image of mosquito
Credit: CDC, James Gathany
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This article waslast modified on October 3, 2019.

This summer, at least ten people have died from Eastern equine encephalitis (EEE). Eastern equine encephalitis virus (EEEV) belongs to a group of viral infections spread by the bites of infected mosquitos—like West Nile virus (WNV). Recent reports have brought attention to this infection because the confirmed cases of EEE this year are notably higher than in most previous years.

EEE is very rare with usually only five to ten human cases confirmed each year, but it can be deadly. So far this year, there have been more than two dozen cases of EEEV infection, according to reporting by Stat, with deaths in Connecticut, Massachusetts, Michigan, and Rhode Island. While some individuals may not develop any symptoms, according to the Centers for Disease Control and Prevention (CDC), approximately 30% of people who develop symptoms of brain infection from EEEV will die. By comparison, there have been over 543 cases of WNV reported so far this year, with 25 deaths—a death rate of about 5%.

Like WNV, EEEV is an arbovirus (arthropod-borne virus) that is transmitted by mosquitoes. Mosquitoes acquire the virus from infected aquatic birds and then transmit the virus when they bite humans (and other mammals, including horses). Importantly, it is not spread directly from person to person or animal to person. The virus is usually found in the Great Lakes region and Eastern and Gulf Coast states, in freshwater swamps. People who live in or visit these areas are most at risk of infection. Most cases have previously been reported from Florida, Massachusetts, New York, and North Carolina.

EEEV can cause two forms of illness:

  • One form affects the body as a whole (systemic), with rapid onset of symptoms such as fever, chills and joint and muscle pain. The illness develops four to ten days after a bite from an infected mosquito and usually resolves completely after about one to two weeks without any notable side effects.
  • The other form can develop suddenly in infants or, in older children and adults, a few days after the systemic illness when the virus infects the brain, causing swelling (encephalitis) as well as high fever, headache, irritability, restlessness, drowsiness, vomiting, diarrhea, convulsions, and coma.

If infected, people older than age 50 and younger than age 15 have a higher risk of serious illness. Even healthy individuals can experience a rapid onset of severe illness, develop encephalitis and die within days. People who survive the encephalitis form of the disease may have mild to severe brain damage.

EEEV infection is typically diagnosed with laboratory tests that detect antibodies made by the body against the virus in blood or spinal fluid. This testing is somewhat specialized and not routinely available at local hospitals or clinics, so samples may be sent to reference laboratories or public health laboratories for testing. Molecular tests that detect the virus itself are not routine but are available through a few select public health laboratories and the CDC.

There is no specific anti-viral treatment for EEE, so treatment is generally supportive. People who are seriously ill may need to be hospitalized and receive help for breathing as well as intravenous (IV) fluids.

Currently, there is no commercially available vaccine for the general public to prevent EEE, so the most effective protection is preventing mosquito bites in the first place:

  • Use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing and follow the directions on the package. Also, remember to reapply the repellent if you will be outdoors for a prolonged period of time.
  • Wear long sleeves and pants if you'll be exposed to mosquitos.
  • Install secure, intact screens on windows and doors to keep mosquitoes out.
  • Eliminate mosquito breeding sites by emptying standing water from flowerpots, buckets, barrels, trash cans, and other containers.
  • Avoid going outside during dawn and dusk, when mosquitos are most active.

According to the CDC, illnesses from insect bites in general have been on the rise in the last few years. Among the possible reasons: an increase in the number of people exposed to areas where mosquitos and other insects are prevalent, suburban reforestation, increased clinician awareness regarding these infections, and the lack of vaccines. (For more details, read the article Illnesses from Insect Bites on the Rise.)

View Sources

Eastern Equine Encephalitis. Commonwealth of Massachusetts. Available online at https://www.mass.gov/service-details/eee-eastern-equine-encephalitis. Accessed September 25, 2019.

What to know about EEE, a mosquito-borne virus on the rise. Stat. September 23, 2019. Available online at https://www.statnews.com/2019/09/23/what-to-know-about-eee/. Accessed September 25, 2019.

A Tenth Person Has Died from EEE Infection. CNN.com. Available online at https://www.cnn.com/2019/09/25/health/eee-tenth-us-death-massachusetts/index.html. Accessed September 25, 2019.

Eastern Equine Encephalitis. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/easternequineencephalitis/index.html. Accessed September 25, 2019.

(September 19, 2019) Marisa Iati. He was 'perfectly healthy' before the mosquito bite. Nine days later, he was brain dead. Washington Post. Available online at https://www.washingtonpost.com/health/2019/09/19/he-was-perfectly-healthy-before-mosquito-bite-nine-days-later-he-was-brain-dead. Accessed September 25, 2019.