolophus bicolor ; house wren, Troglodytes aedon; chickadee, Poecile spp.; and eastern bluebird, Sialia sialis . However, some other birds seem to be quite susceptible, including various owls and raptors. The mortality of birds in the western hemisphere is much higher than typically occurs in the eastern hemisphere, likely due to the increased virulence in the native birds of the western hemisphere. It appears that most of the birds adversely affected are peridomestic (species that are often found in association with human habitations).
Although corvid birds regularly test positive for West Nile Virus, and often die, this does not mean that they are the most important species in maintaining the virus or enhancing dispersal in North America. American crows, for example, though very susceptible, typically comprise a small proportion of the bird community in an area, rarely exceeding 10%. Also, birds differ greatly in their attractiveness to mosquitoes, which affects the transmission potential. Field studies in the eastern USA have shown that American robin, Turdus migratorius , is highly preferred, being fed upon 16 times more often than if mosquitoes displayed no preference, and that virus antibodies are present in over 40% of the robins, documenting exposure to the virus. Importantly, it was estimated that nearly 60% of the infected mosquitoes became infected by feeding on virus - infected robins. In contrast, house sparrow, Passer domesticus , is avoided by mosquitoes and less than 20% showed evidence of exposure. Fish crow, Corvus ossifragus , also is highly selected by mosquitoes and fed upon more often than would be expected. However, their low level of abundance, like that of American crow, precludes them from being important in the epidemiology of the virus. Thus, variability in host - feeding behavior by mosquitoes, host suitability for virus amplification, and bird host abundance all interact to affect disease potential.
An assessment of birds important in introducing West Nile Virus to Europe from Africa, amplifying the virus, and enhancing its spread, suggested that perhaps 100 of the 300 bird species considered may play a role in West Nile epidemiology. Among the potentially important species are cattle egret, Bubulcus ibis ; gulls, Larus spp.; barn swallow, Hirundo rustica ; house martin, Delichon urbica ; common swift, Apus apus; common nightingale, Luscinia megarhynchos ; crows, Corvus spp.; blackcap, Sylvia atricapilla ; and European starling, Sturnus vulgaris . Further research will certainly show that not all are equally important. As in North America, Culex spp. mosquitoes seem most important in disease transmission.
West Nile Virus is not limited to birds, though it is best known as a pathogen of birds. Among other animals infected are eastern fox squirrel, Sciurus niger ; black bear, Ursus americanus ; white - tailed deer, Odocoileus virginianus ; and American alligator, Alligator mississippiensis . It is likely that the list of non - bird hosts will grow as the disease becomes further established in North America.
