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Eastern Equine Encephalitis

The cause of Eastern Equine Encephalitis (EEE) was first identified in the 1930s. From 1964 to 1995, 151 cases were reported in the U.S., with an average of about five cases per year. A few cases of EEE have been diagnosed in Indiana, all in the northern tier of counties in association with swamps. While EEE cases are rare, the fatality rate in humans is about 35%, the highest among arboviruses transmitted in the U.S. Approximately 35% of patients who recover suffer some degree of brain damage and may require permanent institutional care. EEE virus also can cause severe disease and death in horses. As indicated by comments below, much about the transmission cycle of EEE remains unknown.

Causative agent

  • A virus in the Family Togaviridae, genus Alphavirus.

Geographical distribution of cases

  • Eastern seaboard, Gulf Coast, and isolated areas of the midwest, including northern Indiana.
  • Nearly all cases are associated with certain salt and fresh water swamps.
  • NOTE: outbreaks in horses usually precede human cases.

Symptoms of infection

  • Typically flu-like, with severe headache, fever, muscle ache, and nausea.
  • Severe symptoms include encephalitis and/or meningitis.
  • Survivors of encephalitis may suffer permanent neurological sequelae requiring institutional care.

Reservoir of EEE virus

  • Not yet documented, but most likely birds that are associated with swamps.
  • NOTE: scientists do not know where or how EEE virus over winters.
  • NOTE: EEE virus may be introduced into the U.S. by migratory birds coming from southern regions where transmission occurs year round.

Vectors of EEE virus in Indiana

  • Culiseta melanura is the primary vector among birds.
  • Ochlerotatus sollicitans and Coquillettidia perturbans are probable bridge vectors to horses and humans.
  • NOTE: larvae of Co. perturbans develop in bodies of water in which cattails are growing.

Mode of transmission

  • Via the bite of infected female mosquitoes.
  • There is no known human-to-human or horse-to-horse transmission.

Diagnosis of infection

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

Treatment of infection

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

Prevention of infection

  • There is no vaccine for humans, but an effective vaccine exists for horses
  • Take special precautions in areas in which cases of EEE are documented in horses
  • 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

  • There are no effective methods of control for the species of mosquitoes that transmit EEE virus.
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