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Center of Excellence for Emerging and Zoonotic Animal Diseases

Pathogen of the Month




September pathogen of the month: Eastern Equine Encephalitis


What is it?

Eastern Equine Encephalitis is a rare cause of brain infections in humans, and is more prevalent among horses. The Centers For Disease Control says approximately 30 percent of people with EEE die, and many survivors have ongoing neurological problems.


What is its history?

EEE was first identified as a specific disease in the 1930s.


How does the infection spread? The CDC says thatEastern equine encephalitis virus (EEEV) is maintained in a cycle between a particular species of mosquito and avian hosts in freshwater hardwood swamps. The mosquito is not considered to be an important vector of EEEV to humans because it feeds almost exclusively on birds. Transmission to humans requires mosquito species capable of creating a “bridge” between infected birds and uninfected mammals such as some Aedes, Coquillettidia, and Culex species.

Horses, however, are susceptible to EEEV infection and some cases are fatal. EEEV infections in horses, however, are not a significant risk factor for human infection because horses (like humans) are considered to be “dead-end” hosts for the virus.


What are the symptoms? EEEV infection can result in one of two types of illness, systemic or encephalitic (involving swelling of the brain, referred to below as EEE). The type of illness will depend on the age of the person and other host factors.

Systemic infection has an abrupt onset and is characterized by chills, fever, malaise, arthralgia, and myalgia. The illness lasts 1 to 2 weeks, and recovery is complete when there is no central nervous system involvement. In infants, the encephalitic form is characterized by abrupt onset; in older children and adults, encephalitis is manifested after a few days of systemic illness. Signs and symptoms in encephalitic patients are fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, cyanosis, convulsions, and coma.


Are there treatments or vaccines? In horses, vaccinations can prevent onset of the disease. In humans, there is no specific treatment for EEE. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections.


Are there current outbreaks? Outbreaks are underway in several states, with at least one fatality.A Massachusetts women died of EEE in late August. Prior to that incident, the state's Department of Public Health had confirmed a third human case of eastern equine encephalitis (EEE. There are also three suspected cases of the illness in Michigan residents, according to the Michigan Department of Health and Human Services [MDHHS]. In New Jersey, the disease was diagnosed in late August in a horse and also in an alpaca.



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Sources: Centers for Disease Control, CEEZAD