Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025
Definition
Western equine encephalitis (WEE) is a mosquito-borne viral infection that affects the central nervous system, primarily in horses and humans. It is caused by the Western equine encephalitis virus (WEEV), an arbovirus of the Togaviridae family.
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WEE is transmitted to humans and animals through the bite of an infected mosquito, primarily Culex tarsalis.
Symptoms in humans can range from mild flu-like illness to severe neurological damage including encephalitis, which can be fatal.
The virus has an incubation period of 5 to 10 days before symptoms appear.
There is no specific antiviral treatment for WEE; supportive care is the mainstay of therapy.
Vaccination programs exist for horses but there is currently no vaccine available for human use.
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Related terms
Eastern Equine Encephalitis (EEE): A similar mosquito-borne viral disease affecting horses and humans, caused by the Eastern equine encephalitis virus.
St. Louis Encephalitis (SLE): A mosquito-borne viral disease causing inflammation of the brain, caused by St. Louis encephalitis virus.