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What is Eastern equine encephalitis?

Reprinted from Mother Nature Networks (

Written by Mary Jo Dilonardo

Another mosquito-transmitted illness is making headlines as the annoying insect continues to earn its reputation as the world's deadliest animal.

The Eastern equine encephalitis (EEE) virus was confirmed in seven states this summer, according to the Centers for Disease Control and Prevention (CDC). As of late September, the CDC reported 28 cases — including 10 in Massachusetts and eight in Michigan — with seven deaths so far.

What is Eastern equine encephalitis?

According to the CDC, the EEE virus can cause encephalitis, which is rare inflammation of the brain. In the United States, there are usually only five to 10 cases of EEE reported each year. It can only be transmitted by the bite of an infected mosquito. Some of the mosquitoes that can spread the virus to humans include the Aedes, Coquillettidia and Culex species. EEE can't be spread from person to person or from horses to people.

The disease was first recognized in 1831 when 75 horses died of a similar condition in Massachusetts, according to a Purdue University newsletter edited by virologist Dr. Roman Pogranichniy. The virus was isolated two years later when a significant outbreak of encephalomyelitis occurred in horses in parts of Delaware, Maryland, New Jersey and Virginia, along the coast.

Symptoms of EEE

Early symptoms of EEE are very similar to the flu. They often begin with headache, high fever, chills, muscle aches and vomiting. The illness can progress with more severe symptoms including seizures, disorientation and coma. About one-third of those who get the virus die, according to the CDC. Many of those who survive can have mild to severe brain damage.

Who is at risk of EEE?

Most cases of EEE have been reported from Gulf Coast and Atlantic states, although the virus has also been found in the Great Lakes area and scattered throughout the country. It's most commonly contracted in late spring through early fall, reports the CDC. Anyone who is in an area where the virus is spreading can become infected. The risk is greater for people who spend more time outdoors, particularly in areas where there are mosquitoes.

Is there a treatment?

Although there is an effective vaccine for horses, there is no EEE vaccine for people.

There is also no specific antiviral treatment for EEE, so the only treatment is supportive care and, of course, prevention. Ward off and kill mosquitoes by using insect repellent, fans, protective clothing and preventative measures to stop them from breeding in your yard.

"The key to staying safe is protection,” said Sam Telford, professor at Cummings School of Veterinary Medicine at Tufts University and expert on disease spread by ticks and mosquitoes. “Even though EEE is rare, there’s no way to know where and when it can strike.”

Editor's note: This story has been updated with new information since it was originally published in August 2019.

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