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Eastern Equine Encephalitis

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Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) is a mosquito-borne viral disease. As the name suggests, EEE occurs in the eastern half of the US. Because of the high case fatality rate, it is regarded as one of the more serious mosquito-borne diseases in the United States.

TRANSMISSION: What is the EEE transmission cycle? How do people become infected with EEE virus?

  • EEE virus is transmitted to humans through the bite of an infected mosquito
  • The main EEE transmission cycle is between birds and mosquitoes
  • Several species of mosquitoes can become infected with EEE virus. The most important mosquito in maintaining the enzootic (animal-based, in this case bird-mosquito-bird) transmission cycle is Culiseta melanura
  • Horses can become infected with, and die from, EEE virus infection


  • Eastern equine encephalitis virus is a member of the family Togaviridae, genus Alphavirus
  • Closely related to Western and Venezuelan equine encephalitis viruses

HUMAN CLINICAL FEATURES: What type of illness can occur?

  • Symptoms range from mild flu-like illness to encephalitis (inflammation of the brain), coma and death
  • The EEE case fatality rate (the % of persons who develop the disease who will die) is 35%, making it one of the most pathogenic mosquito-borne diseases in the US
  • It is estimated that 35% of people who survive EEE will have mild to severe neurologic deficits

INCIDENCE: How many and where have human disease cases occurred?

  • 200 confirmed cases in the US 1964-present
  • Average of 4 cases/year, with a range from 0-14 cases
  • States with largest number of cases are Florida, Georgia, Massachusetts, and New Jersey
  • The enzootic (animal-based) transmission cycle is most common to coastal areas and freshwater swamps
  • Human cases occur relatively infrequently, largely because the primary transmission cycle takes place in swamp areas where populations tend to be limited

RISK GROUPS: Who is at risk for developing EEE?

  • Residents of and visitors to endemic areas (areas with an established presence of the virus)
  • People who engage in outdoor work and recreational activities
  • Persons over age 50 and younger than age 15 seem to be at greatest risk for developing severe disease

PREVENTION: How can people avoid infection with EEE virus?

  • A vaccine is available to protect equines
  • People should avoid mosquito bites by employing personal and household protection measures, such as using insect repellent containing DEET, wearing protective clothing, taking precautions from dusk to dawn when mosquitoes are most likely to bite, and controlling standing water that can provide mosquito breeding sites. For more information about preventing mosquito-borne disease see our protection information.

SURVEILLANCE: How is EEE monitored?

  • EEE is reportable under the National Notifiable Diseases Surveillance System
  • In 2003, EEE cases are being reported to ArboNet for the first time. ArboNet is the national, electronic surveillance system established by CDC to assist states in tracking West Nile virus and other mosquito-borne viruses.
  • Data for 1964-2000 is posted on this website, by state, as is a map of cases from 1964-1997


  • Risk of exposure to infected mosquitoes increases as population expands into areas with an established presence of the virus


  • There is no licensed vaccine for human use
  • There are no effective therapeutic drugs
  • Unknown overwintering cycle
  • Control measures expensive
  • Limited financial support of surveillance and prevention