Frequently Asked Questions
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- What is La Crosse encephalitis?
- How do people get infected with LACV?
- Where and when have most cases of LACV disease occurred?
- Who is at risk for infection with LACV?
- How soon do people get sick after getting bitten by an infected mosquito?
La Crosse encephalitis is a rare disease that is caused by a virus spread by infected mosquitoes. La Crosse encephalitis virus (LACV) is one of a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis). In the United States, an average of 63 LACV disease cases are reported each year.
LACV is transmitted by the bite of an infected mosquito. Most people become infected from the “treehole mosquito” (Aedes triseriatus). LACV is not transmitted directly from person to person.
Most cases of LACV disease have been reported from upper Midwestern, mid-Atlantic and southeastern states. LACV disease cases occur primarily from late spring through early fall, but in subtropical areas where the mosquito is found (e.g., the Gulf states), rare cases can occur in winter.
Anyone bitten by a mosquito in an area where the virus is circulating can get infected with LACV. The risk is highest for people who live, work or recreate in woodland habitats, because of greater exposure to potentially infected mosquitoes.
It takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of LACV disease.
Most persons infected with LACV 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 age 16, and is often accompanied by seizures. Coma and paralysis occur in some cases.
Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.
There is no specific treatment for LACV disease. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections.
Prevent mosquito bites. There is no vaccine or preventive drug.
- Use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing. The repellent/insecticide permethrin can be used on clothing to protect through several washes. Always follow the directions on the package.
- Wear long sleeves and pants when weather permits.
- Have secure, intact screens on windows and doors to keep mosquitoes out.
- Eliminate mosquito breeding sites by emptying standing water from flower pots, buckets, barrels, and other containers. Drill holes in tire swings so water drains out. Empty children’s wading pools and store on their side after use.
- LACV can survive the winter in the mosquito eggs that will hatch into infected mosquitoes in the spring. Cleaning potential breeding sites such as old tires or tin cans can reduce the number of infected eggs developing into infected mosquitoes. As the Aedes triseriatus mosquito prefers treeholes for breeding sites, you can reduce mosquitoes by filling treeholes in/around your yard with soil.
If you or anyone in your household has symptoms that are causing you concern, consult a healthcare provider for proper diagnosis.
- Page last reviewed: April 11, 2016
- Page last updated: November 20, 2017
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