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Western Equine Encephalitis

  • Definition

    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.
  • Causes

    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
  • Symptoms

    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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  • Diagnosis

    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:
  • Treatment

    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
  • Prevention

    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.

    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: Updated November 7, 2005. Accessed January 4, 2013.

    Meningitis and encephalitis fact sheet. National Institutes of Neurological Disorders and Stroke website. Available at: 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: Accessed January 4, 2013.

    10/1/2013 DynaMed Systematic Literature Surveillance 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.

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