What is Eastern Equine Encephalitis?

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  • Written By: Soo Owens
  • Edited By: Susan Barwick
  • Last Modified Date: 03 October 2019
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Eastern equine encephalitis (EEE)is a rare viral disease transmitted through the bite of an infected mosquito. In most cases no symptoms are present. Severe cases of EEE can lead to encephalitis, the inflammation of the brain, which is most often fatal. People between the ages of 15 and 50 years old are the most susceptible.

Eastern equine encephalitis is an arthropod borne, arboviral, disease, meaning it is contracted through interaction with a blood-sucking arthropod, in this case the mosquito. The virus typically originates in a mosquito-bird-mosquito cycle involving the mosquito species Culiseta melanura. The virus is then occasionally spread to horses and humans through the Coquillettidia perturbans or the more common Aedes canadensis species.

Eastern equine encephalitis is divided into two variants, North and South American. EEE is most commonly found in those states east of the Mississippi River, especially in areas near or containing large freshwater water supplies, swamps, and marshes that are surrounded by wooded areas. Outside of the U.S., eastern equine encephalitis has also been found in gulf coastal areas. The mosquitoes that transmit the virus from birds usually die in the winter months, limiting the range of infections to the fall and summer periods.


The spread of eastern equine encephalitis is dependent on arthropods and, thus, is very limited in geographical scope. No substantial amount of the virus is transmitted after infection. As a result, it is highly unlikely that another mosquito could contract the disease from a horse or human who has been infected.

The first symptoms of eastern equine encephalitis are high fever, usually 103°F to 106°F (39.5°C to 41.11°C), headache, nausea and diarrhea, each occurring five to ten days after infection but not always simultaneously. The lack of specific early symptoms makes EEE difficult to diagnose right away. The disease progresses rapidly to central nervous system disfunction and death.

The most dangerous and serious development to occur as a result of EEE is the onset of encephalitis. The early symptoms of this do not differ much from the first symptoms and manifest as headaches, fever, confusion, drowsiness, and fatigue. As the encephalitis progresses the symptoms may expand to include seizures, in roughly 50% of those infected, and tremors, hallucinations, photophobia, and memory problems.

There is no known cure or treatment for eastern equine encephalitis. The mortality rate is estimated to be between 30%-70%. The morbidity rate is around 90%, with most survivors becoming permanently disabled. Only 10% of those infected recover completely. Since 1964, there have only been 163 confirmed cases of EEE, with most years reporting fewer than five cases.

The disease is preventable. Ensuring that one's body is fully clothed and using insect repellent are the best known preventative measures. One should also remain indoors between the hours of dusk and dawn, when mosquitoes are most active, during the summer months.


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