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La Crosse VirusAedes triseriatus mosquito

La Crosse virus (LACV) is an arthropod-borne virus (arbovirus) spread by the bite of infected mosquitoes.  Infection with LACV can lead to severe febrile illness, encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).  While many people infected with LACV have no apparent symptoms, severe disease occurs most often in children under 16 years of age.  In rare cases, long-term disability or death can result from LACV.  There is no specific treatment for LACV infection, and care is based on symptoms.

Most people are infected in Ohio by the eastern treehole mosquito, Aedes triseriatus, an aggressive daytime biting mosquito commonly found in wooded areas.  LACV is endemic in Ohio, and Ohio has reported more human cases than any other state in the United States, averaging about 20 cases per year (see current human case map).

There is no vaccine against LACV.  Reducing exposure to mosquito bites is the best defense against getting infected with LACV or other mosquito-borne viruses.  There are several approaches you and your family can use to prevent and control mosquito-borne diseases:

  • Use repellent:  When outdoors, use Environmental Protection Agency (EPA) registered insect repellent on exposed skin as well as on clothing (mosquitoes will bite through thin cloth).
    • Permethrin is a repellent/insecticide that can be applied to clothing and will provide excellent protection through multiple washes.  You can treat clothing yourself (always follow the directions on the package!) or purchase pre-treated clothing.  For best protection, it is still necessary to apply other repellent to exposed skin.
  • Wear protective clothing:  Wear long sleeves, pants and socks when weather permits.
  • Avoid peak biting hours:  Avoid outdoor activity or use protective measures when mosquitoes are active (Aedes triseriatus mosquitoes are most active during daytime - from dawn until dusk).
  • Install and repair screens:  Have secure, intact screens on windows and doors to keep mosquitoes out.
  • Keep mosquitoes from laying eggs near you:  Mosquitoes can lay eggs even in small amounts of standing water.  While Aedes triseriatus prefers treeholes, it will also lay eggs in artificial containers.  You can fill treeholes in/around your yard with soil.  Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets, barrels and tires.  Change the water in pet dishes, and replace the water in bird baths weekly.  Drill holes in tire swings so water drains out.  Empty children's wading pools and store on their sides after use.

Animals associated with La Crosse virus in Ohio include:

  • Mosquitoes:  Mosquitoes become infected with LACV primarily through taking blood meals from infected mammals, especially squirrels and chipmunks.  However, the virus can be transmitted from infected female mosquitoes to their eggs, known as vertical transmission, which results in infected offspring.  Because of this vertical transmission, LACV can persist in an area for years if mosquito breeding is not controlled.
  • Squirrels and chipmunks:  Squirrels, chipmunks and other small mammals are amplifying hosts for LACV, meaning they serve as a source of infection to mosquitoes that bite them and humans.

La Crosse virus resources:

Ohio statistics and maps:


Disease reporting and surveillance:

Mosquito-borne disease prevention literature:


Page Updated:  01/30/2018

Zoonotic Disease Program