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Western Equine Encephalitis

The cause of Western Equine Encephalitis (WEE) was first identified in the 1930's in California. From 1964 to 1995, 639 cases of WEE were reported, nearly all in the western regions of the U.S. and Canada. However, the incidence of WEE has increased with the expansion of irrigated agriculture that creates habitats conducive both to reservoir birds and vector mosquitoes. The vast majority of cases are not serious, but WEE can cause encephalitis in humans, with a fatality rate of abut 3%. Children, especially those under one year old, are affected more severely than adults and 5 to 30% of children may suffer permanent sequelae. WEE virus also can cause severe disease and death in horses.

Causative agent

  • A virus in the Family Togaviridae, genus Alphavirus.

Geographical distribution of cases

  • North America.
  • NOTE: transmission of WEE virus in Indiana might occur in southwestern counties with irrigated agriculture.

Symptoms of infection

  • Typically flu-like, including headache, fever, muscle ache, and nausea.
  • Severe symptoms include encephalitis and/or meningitis.
  • Disease is most severe in very young children.
  • Children who recover from encephalitis may suffer permanent neurological sequelae.

Reservoir hosts of WEE virus

  • Numerous species of birds, including the house sparrow.

Vectors of WEE virus

  • Culex tarsalis is the primary vector in the U.S.
  • NOTE: Females seek blood meals from early evening well into night when your risk of exposure to infected mosquitoes is greatest.
  • NOTE: larvae commonly develop in water associated with irrigated agriculture and are also associated with livestock production where they develop in hoof prints that hold water.
  • NOTE: C. tarsalis females are capable of flying as far as 10-15 miles from larval developmental sites and therefore can be vectors of WEE in residential areas far removed from irrigated agriculture.

Mode of transmission

  • Via the bite of infected female C. tarsalis.
  • NOTE: there is no known human-to-human or horse-to-horse transmission.

Diagnosis of infection

  • Symptoms listed above together with their onset in late summer-early fall.
  • Laboratory tests that detect antibodies to WEE 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 for humans, but an effective vaccine exists for horses
  • Avoid outdoor activities in evening and into night when C. tarsalis females feed
  • 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

  • Elimination of larval developmental sites often is not feasible.
  • If possible, manage irrigation water to avoid creating flooded habitats that support Culex tarsalis larva.
  • Larvicides as needed based on monitoring aquatic sites in which Culex larvae develop.
  • Adulticides if warranted by monitoring WEE in birds, in Culex mosquitoes, and disease cases in humans
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