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 ass ociated with encephalitic illness and deaths in humans, horses and birds in the US outbreak. The disease in horses is manifested primarily by ataxia o f variable severity. Although a significant number of infections are inappa rent, equine case fatality rates may exceed 40%. The virus has overwintered in the United States, and poses a threat of future disease occurrences.