Equine West Nile Viral Encephalomyelitis – NAVLE Study Guide

Overview and Clinical Importance

West Nile Virus (WNV) is a mosquito-borne flavivirus that causes neuroinvasive disease in horses and is the most common arboviral encephalitis in North America. First identified in the United States in 1999, WNV has become endemic throughout the continent. The virus is maintained in a bird-mosquito-bird transmission cycle, with horses and humans serving as incidental "dead-end" hosts. Understanding WNV is essential for NAVLE success as it represents a significant component of equine infectious neurological diseases.

Approximately 80% of infected horses remain asymptomatic, while 20% develop clinical disease. Of those showing clinical signs, the case fatality rate is approximately 33%, with an additional 10-20% recovering with residual neurologic deficits. The American Association of Equine Practitioners (AAEP) classifies WNV vaccination as a core vaccine for all horses in North America.

Etiology and Virology

West Nile virus is a single-stranded, positive-sense RNA virus belonging to the family Flaviviridae and genus Flavivirus. It is a member of the Japanese encephalitis serocomplex, which includes St. Louis encephalitis virus, Murray Valley encephalitis virus, and Japanese encephalitis virus. The virus is approximately 45-50 nm in diameter with an icosahedral envelope.