Migratory birds and spread of West Nile virus in the Western Hemisphere

West Nile virus, an Old World flavivirus related to St. Louis encephalitis virus, was first recorded in the New World during August 1999 in the boroug h of Queens, New York City. Through October 1999, 62 patients, 7 of whom di ed, had confirmed infections with the virus. Ornithophilic mosquitoes are t he principal vectors of West Nile virus in the Old World, and birds of seve ral species, chiefly migrants, appear to be the major introductory or ampli fying hosts. If transovarial transmission or survival in overwintering mosq uitoes were the principal means for its persistence, West Nile virus might not become established in the New World because of aggressive mosquito supp ression campaigns conducted in the New York area. However, the pattern of o utbreaks in southern Europe suggests that viremic migratory birds may also contribute to movement of the virus. If so, West Nile virus has the potenti al to cause outbreaks throughout both temperate and tropical regions of the Western Hemisphere.