Western Equine Encephalitis

Vector-Borne Disease
Public Health

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What you need to know

​​About Western Equine Encephalitis

Western Equine Encephalitis (WEE) is a severe viral illness spread by the bite of an infected mosquito and can affect humans and horses. WEE is very, very rare in the United States, with most cases occurring west of the Mississippi River, west of the Rocky Mountains.   

Learn more about how to prevent mosquito bites at our Mosquito-borne Disease Prevention Page. ​

How is Western Equine Encephalitis spread to humans?

A mosquito becomes infected by feeding on a bird that has WEE. Once the mosquito has the virus it feeds on other birds, mammals or humans and transfers the virus. The vectors for human transmission are the Culex tarsalis, Culiseta, and Aedes mosquito species

Because it is difficult to tell how many mosquitoes may be infected with a virus, it is important to prevent exposure to mosquitoes at all times.

Signs & Symptoms of Western Equine Encephalitis

Most people who become infected with Western Equine Encephalitis will have no symptoms, or mild flu like illness. A small percentage of people who are infected develop encephalitis. Of encephalitis cases, 5-15% are fatal. Symptoms usually occur within 5-15 days after being bitten by an infected mosquito.

Severe illness can occur in people of any age; however, people over 50 years of age and infants and very young children are at greater risk for severe illness if they are infected. People with compromised immune systems are also at greater risk.

Mild signs and symptoms

  • Fever
  • Chills
  • Muscle aches
  • Joint pain
  • Malaise

Severe signs and symptoms

  • High Grade Fever
  • Headache
  • Irritability
  • Restlessness
  • Drowsiness
  • Loss of appetite
  • Vomiting
  • Diarrhea
  • Bluish discoloration of the skin
  • Convulsions
  • Coma

Western Equine Encephalitis Treatment

There is no specific vaccine or treatment for Western Equine Encephalitis infection. Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms. Most people fully recover from the virus. However, some severe cases may require hospitalization.

If you think you or a family member might have Western Equine Encephalitis infection, talk with your health care provider.

For more information on Western Equine Encephalitis, contact your local health department or call the DHHS Office of Epidemiology at (402) 471-2937.​