West Nile Virus Outbreak Among Horses in New York State, 1999 and 2000

West Nile VirusAn outbreak of arboviral encephalitis attributable to West Nile (WN) virus was first recognized in the United States in 1999, when dead crows were reported in and near New York City, and a zoological park noted that some of their exhibition birds had died.These events coincided with initial public health reports of human encephalitis cases diagnosed as St. Louis encephalitis virus in New York City (1).The successful isolation of virus from dead birds allowed the subsequent identification of WN virus as the etiologic agent of both human and animal disease (2).WN virus is primarily transmitted between mosquitoes and birds, but transmission to mammals can occur when infection occurs in mosquito species that feed on birds and mammals.Encephalitis has been confirmed only in humans and horses (1,3-10).During 1999, 20 equine cases of WN virus encephalitis were confirmed in the United States, all in New York.In 2000, 23 equine cases were identified in New York, with more cases identified in New Jersey, Delaware, Rhode Island, Massachusetts, Connecticut, and Pennsylvania.We summarize these findings. The Study 1999 InvestigationsDuring fall and early winter 1999, veterinarians with the U.S. Department of Agriculture and the New York Department of Agriculture and Markets (NYSDAM) investigated reports of an unusual cluster of neurologic illness in horses on Long Island.Investigations were initiated by reports from practicing veterinarians.Other veterinarians were contacted to determine whether other similar cases were occurring in the area.Tissue and blood samples were collected and submitted to the National Veterinary Services Laboratories (NVSL) for testing and forwarded to the Centers for Disease Control and Prevention (CDC) for confirmation, as necessary.