The virus is transmitted to people and horses by bites from infected mosquitoes (Culex tarsalis and Aedes taeniorhynchus) and birds during wet, summer months.

In North America, WEE is seen primarily in U. S. states and Canadian provinces west of the Mississippi River.

Unlike Eastern equine encephalitis, the overall mortality of WEE is low (approximately 4%) and is associated mostly with infection in the elderly.

WEE was discovered in 1930 when a number of horses in the San Joaquin Valley of California, USA died of a mysterious encephalitis.

Karl Friedrich Meyer investigated but was not able to isolate the pathogen from necropsies of horses that had been dead for some time and needed samples from an animal in the earlier stages of disease.