West Nile encephalomyelitis

West Nile encephalomyelitis

a disease caused by West Nile virus, a mosquito borne flavivirus which infects birds, humans and other animals, particularly horses. Originally isolated in Uganda in 1937, it is endemic in Africa, the Middle East and West Asia. In 1999 it was introduced into New York City and has spread to all states and to Canada and Central America. A large number of bird species are the natural host for the virus, in the United States the American crow Corvus brachyrhynchos, the fish crow, Corvus ossifragus, and the blue jay, Cyanocitta cristata. Culex pipiens is prominent as a vector, but other Culex, Aedes and Anopheles spp mosquitoes are also recognized as vectors depending on the locale. Clinical signs in humans vary from inapparent or a mild febrile illness to acute, fatal myeloencephalitis and hepatitis. In horses, encephalomyelitis is the major sign with hind leg weakness, flaccid paralysis of the lower lip, impaired vision, ataxia, head pressing, aimless wandering, recumbency and death.