EEEK

By Peter Galvin, MD

Eastern equine encephalitis (EEE) is a potentially life-threatening viral infection spread by mosquitoes in the eastern U.S. from Texas to Maine. It is seen mostly in coastal and wooded areas near marshes and swamps in the Atlantic, Gulf Coast, and Great Lakes regions of the U.S. The disease is seasonal, with most cases seen from June through October. The highest number of cases occur in late August. In 2024, 19 cases of EEE were reported in the U.S. The number of reported cases is low because 95% of those infected have no symptoms. Those who do develop symptoms will experience fever, chills, muscle aches and joint pain, which typically occur four to 10 days after being bitten. In those who have symptoms, 5% will go on to develop neuroinvasive disease.

Those at risk of EEE are those who live or work in areas near swamps and marshes. These include people working in agriculture or participating in outdoor activities such as camping and gardening. Neurological disease (neuroinvasive disease) includes swelling of the brain (encephalitis) or inflammation of the tissues surrounding the brain and spinal cord (meningitis). Neurological symptoms include headache, confusion, neck pain or stiffness, sensitivity to light (photophobia), seizures and/or coma. Those at the highest risk of neurological disease are younger than 15, older than 50, and those who are immunocompromised (such as organ transplant recipients and those on immunosuppressive medications).

EEE is transmitted by the bite of an infected mosquito (usually the Culiseta melanura mosquito). The mosquito is known as the vector. The reservoir for the virus (where it originates) are passerine birds, known as perching birds, which have three forward toes and one rear toe. These birds make up over 50% of the U.S. bird population. After the mosquito bites an infected bird, it transmits the virus by biting a horse or a human. While human fatality from EEE is very rare, the fatality rate in horses is 70% to 90%. The first known epidemic in horses occurred in the 1830s, and the first known human epidemic occurred in 1933 in coastal New Jersey, Delaware, Maryland, and Virginia.

EEE can be detected by a blood test for antibodies. Patients with severe symptoms and disease require hospitalization for supportive care, which often includes mechanical ventilation. EEE does not respond to antiviral medications, and there is no vaccine for it (although there is a vaccine for horses). From 2005 to 2024 there were 193 cases of EEE reported to the CDC, 94% had neuroinvasive disease, and 41% died. Those who survive neurological disease have a greater than 50% chance of long-term complications, including mild to severe problems with thinking and memory, personality disorders, seizures, paralysis, and drooping or difficulty moving one or both sides of the face.

Prevention of EEE involves measures to prevent mosquito bites by wearing long-sleeved clothing and using FDA-registered insect repellants. Surveillance by state and local health officials of mosquitoes and sick animals, such as horses and emus, can help identify the virus in certain areas and guide prevention measures using compounds to kill developing and adult mosquitoes.

For more information go to the website of the Centers for Disease Control and Prevention at www.cdc.gov

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