Eastern equine encephalitis (EEE) virus is spread to people by the bite of an infected mosquito. Only a few cases are reported in the United States each year. Although uncommon, EEE is one of the most severe mosquito-transmitted diseases in humans. About 30% of those infected who have serious symptoms die and half of the survivors have lasting neurological symptoms.
EEE tends to occur in the late summer and early fall months. EEE can infect humans, birds, mosquitoes, horses and some other mammals.
Which Mosquito Is the Primary Vector?
The mosquito species that is the primary EEE carrier is found in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region. It does not bite people. Sometimes, though, the virus escapes from its marsh habitat by means of other mosquito species that feed on both birds and mammals that have been infected. Experts suggest that EEE transmission to humans is most likely associated with the “cattail mosquito."
Where EEE Is Found
From 2009-2022 there were 129 reported human cases, with more than 60% of these occurring in five states – MA, MI, FL, GA, and NC. Most cases occur in the Atlantic and Gulf Coast states. (See map below.)
Most people infected with this rare virus never develop any symptoms. Others get only a mild flu-like illness. The time from an infected mosquito bite to onset of illness (incubation period) ranges from 4 to 10 days. Milder symptoms include:
- Sore throat
Though rare, EEE can be serious. Fewer than one percent of those infected develop severe symptoms such as encephalitis or meningitis (inflammation of the brain or surrounding tissues). One in three individuals who develop these serious EEE symptoms die. About half of the survivors have mild to severe brain damage.
More Severe EEE Symptoms May Include
- High fever
- Bluish discoloration of the skin
Who Is Most at Risk?
Anyone can get EEE, but those at increased risk include individuals living in or visiting areas where the disease is circulating in the environment and occurs most often (see map). Also at risk are outdoor workers and participants in outdoor recreational activities in EEE-prone areas. Persons over age 50 and under age 15 seem to be at greatest risk for developing severe disease when infected with EEE.
There are no vaccines to prevent or medicines to treat EEE. Hospitalization and supportive care may be needed.
Mosquitos capable of infecting humans with EEE are active principally during the early evening hours, but have been known to bite in shady areas where mosquitoes rest during daylight hours. Use repellents containing DEET (or other EPA-registered active ingredients) according to label directions to help prevent bites. The American Academy of Pediatrics and the Centers for Disease Control and Prevention note that products containing up to 30% DEET are appropriate for children over two months of age. For more information, please visit our prevention page.