West Nile encephalitis virus caused an outbreak and deaths in humans and horses in the northeastern United States in the late summer and early fall of 1999. Recent isolations of the virus from adult mosquitoes and a dead bird in New York state confirm that the West Nile virus successfully overwintered and has become established in the United States and perhaps elsewhere in the Americas, with the potential for future outbreaks of this disease in humans and horses.

West Nile encephalitis is a mosquito-borne viral disease whose global distribution previously was restricted to Africa, the Middle East, parts of Asia, and on occasion, southern Europe, according to Dr. Peter Timoney, director of the Gluck Equine Research Center in Kentucky. Although many cases of infection are subclinical, this virus can cause illness and death in humans and equids, as was attested to in the disease outbreak that occurred in New York state last year. That outbreak was the first-ever detection of West Nile virus in the New World.

While it has not been possible to confirm the source of West Nile virus for that outbreak–or how this pathogen was spread to the United States in the first place–the prevailing opinion is that the virus was introduced through the illegal importation of an infected bird or birds. The genetic sequence of the strain of West Nile virus recovered during the outbreak is virtually identical with that of a virus isolate made from a goose in Israel in 1998.

Because the virus was able to live through the winter in the United States, there is the potential for another outbreak this spring when climatic conditions are right for mosquitoes to hatch and feed, and thus spread the virus. There also is the potential that this deadly virus could spread to other parts of North and South America. The federal government is working on ways to combat this disease in horses and humans. Required research will be undertaken in order to develop prevention protocols and possible vaccinations