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La Crosse Encephalitis: About

La Crosse encephalitis is an illness spread by mosquitoes. In Wisconsin, it is spread to humans by the bite of an infected Aedes triseriatus mosquito, also known as the eastern tree hole mosquito.

La Crosse encephalitis mostly occurs in the Midwestern, mid-Atlantic, and Southeastern parts of the U.S. The first case of La Crosse encephalitis was identified in a Minnesota resident who was diagnosed in La Crosse, Wisconsin, in 1960.

Anyone can get La Crosse encephalitis, but people who spend more time outdoors are at higher risk of being bitten by an infected mosquito. Severe disease is most common in children under 16 years old. Mosquitoes are usually active in Wisconsin from May to October, but the risk of La Crosse encephalitis is highest during the months of July through September.


LaCrosse Encephalitis map

Mosquitoes can be found in areas with standing water, which they need to breed. It is important to remove standing water sources, such as containers, leaves, and yard debris, to reduce breeding habitats in your yard. For more tips, please visit our Mosquito Bite Prevention page.

La Crosse encephalitis virus is spread to humans through the bite of an infected mosquito.

  • Mosquitoes become infected with La Crosse encephalitis virus by feeding on infected small mammals, such as chipmunks and squirrels, in forest habitats.
    A hole in a tree trunk.
    • After feeding on a mammal that has La Crosse encephalitis virus, the virus will end up inside of the mosquito.
    • Once it has the virus, a mosquito can spread the virus to other mammals, including humans, when they take another blood meal.
  • The eastern tree hole mosquito is an aggressive daytime-biting mosquito, especially in or near woods infested with these mosquitoes. Female mosquitoes prefer to lay their eggs in pools of water in tree holes.
  • The mosquitoes that spread La Crosse encephalitis are usually most active during the late summer, into the fall.
  • Few mosquitoes actually carry the virus, but it is important to take prevention measures when spending time outside.

La Crosse encephalitis is preventable. Visit our Mosquito Bite Prevention page to learn how to prevent mosquito bites.

Symptoms can show up five to 15 days after being bitten by an infected mosquito.

Most people who are infected with La Crosse encephalitis virus never develop obvious signs of infection. Others may develop mild symptoms like fever, fatigue, and headache. Infection with the virus can lead to severe illness, including neurological symptoms, but this is rare. Severe illness is more likely to develop in children under 16 years old. Death from an infection with La Crosse encephalitis is rare, but has been documented. Most patients recover completely. However, lasting symptoms of the disease, including seizures, paralysis, and cognitive abnormalities, have been reported in some cases. If you have had La Crosse encephalitis, you cannot get it again.

Mild signs and symptoms
  • Fever
  • Headache
  • Nausea
  • Vomiting
  • Fatigue (tiredness)
  • Lethargy (reduced activity or alertness)
Severe signs and symptoms
  • Seizures
  • Hemiparesis (partial paralysis of one side of the body)
  • Cognitive and neurobehavioral abnormalities
  • Encephalitis (inflammation of the brain)
    • Increasing lethargy
    • Altered mental status
  • Coma

There is currently no treatment or vaccine for La Crosse encephalitis. Over-the-counter pain relievers may be given to relieve the symptoms. In severe cases, patients may need to be hospitalized to receive supportive treatment.

If you believe you or a family member may have La Crosse encephalitis, contact your doctor immediately.

Department of Health Services resources
Centers for Disease Control and Prevention (CDC) resources

Mosquito Bites Are Bad: An educational activity book for kids about preventing illnesses spread by mosquitoes.

Partner resources
La Crosse encephalitis is preventable. Visit our Mosquito Bite Prevention page to learn how to protect yourself from La Crosse encephalitis and other illnesses spread by mosquitoes.

Questions about illnesses spread by mosquitoes? Contact us!
Phone: 608-267-9003 | Fax: 608-261-4976

Last revised May 8, 2023