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Eastern Equine Encephalitis

Eastern equine encephalitis (EEE) is caused by infection with eastern equine encephalitis virus (EEEV). The natural reservoir for EEEV is tree-perching birds found in forested wetlands. The virus is spread between these birds and other small mammals, reptiles and amphibians by Culiseta melanura mosquitoes. Occasionally, EEEV spills over from its usual reservoirs to infect dead-end hosts such as humans and horses. Humans are become infected with EEEV when they are bitten by Aedes or Culex mosquitos that have taken blood meals from these birds. Horses are also susceptible to EEEV and can die from infection.

After inoculation of virus- infected mosquito saliva into perivascular dermal tissue, EEEV infects Langerhans and dendritic cells. The infected cells migrate to regional lymphoid tissue, where EEEV replicates and leads to systemic viremia. Approximately 95% of individuals remain asymptomatic after infection. A minority of patients develop nonspecific symptoms after an incubation period of 3 to 10 days. Symptoms include fever, malaise, intense headache, muscle aches, and nausea and vomiting. Symptoms may last as long as two weeks. Most patients recover uneventfully, but other individuals progress to encephalitis. Imaging typically shows involvement of the basal ganglia and thalami. Approximately 30% of encephalitis cases are fatal.

IgG and IgM antibodies to EEEV are detected by indirect immunofluorescent assays. Dilutions of cerebrospinal fluid or serum are incubated with EEEV infected cells. If antibodies to EEEV are present, they bind to the infected cells and are then detected by a fluorescein-labeled antibody directed to human IgG or IgM. Serologic tests may be negative at the onset of nonspecific symptoms or neurologic changes.

No antiviral drug has been demonstrated to have efficacy against EEEV. The mainstay of treatment is supportive care, usually in an intensive care unit. Patients do not need to be isolated.


Morens DM, Eastern Equine Encephalitis Virus — Another Emergent Arbovirus in the United States. New Engl J Med 2019;381:1989-92.

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