West Nile encephalitis

West Nile viral encephalitis in horses, previously reported in Africa, Asia and Europe, occurred for the first time in the Western Hemisphere in 1999. West Nile virus, a flavivirus maintained by a bird-mosquito cycle, was associated with encephalitic illness and deaths in humans, horses and birds in the US outbreak. The disease in horses is manifested primarily by ataxia of variable severity. Although a significant number of infections are inapparent, equine case fatality rates may exceed 40%. The virus has overwintered in the United States, and poses a threat of future disease occurrences.