La Crosse Encephalitis Fact SheetRevised July, 2013
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What is La Crosse encephalitis?
LaCrosse (LAC) encephalitis is an illness transmitted to people through the bite of a mosquito infected with LAC encephalitis virus.
How serious is La Crosse encephalitis?
- Most people infected with this virus will have no symptoms or a mild flu-like illness. A small percentage of people (especially children) may develop encephalitis. Approximately 1-3% of these encephalitis cases are fatal, and 15%-30% of patients have long-term nervous system problems.
- Most of the severe cases start with headache, fever, nausea, and lethargy. The illness may rapidly progress into disorientation, seizures, and coma. There is no treatment for the illness other than supportive care until the illness is over.
What kind of mosquito transmits La Crosse encephalitis?
- Minnesota has approximately 50 species of mosquitoes, but not all mosquitoes feed on people. The mosquito primarily responsible for transmitting LAC encephalitis to people is the Eastern Tree Hole mosquito (Aedes triseriatus).
- The Tree Hole mosquito, found almost exclusively in wooded or shaded areas, usually does not fly far from the area where it was hatched. It feeds during the day, unlike many of our pest mosquitoes that feed mostly at dusk and dawn.
- Tree Hole mosquitoes reproduce in water-holding tree holes (rainwater that collect between the main trunks of trees with two or more trunks). They also reproduce in waste tires, buckets, cans, and any other container that can hold rainwater.
- The female Tree Hole mosquito can pass LAC encephalitis virus to her eggs. In this way, the virus is maintained in the same areas year after year. If large numbers of water-holding containers are present in an area where LAC virus is also present, there may be significant numbers of infected mosquitoes by late summer. The highest risk for LAC
encephalitis begins the middle of July and lasts through early September.
Who is at risk for LaCrosse encephalitis?
- Severe cases occur primarily in children (average case age is six years old).
- In Minnesota, cases of LAC encephalitis have been found in 20 southeastern counties from the Twin Cities to the Iowa border. Zero to 13 cases are reported each year in Minnesota.
- Most people are exposed to infected mosquitoes in wooded habitat, as the mosquito that transmits the virus is found in hardwood forests.
What can people do to prevent LaCrosse encephalitis?
- The best way to prevent LaCrosse encephalitis is to remove water-holding containers from your property. It is easier to do this in the spring before growing vegetation obscures the containers.
- Bird baths should have their water changed every week to prevent mosquito breeding, and gutters should be checked to make sure they are not plugged.
- Water-holding tree holes should be filled with dirt, sand, or a pliable insulation cement to prevent further mosquito breeding.
- If children play in or near wooded areas during the day, they should:
- use repellents containing DEET (up to 30% DEET) according to label directions
- wear long-sleeved shirts and long pants (light-colored clothing works best)