Technical Fact Sheet


La Crosse virus (LACV) is a member of the California serogroup, in the genus Bunyavirus, family Bunyaviridae.


LACV is transmitted to humans through the bite of an infected mosquito (Aedes triseriatus, the eastern treehole mosquito). The virus has a complex life cycle involving Ae. triseriatus and small mammals such as chipmunks and squirrels. Humans are not thought to be involved in the transmission cycle as they develop only low levels of circulating virus in the bloodstream. Ae. triseriatus lays its eggs in treeholes and man-made containers; it typically bites during the day.

Geographic distribution

An average of 70 cases of LAC encephalitis are reported each year in the United States. Historically, most cases of LACV disease were reported from the upper Midwestern states. Recently, more cases have been reported from mid-Atlantic and southeastern states. LACV disease cases occur primarily from late spring through early fall, but in subtropical endemic areas (e.g., the Gulf states), rare cases can occur in winter.

Risk factors

All people in areas where virus activity has been identified are at risk of getting infected with LACV. However, most cases of severe LACV disease occur in children under 16 years of age. People who live in or visit woodland habitats and those who work outside or participate in outdoor recreational activities in areas where the virus is circulating are at increased risk due to mosquito exposure. LACV infection is thought to confer life-long immunity against re-infection.

Incubation period

Usually 5-15 days


Most persons infected with LACV have no apparent illness. Initial symptoms in those who become ill include fever, headache, nausea, vomiting, fatigue, and lethargy. Severe LACV disease (encephalitis) occurs most commonly in children under the age of 16, often accompanied by seizures. Coma and paralysis occur in some cases.


No specific antiviral treatment for LAC encephalitis is available. Patients with suspected LAC encephalitis should be hospitalized, appropriate serologic and other diagnostic tests ordered, and supportive treatment (including seizure control) provided. Most patients seem to experience full recovery. Neurologic sequelae (e.g., recurrent seizures, hemiparesis, and cognitive and neurobehavioral abnormalities) of varying duration have been reported in some cases.

Mortality rate

Less than 1% of LAC encephalitis cases are fatal.


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, pants and socks when weather permits.
  • Have secure intact screens on windows and doors to keep mosquitoes out.
  • Mosquitoes lay eggs in standing water. Eliminate mosquito breeding sites by emptying standing water from flower pots, buckets, barrels, tires and other containers. Drill holes in tire swings so water drains out. Keep children’s wading pools empty and stored on their side when they aren’t being used.