Western equine encephalitis

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Western Equine Encephalitis

Sleeping Sickness

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.

How Western Equine Encephalitis is Transmitted

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 and Culesita). 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.

Incubation Period for Western equine encephalitis

The incubation period is 5 to 10 days for WEE.


Head pressing
Impaired vision
Difficulty swallowing
Altered behavior
Grinding teeth


  • History
  • Clinical signs
  • Physical exam
  • Laboratory test



Report diseaseWEE is a reportable disease, meaning that if you suspect that your horse has this disease, by law you need to report it to your veterinarian, or a state or federal veterinarian.
Supportive treatment


  • Vaccination
  • Biosecurity


Mortality rate is lower than with EEE, and is generally less than 30% but horses which develop neurological signs have a poorer prognosis.

Scientific Research

General Overviews

Risk Factors

  • Outbreaks of WEEV occur most frequently in years when, due to a complex of ecological factors, the prevalence of infection in mosquitoes is particularly high.