an equine encephalomyelitis found in the western U.S. and parts of South America, transmitted by mosquitoes and caused by the western equine encephalomyelitis virus (a species of Alphavirus in the family Togaviridae); the infection is similar to but milder than eastern equine encephalomyelitis in humans and is, as a rule, inapparent, but some cases with central nervous system involvement have been fatal.western equine encephalomyelitis virus.