Summary Of West Nile Virus In The United States

Courtesy of USDA, APHIS, Veterinary Services, Emergency Programs
Infectious Disease Table Topics, AAEP Convention
(Report of November 18, 1999)

West Nile Virus (WNV) has been identified in birds, mosquitoes,

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Courtesy of USDA, APHIS, Veterinary Services, Emergency Programs
Infectious Disease Table Topics, AAEP Convention
(Report of November 18, 1999)

West Nile Virus (WNV) has been identified in birds, mosquitoes, people, and horses in a limited area of the northeastern United States. Specifically, this area consists of parts of Connecticut, New York, New Jersey, and one county in Maryland. The available literature on this virus demonstrates that the transmission cycle is primarily a mosquito-bird cycle, with occasional incursions into other vertebrates as terminal hosts only. The literature supports the conclusion that horses are terminal hosts for WNV and do not maintain a sufficient viremia to infect either other mammals (including humans) or mosquitoes.


An outbreak of human encephalitis of then unknown etiology began in New York City in early August 1999. On September 14, 1999, a virus was isolated from tissues of exotic birds from the Bronx Zoo and of wild birds from the New York City area. This virus was later identified as WNV and confirmed as the cause of the human encephalitis outbreak. The virus was also confirmed in clinically ill horses located in the Riverhead area of Suffolk County on Long Island, New York, on October 18, 1999.


Surveillance has continued in the affected area and in additional States down the eastern coast of the U.S. to monitor possible spread of WNV. This surveillance consists of investigating suspect cases in horses, testing wild birds, mosquito collection and testing, and sentinel chicken testing. To date, no apparent spread has been detected in horses, no cases have been detected in commercial poultry, and the number of wild birds and mosquitoes testing positive continues to decrease.


Studies have been initiated to obtain additional information on the virus. Specifically, small inoculation studies in horses, chickens, and turkeys have begun. Information gathered from these studies will be shared when available

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Written by:

Stephanie L. Church, Editorial Director, grew up riding and caring for her family’s horses in Central Virginia and received a B.A. in journalism and equestrian studies from Averett University. She joined The Horse in 1999 and has led the editorial team since 2010. A 4-H and Pony Club graduate, she enjoys dressage, eventing, and trail riding with her former graded-stakes-winning Thoroughbred gelding, It Happened Again (“Happy”). Stephanie and Happy are based in Lexington, Kentucky.

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