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LaCrosse Encephalitis

The cause of LaCrosse Encephalitis (LAC) was discovered in 1963 near LaCrosse, Wisconsin. Since 1964, an estimated 70-75 cases per year are reported in the U.S. Several cases of LAC are reported in Indiana yearly, almost all in children, but many more cases go unrecognized and most likely are diagnosed as "summer flu." LAC virus is unusual among Indiana arboviruses in having small mammals, not birds, as its reservoir. Although the vast majority of cases are not serious, LAC can cause severe illness, especially in children under age 16. The fatality rate in clinical cases is less than 1%, but survivors may suffer permanent sequelae. Similar to SLE virus, LAC virus does not cause disease in horses.

Causative agent

  • A virus is the Family Bunyaviridae, genus Bunyavirus.

Geographical distribution of cases

  • Upper midwest, including Indiana, in association with deciduous forests.
  • Recently, an increasing number of cases have been recognized in the mid-Atlantic and southeastern U.S.

Symptoms of infection

  • Typically flu-like, including headache, fever, muscle ache, and nausea.
  • Severe symptoms include encephalitis and/or meningitis.
  • Severe symptoms also include seizures, paralysis, and coma, with neurological sequelae in survivors.

Reservoir hosts of LAC virus

  • Chipmunks, squirrels, field mice, and rabbits.
  • Also, LAC virus over winters in the eggs of the vector mosquito.
  • NOTE: virus amplification occurs in chipmunks and in tree squirrels.

Vector of LAC virus in Indiana

  • Ochlerotatus triseriatus, the eastern tree-hole mosquito.
  • This species is common in woodlands, but also occurs in wooded areas in and adjacent to residential areas.
  • NOTE: females seek blood meals during the day under low light intensity (typically in shaded woods) when your risk of exposure to infected mosquitoes is greatest.
  • NOTE: larvae develop in tree holes and other containers, including discarded tires and wooden rain barrels.

Mode of transmission

  • Via the bite of infected female O. triseriatus.
  • NOTE: there is no known human-to-human transmission.

Diagnosis of infection

  • Symptoms listed above together with their onset in early to mid-summer.
  • Laboratory tests that detect antibodies to LAC virus in a patient's blood.

Treatment of infection

  • Supportive only; there are no anti-viral drugs currently available.

Prevention of infection

  • There is no vaccine.
  • Avoid outdoor summer activities in shaded areas during the day when O. triseriatus females feed.
  • Keep playground equipment in sunlit areas and away from shaded edges of woodlots.
  • Wear clothing consisting of a long sleeved shirt, long pants, and a hat.
  • Use a repellent recommended by CDC and treat clothing with permethrin.

Control of vectors

  • Eliminate larval developmental sites such as discarded tires and wooden barrels that hold water.
  • Do not use tires as swings in playgrounds.
  • Fill tree holes known to be larval developmental sites.
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