Western equine encephalitis (WEE) is a mosquito-borne zoonotic disease of horses caused by the western equine encephalomyelitis virus (WEEV). WEEV is of the Alphavirus
genus within the family Togaviridae. It was first reported in 1930, in a horse living in California. WEEV affects humans and horses
WEE can cause severe encephalitis in horses.
WEEV is a concern for horses that live in areas west of the Mississippi River in the United States, across south-central Canada from approximately Lake Superior to the Rocky Mountains, and in British Columbia. Most cases of infection with WEEV appear between the summer months of June through August, just after Culex tarsalis
mosquitoes reach their highest population density.
WEE is transmitted primarily by Culex tarsalis
mosquitoes, however it has also been found in a variety of mosquito species across 5 different genera (Aedes, Anopheles, Coquillettidia, Culex
). Birds serve as the most important vertebrate host for WEE which involves a mosquito-vertebrate-mosquito cycle. The primary amplifying hosts are the house sparrow (Passer domesticus
) and the house finch (Carpodacus mexicanus
). The red-winged blackbird, magpie, blacktail jackrabbit, kangaroo rat, Western gray squirrel, and prairie dog are also amplifying hosts.
The incubation period is 5 to 10 days for WEE.