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St. Louis Encephalitis

The cause of St. Louis Encephalitis (SLE) was discovered in 1933. From 1964 to 1998, 4,478 cases of SLE were reported in the U.S., including 369 in Indiana. The last major outbreak of SLE in Indiana occurred in 1975. SLE virus is related to WNV and, like WNV, SLE has a bird reservoir and Culex mosquitoes as vectors. The vast majority of infections do not cause serious disease and go undiagnosed, but SLE virus can cause severe illness and death in humans, especially the elderly in whom the fatality rate is 3-30%. Children typically do not develop serious disease, but those who do have a relatively high rate of encephalitis. Unlike WNV, SLE virus does not cause disease in horses.

Causative agent

  • A virus in the Family Flaviviridae, genus Flavivirus.

Geographical distribution of cases

  • North America and South America.
  • Most cases in the U.S. have occurred in the midwest and southeast.

Symptoms of infection

  • Typically flu-like, including headache, fever, muscle ache, and nausea.
  • Severe symptoms include encephalitis and/or meningitis.

Reservoir hosts of SLE virus

  • Numerous species of birds, including the house sparrow.
  • NOTE: SLE virus also may over winter in hibernating female Culex mosquitoes.

Vectors of SLE virus in Indiana

  • Culex pipiens and C. quinquefasciatus.
  • NOTE: female Culex feed from early evening well into night, when your risk of exposure to infected mosquitoes is greatest.
  • NOTE: Culex larvae typically develop in water with a high organic content, including highly polluted habitats, and are also noted for developing in small containers such as discarded tires, cans, rain gutters, etc.

Mode of transmission

  • Via the bite of infected Culex females.
  • NOTE: there is no known human-to-human transmission.

Diagnosis of infection

  • Symptoms listed above together with their onset in late summer-early fall when Culex numbers are high.
  • Laboratory tests that detect antibodies to SLE 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 evening and into night when Culex females seek blood meals.
  • 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 containers in yards and on business properties that hold water.
  • Larvicides as needed based on monitoring aquatic sites in which Culex larvae develop.
  • Adulticides if warranted by disease cases in humans.
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