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Mosquito-borne Eastern Equine Encephalitis virus: What to know

EEE one of most dangerous mosquito-borne diseases

Eastern Equine Encephalitis/Mosquito.
Eastern Equine Encephalitis/Mosquito. (CDC/Pexels)

Cases of the rare mosquito-borne virus Eastern Equine Encephalitis, known as EEE, are popping up in Michigan and other U.S. states.What is Eastern equine encephalitis (EEE)?

EEE is a rare disease that is caused by a virus spread by infected mosquitoes. EEE virus (EEEV) is one of a group of mosquito-transmitted viruses that can cause inflammation of the brain (encephalitis). In the United States, approximately 5-10 EEE cases are reported annually.

Between 2010 and 2019, Michigan reported 17 cases of EEE, second-most in the U.S., behind Massachusetts. In 2019, the U.S. confirmed the most cases of EEE in a single-year, with 38 human cases.

EEE is one of the most dangerous mosquito-borne diseases in the United States, with a 33 percent fatality rate in people who become ill and a 90 percent fatality rate in horses that become ill.

Here some answers to frequently asked questions on EEE from the Centers for Disease Control:

How do people get infected with EEEV?

EEEV is transmitted through the bite of an infected mosquito. Disease transmission does not occur directly from person to person.

Where and when have most cases of EEE occurred?

Most cases of EEE have been reported from Atlantic and Gulf Coast states. Cases have also been reported from the Great Lakes region. EEE cases occur primarily from late spring through early fall, but in subtropical endemic areas (e.g., the Gulf States), rare cases can occur in winter.

Who is at risk for infection with EEEV?

Anyone in an area where the virus is circulating can get infected with EEEV. The risk is highest for people who live, work, or participate in outdoor activities in or around wooded swampy areas in the eastern United States.

Persons younger than age 15 and over age 50 are at greatest risk of severe disease following infection.

How soon do people get sick after getting bitten by an infected mosquito?

It takes 4 to 10 days after the bite of an infected mosquito to develop symptoms of EEE.

What are the symptoms of EEEV disease?

Most people who become infected with EEEV show either no or mild symptoms such as fever and joint or muscle pain. Severe cases of EEEV infection include encephalitis (swelling of the brain) or meningitis (swelling of the membranes that surround the brain and spinal cord) and begin with the sudden onset of fever, chills, headache, and vomiting. The illness may then progress into disorientation, seizures, and coma. Approximately a third of patients who develop encephalitis due to EEEV infection die, and many of those who survive have mild to severe brain damage.

How is EEE diagnosed?

Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.

What is the treatment for EEE?

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.

How can people reduce the chance of getting infected with EEEV?

Prevent mosquito bites. There is no approved vaccine for people or preventive drug.

  • Use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing. The repellent/insecticide permethrin can be used on clothing to protect through several washes. Always follow the directions on the package.
  • Wear long sleeves and pants when weather permits.
  • Have secure, intact screens on windows and doors to keep mosquitoes out.
  • Eliminate mosquito breeding sites by emptying standing water from flower pots, buckets, barrels, and other containers. Drill holes in tire swings so water drains out. Keep children’s wading pools empty and on their sides when they aren’t being used.

What should I do if I think a family member might have EEE?

Consult your healthcare provider for proper diagnosis.

Where did EEE come from?

According to Vector Disease Control International, eastern equine encephalitis virus (EEEV) is a zoonotic alphavirus and arbovirus, and was first recognized in horses in 1831 in Massachusetts. The first confirmed human cases were identified in New England in 1938.

How is EEE transmitted?

Eastern equine encephalitis virus (EEEV) is maintained in a cycle between Culiseta melanura mosquitoes and avian hosts in freshwater hardwood swamps. Cs. melanura 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 AedesCoquillettidia, and Culex species.

Horses are susceptible to EEEV infection and many 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 (i.e., the concentration of virus in their bloodstreams is usually insufficient to infect mosquitoes). There is a vaccine to prevent EEEV infection in horses.

Eastern equine encephalitis virus (EEEV) transmission.
Eastern equine encephalitis virus (EEEV) transmission. (CDC)

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