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Western Equine Encephalitis


Western equine encephalitis (WEE) is a virus spread by a bite from an infected mosquito. While WEE is rare, an infection can be serious or fatal.


WEE is caused by a virus. The virus is passed through the bite of an infected mosquito.

Risk Factors

Factors that may increase your chance of WEE include:

  • Living in or visiting the plains regions of western and central United States
  • Doing activities outdoors and not using insect repellent


Most people with WEE do not have any symptoms.

If symptoms do occur, they appear within 5-10 days after infection and include:

  • Headache
  • Fever
  • Neck stiffness
  • Chills
  • Fatigue
  • Joint and muscle pain
  • Vomiting

WEE can lead to more serious, life-threatening symptoms like inflammation of the brain (encephalitis), seizures, and coma . These serious symptoms are more common in infants and older adults.

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In addition to taking your medical history and doing a physical exam, your doctor will ask you:

  • What kind of symptoms you are experiencing
  • Where you have been living or traveling
  • Whether you have been exposed to mosquitoes

Your doctor may need to test your bodily fluids. This can be done with:

  • Blood tests
  • Cerebrospinal fluid tests to evaluate the fluid that surrounds the brain and spinal cord

Imaging tests to evaluate the brain can be done with:


Because the infection is viral, there is no specific treatment for WEE. Treatment will focus on managing your symptoms and related complications through:

  • IV fluids
  • Antiseizure medications
  • Medications to decrease brain swelling
  • Breathing support—mechanical ventilation


There is no vaccine for humans. There is a vaccine for horses. Prevention of WEE focuses on controlling mosquitoes and avoiding mosquito bites. Steps you can take to avoid mosquito bites include:

  • Stay inside between dusk and dark, when mosquitoes are most active.
  • Wear long pants and long-sleeved shirts when outside.
  • Use an insect repellent with DEET.
  • Repair screens to prevent mosquitoes from entering the house.
  • Use proper mosquito netting at night. Look for netting treated with insecticide.
  • Remove standing water (such as birdbaths, clogged gutters) to prevent mosquito breeding.

Revision Information

  • Centers for Disease Control and Prevention


  • National Institute of Allergy and Infectious Diseases


  • Health Canada


  • Healthy Alberta


  • Fact sheet: Western equine encephalitis. Centers for Disease Control and Prevention website. Available at: http://www.cdc.gov/ncezid/dvbd. Updated November 7, 2005. Accessed January 4, 2013.

  • Meningitis and encephalitis fact sheet. National Institutes of Neurological Disorders and Stroke website. Available at: http://www.ninds.nih.gov/disorders/encephalitis%5Fmeningitis/detail%5Fencephalitis%5Fmeningitis.htm. Updated February 16, 2011. Accessed January 4, 2013.

  • Reimann CA, Hayes EB, DiGuiseppi C, et al. Epidemiology of neuroinvasive arboviral disease in the United States, 1999-2007. Am J Trop Med Hyg. 2008;79(6):974-979.

  • Western equine encephalitis fact sheet. Minnesota Department of Public Health website. Available at: http://www.health.state.mn.us/divs/idepc/diseases/weencephalitis/wee.html. Accessed January 4, 2013.

  • 10/1/2013 DynaMed Systematic Literature Surveillance http://www.ebscohost.com/dynamed: Reimer LJ, Thomsen EK, Tisch DJ, et al. Insecticidal bed nets and filariasis transmission in Papua New Guinea. N Eng J Med. 2013;369(8):745-753.