Font Size:

La Crosse Encephalitis

La Crosse Encephalitis virus (LACV) is transmitted to humans by an infected mosquito. LACV is cycled between mosquitoes and a host (usually small mammals such as a chipmunk). The mosquito that vectors this disease is Aedes triseriatus (commonly known as the eastern treehole mosquito). The habitat for the animals that carry this disease is a deciduous forest, which is a forest with trees that lose their leaves each year. Once a human is infected with LACV, it cannot be spread to other humans (not by human to human contact, nor by a mosquito biting one person and then another). Aedes triseriatus is considered an aggressive day-time biting mosquito, and is often in infested woods or at the edge of the wood-line. In the United States, this disease most commonly occurs in the upper Midwestern, northeastern, mid-Atlantic, and southeastern states. The CDC reports that on average, there are 68 cases of LACV neuroinvasive disease a year. Two human cases of LACV have been reported in Alabama since 2008.

About La Crosse Encephalitis Infection

  • Often when people become infected they show no symptoms.
  • The incubation period for this virus is usually 5-15 days.
  • Symptoms of onset illness include fever, headache, vomiting, fatigue, and lethargy.
  • Most commonly, severe cases of LACV occur in children under the age of 16.
  • In cases of children with LACV, the patient usually suffer from seizures as well.
  • In some cases, coma and paralysis occur.
  • Less than 1% of LACV cases are fatal.
  • People that live in or visit areas that have Aedes triseriatus mosquitoes are at risk of LACV.


  • There is no vaccination for LACV.
  • There are no specific antiviral medications.
  • Suspected LACV cases should be hospitalized, diagnostic tests and appropriate serologic tests ordered.
  • Upon hospitalization, proper supportive treatment can be administered (including seizure control).
  • Most patients make a full recovery.
  • If you or your family member develop the symptoms of LACV contact your healthcare provider.

Detect, Test, and Report

The best preventive measure for residents living in areas infested with disease vectoring mosquito species is to eliminate the places where the mosquito lays her eggs, primarily artificial containers that hold water. In the case of the mosquito that vectors LACV, Aedes triseriatus (commonly known as the eastern treehole mosquito), breeding is possible in treeholes and also all of the common household problem areas.

Items that collect rainwater or to store water (for example, plastic containers, 55-gallon drums, buckets, or used automobile tires) should be covered or properly discarded. Pet and animal watering containers and vases with fresh flowers should be emptied and cleaned (to remove eggs) at least once a week. This will eliminate the mosquito eggs and larvae and reduce the number of mosquitoes present in these areas. For more info on how to control mosquitos around your home Vector Control.

Using air conditioning or window and door screens reduces the risk of mosquitoes coming indoors. Proper application of mosquito repellents on exposed skin and proper clothing decreases the risk of being bitten by mosquitoes. See Avoid the Bite for more information about preventing mosquito-borne diseases. To learn more about insect repellents, visit this Environmental Protection Agency website: Insect Repellent Information from the EPA

Additional Resources

For questions or concerns regarding insect-borne disease in Alabama, see the Contact Us page, or email us at

Page last updated: December 7, 2021