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Mosquito-Borne Encephalitis

Viruses cause disease around the world


Updated June 16, 2006

Some types of mosquitoes carry viruses which can cause potentially severe illness in humans. If you live in the United States, you've probably heard of West Nile virus. But that disease is caused by just one of several mosquito-borne viruses which occur around the world. (In fact, West Nile usually occurs in Africa, West Asia, and the Middle East.)

The diseases
Some of the encephalitis diseases caused by mosquito-borne viruses are:

  • La Crosse encephalitis
    Occurs in: United States (Midwest, mid-Atlantic, southeastern)
    Symptoms: fever, headache, nausea, vomiting, lethargy
    Severe disease: most common in children under age 16, consists of seizures, coma, paralysis, brain damage
    Death rate: less than 1% of cases
  • Eastern equine encephalitis
    Occurs in: United States (eastern, Gulf Coast, Midwest)
    Symptoms: fever, muscle pains, headache
    Severe disease: seizures, coma, brain damage
    Death rate: as high as 1/3 of all cases
  • Japanese encephalitis
    Occurs in: throughout Asia
    Symptoms: fever, headache, vomiting
    Severe disease: encephalitis, brain damage
    Death rate: 10-30% of cases (A vaccine is available for this disease.)
  • St. Louis encephalitis
    Occurs in: lower 48 states of United States
    Symptoms: fever, headache
    Severe disease: meningoencephalitis
    Death rate: 5-15% of cases
  • Venezuelan equine encephalitis
    Occurs in: Central and South America
    Symptoms: flu-like symptoms
    Severe disease: encephalitis
  • Western encephalitis
    Occurs in: western United States and Canada
    Symptoms: usually mild, but may have fever, headache, nausea, vomiting, malaise
    Severe disease: altered mental status, meningitis, brain damage Death rate: about 3% of cases
  • West Nile virus
    Occurs in: Africa, West Asia, Europe, Middle East, United States
    Symptoms: fever, headache
    Severe disease: encephalitis
    Death rate: low

How to protect yourself
1. Personal protective measures

  • stay out of mosquito-infested areas such as swamps
  • mosquitoes are active in the early evening, so avoid being outdoors then
  • wear long pants (not shorts) and long-sleeved shirts in wooded areas or near lakes and ponds
  • wear mosquito repellent
  • make sure your yard doesn't have standing water in buckets, pools, wheelbarrows, jars, etc., where mosquitoes can breed

2. Public health measures

  • spray insecticide where mosquitoes breed, such as lakes, ponds, roadside ditches, etc.
  • educate people about eliminating standing water and protecting themselves when outdoors
  • monitor the mosquito population and test for viruses

Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases, Arboviral Encephalitides.

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