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EEE Virus: Mosquito-Borne

EEE Virus: Mosquito-Borne

Eastern Equine Encephalitis (EEE) is a rare cause of brain infections (encephalitis). Only a few cases are reported in the U.S. each year. Most occur in eastern or Gulf Coast states. Approximately 30% of people with EEE die, and many survivors have ongoing neurologic problems.

EEE was first recognized in horses in 1831 in Massachusetts. The first confirmed human cases were identified in New England in 1938.

According to CNN (9-25-19), 10 people have died from this mosquito-borne illness in 2019. EEE virus has caused 27 human cases in 6 states since August 2019. In 2018 there were 6 reported cases.

EEE is transmitted to humans (and horses) through the bite of an infected mosquito

  • The primary transmission cycle takes place in and around swampy areas

  • About 4-5% of human EEE virus infections result in EEE

  • Infection is thought to confer life-long immunity against re-infection

  • In the U.S., an average of 7 human cases of EEE are reported annually

The incubation period of EEE (the time from infected mosquito bite to onset of illness) ranges from 4 to 10 days. It can result in one of two types of illness:

Systemic (affecting the entire body)

  • Chills

  • Fever

  • Fatigue

  • Muscle and joint pain

The illness lasts 1 to 2 weeks, and recovery is complete when there is no nervous system involvement.

Encephalitis: (affecting the brain)

  • Fever

  • Headache

  • Irritability

  • Restlessness

  • Drowsiness

  • Loss of appetite

  • Vomiting and diarrhea

  • Cyanosis (bluish coloring)

  • Convulsions

  • Coma

About 1/3 will die from EEE. Many are left with disabling and progressive mental and physical complications, which can range from minimal brain dysfunction to severe intellectual impairment, personality disorders, seizures, paralysis, and cranial nerve dysfunction. Many patients with severe complications die within a few years.


  • Preliminary diagnosis is often based on the patient’s symptoms, places and dates of travel, activities, and epidemiologic history of the location where the infection occurred

  • Laboratory test of blood and spinal cord fluid

Treatment: There is no vaccine or anti-viral treatment available

Prevention: Prevent mosquito bites

Insect Repellent: Use Environmental Protection Agency (EPA)-registered insect repellents with one of the active ingredients below:

  • DEET

  • Picaridin

  • IR3535

  • Oil of lemon eucalyptus

  • Para-menthane-diol (PMD)

  • 2-undecanone

Always follow the product label instructions.

Reapply insect repellent as directed.

If you are also using sunscreen, apply sunscreen first and insect repellent second.


  • Wear long-sleeved shirts and long pants

  • Apply insect repellent to clothing and gear. Don’t apply directly to the skin


  • Use screens on windows and doors. Repair holes in screens to keep mosquitoes outdoors

  • Use air conditioning, if available

  • Stop mosquitoes from laying eggs in or near water: Once a week, empty and scrub, turn over, cover, or throw out items that hold water, such as tires, buckets, planters, toys, pools, birdbaths, flowerpots, or trash containers

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