Frequently Asked Questions

What is La Crosse encephalitis?

La Crosse (LAC) encephalitis is a rare disease caused by the bite of a mosquito infected with LAC virus. LAC virus is one of a group of mosquito-borne viruses that can cause inflammation of the brain (encephalitis). In the United States, 50-150 LAC cases are reported each year.

How do people get infected with LAC virus?

LAC virus is spread by the bite of an infected mosquito. Most people become infected from the “treehole mosquito” (Aedes triseriatus). LAC virus is not spread by coughing, sneezing, touching, or other contact with someone who is infected. It is not known if LAC virus can be spread through blood or tissue donation.

Where and when have most cases of LAC occurred?

Most cases of LAC virus disease have been reported from upper Midwestern, mid-Atlantic and southeastern states. LAC cases occur primarily from late spring through early fall, but in warmer states where the mosquito is found, like the Gulf states, cases can occur in winter as long as mosquitoes are active.

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Who is at risk for infection with LAC virus?

Anyone in an area where the virus is found can be infected with LAC virus. The mosquitoes that spread LAC virus live in tree holes. You are at higher risk for getting LAC virus from an infected mosquito if you live, work, or spend time outdoors in the woods.

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How soon do people get sick after getting bitten by an infected mosquito?

It takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of LAC encephalitis.

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What are the symptoms of LAC?

Most people infected with LAC virus have no apparent illness.

Initial symptoms in those who become ill include fever, headache, nausea, vomiting, and tiredness.

Severe disease (involving encephalitis, an inflammation of the brain) occurs most commonly in children under 16 years of age and is often accompanied by seizures. Coma and paralysis occur in some cases.

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How is LAC encephalitis diagnosed?

Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.

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What is the treatment for LAC virus disease?

There is no specific treatment for LAC virus disease. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Severe illness is treated by supportive therapy that can include hospitalization, respiratory support, IV fluids, and prevention of other infections.

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How can people reduce the chance of getting infected with LAC virus?

There is no vaccine to prevent or medicine to treat LAC infection. The best way to reduce your chance of infection is by preventing mosquito bites.

  • Use an Environmental Protection Agency (EPA)-registered insect repellent and wear long-sleeved shirts and pants when outside.
  • Repair or install screens on windows and doors to keep mosquitoes outside.
  • The mosquito that spreads LAC virus, Aedes triseriatus, prefers to lay its eggs in tree holes. You can reduce the number of mosquitoes around your home by that collect water.
  • LAC virus can survive the winter in mosquito eggs that will hatch into infected mosquitoes in the spring. Stop mosquitoes from laying eggs in or near water. Once a week, empty and scrub, turn over, cover, or throw out items that hold water.
    • Drill holes in tire swings so water drains out. Empty children’s wading pools and store on their side after use.

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What should I do if I think a family member might have LAC encephalitis?

If you or anyone in your household has symptoms of LAC encephalitis, talk to your healthcare providers.

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