La Crosse encephalitis

What is La Crosse encephalitis?

La Crosse (LAC) encephalitis is a mosquito-borne virus that was first described in La Crosse, Wisconsin in 1963. Since then, it has been reported in several Midwestern and Mid-Atlantic states. The LAC virus is one of many mosquito-transmitted viruses that can cause an inflammation of the brain (encephalitis). About 80-100 cases of this condition are reported each year in the United States. Most cases occur in children younger than age 16. While most people who become infected have no symptoms, those who do become ill may have fever, headache, vomiting and lethargy (tiredness). Severe cases develop encephalitis accompanied by seizures. Coma and paralysis occur in some cases. There is no specific treatment for LAC encephalitis. Supportive therapy is provided to those who develop severe cases of the disease.

Last updated on 05-01-20

How can people reduce the chance of getting La Crosse encephalitis virus (LACV)?

The best way to prevent LACV is to prevent mosquito bites. There is currently no vaccine or preventive medication. Mosquito bites may be prevented by following these recommendations:

  • Use insect repellent containing DEET, oil of lemon eucalyptus or other proven mosquito deterrents on exposed skin and clothing,
  • Wear long sleeves and pants when outside,
  • Utilize screens on windows and doors to keep mosquitoes outdoors, and
  • Eliminate mosquito breeding sites by emptying standing water from buckets, barrels, toys and other containers in your yard.

Last updated on 05-01-20

How do people get infected with La Crosse encephalitis virus (LACV)?

LACV is transmitted by the bite of an infected tree-hole mosquito. Mosquitoes become infected by biting small mammals like chipmunks or squirrels that are infected with the virus. The virus can be spread by an infected mosquito to its eggs. Tree-hole mosquitoes live not only in tree holes, but also in containers that you might have in your backyard, like buckets, tires, toys and tarps. Tree-hole mosquitoes are most active during daytime hours near wooded areas. The virus cannot be transmitted from human to human.

Last updated on 05-01-20

How might La Crosse (LAC) encephalitis be treated?

There is no specific treatment for LAC encephalitis. Severe cases are treated with supportive therapy which may include hospitalization, respiratory support, IV fluids and prevention of other infections.

Last updated on 05-01-20


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The RareGuru disease database is regularly updated using data generously provided by GARD, the United States Genetic and Rare Disease Information Center.

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