La Crosse Encephalitis

La Crosse Encephalitis is a rare viral disease of the brain caused by the La Crosse virus. It usually affects children. The virus is carried by a tree hole mosquito (Aedes triseriatus), chipmunks, and squirrels.

The majority of infections are subclinical or result in mild illness about five to fifteen days following the bite of an infected mosquito. However this illness may progress to seizures, coma, and death. Subclinical means the person is not sick enough to have problems. This is fatal less than 1% of the time. It is found mostly in the Great Lakes states and the mid-Atlantic states. Rural populations are most often affected. No vaccines are available. The mosquito is found in woodland habitats and lives in artificial containers (anything that will contain water) and tree holes.


This is usually a mild illness, but symptoms can range from feeling OK to:

  • Fever

  • Headache

  • Nausea

  • Vomiting

  • Fatigue

All of this can progress to seizures, coma and possibly permanent brain damage and or death.


Your caregiver will be able to tell what is wrong by doing tests on blood and spinal fluid.


There is no specific treatment. Treatment is mostly supportive.


  • Avoiding bites by infected mosquitoes.

  • Mosquito control.

  • Eliminate breeding sites (water-filled tires, buckets, cans, etc.) near houses.

  • When outside, wear long pants and long-sleeved shirts.

  • Treat exposed skin with an insect repellent.