Equine Encephalomyelitis Study Guide

Overview and Clinical Importance

Equine encephalomyelitis refers to a group of mosquito-borne viral diseases caused by alphaviruses in the family Togaviridae. The three major viruses are Eastern equine encephalomyelitis virus (EEEV), Western equine encephalomyelitis virus (WEEV), and Venezuelan equine encephalomyelitis virus (VEEV). These diseases represent significant causes of severe neurological disease in horses with high mortality rates, making them critical topics for the NAVLE examination.

These viruses cycle naturally between mosquito vectors and avian or rodent reservoir hosts. Horses and humans are considered incidental dead-end hosts because viremia is typically insufficient to infect feeding mosquitoes. EEE/WEE/VEE vaccines are AAEP core vaccines, emphasizing the importance of understanding these diseases for preventive medicine.

Etiology and Virology

Viral Classification

All three encephalomyelitis viruses belong to the genus Alphavirus within the family Togaviridae. They are enveloped, single-stranded, positive-sense RNA viruses approximately 70 nm in diameter with icosahedral nucleocapsid symmetry.