West Nile Virus Disease Guidelines

West Nile virus (WNV) is a mosquito-borne flavivirus that is endemic in the avian population of North America. In the late 1990’s WNV spread to North America and became more virulent, capable of causing fever and neuroinvasive disease in horses and humans with regularity; previously WNV infection generally resulted in fever in humans. West Nile virus is now considered endemic in Canada, the United States, Mexico, and the Caribbean.

Once a horse is infected, viremia is established, and the virus is thought to penetrate the blood brain barrier and enter the central nervous system. Mortality rates for horses diagnosed with WNV are reported to be 22-44%. Previously vaccinated but improperly boostered horses may experience less severe disease than horses with no vaccine history at all. A history of no vaccination or incomplete vaccination, persistent recumbency, and facial or tongue paralysis is associated with a poor outcome, and mortality rates increase in older horses. Treatment is supportive, and though the administration of hyperimmune WNV plasma makes good clinical sense, there are no studies in horses supporting its use.