a group A arbovirus of the genus Alphavirus, family Togaviridae, occurring in the western U.S. and parts of South America; it occurs naturally, usually as a symptomless infection in birds, but causes western equine encephalomyelitis in horses and humans following transfer by the bites of mosquitoes, chiefly Culex tarsalis.western equine encephalomyelitis. SYN: WEE virus.