Eastern Equine Encephalitis

Eastern equine encephalitis (EEE, or "Triple E") is a rare but serious disease caused by a virus that spreads through the bite of an infected mosquito. The disease also commonly affects horses, but it has been reported in other animals like alpacas, donkeys, pheasants, and emus.

There are no human vaccines to prevent or medicines to treat EEE. The best way to reduce your risk of infection with EEE virus is by preventing mosquito bites: limit time outdoors at dawn and dusk, use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear with permethrin, and control mosquitos indoors and outdoors.

EEE Virus in Vermont

EEE was first detected in Vermont in 2011. Each year during June through mid-October, mosquitoes are collected from sites around the state and tested for EEE virus at the Health Department Laboratory.

During the 2024 mosquito surveillance season, 86 groups of mosquitoes tested positive for EEE virus across 16 towns, and there were two human cases of EEE reported in Chittenden County - the first human cases in Vermont since 2012.

EEE virus activity in Vermont clusters near acidic, hardwood swamps, most commonly in Franklin, Grand Isle, Addison, and northern Rutland counties. However, EEE virus could be circulating in other parts of the state, so all Vermonters should take precautions to prevent mosquito bites.

EEE has been reported in several animals in Vermont. In 2024, two horses from Addison and Orleans counties died from the disease.

This map shows areas of risk for EEE virus transmission based on the latest surveillance information. People in these areas should be aware and take extra precautions against mosquito bites.
State and local officials enhance their response as the level of risk increases in these areas during mosquito season.
Mosquitos can pose a risk of transmitting illnesses to humans anywhere in Vermont. So everyone should take steps to prevent mosquito bites during mosquito season.
For more about EEE virus risk categories and response, see the Arbovirus Surveillance Plan.

See the weekly surveillance results

What you can do

The best way to prevent EEE, and other viruses like West Nile Virus, is to take steps to prevent mosquito bites and control mosquitos.

Anyone who spends time in an area where EEE virus activity has been identified is at risk of infection. People should take steps to avoid mosquito bites, especially in late July through September. People who engage in outdoor work and recreational activities, especially near wetlands and swamps, are at increased risk of infection.

Symptoms

Most people infected with EEE virus do not develop symptoms. For those who develop symptoms, the time from an infected mosquito bite to onset of illness (incubation period) ranges from four to 10 days. You can't tell if you have EEE virus just by looking at the mosquito bite. The bite itself looks like any other mosquito bite.

EEE virus infection can result in febrile (fever) illness or neurologic disease. The type of illness will depend on the age of the person and other factors.

Talk to your health care provider if you have any concerning symptoms, including neurological symptoms listed below.

Febrile (Fever) Illness

Symptoms may include:

Fever
Chills
Body aches
Joint pain

The illness lasts one to two weeks, and most people recover completely when there is no central nervous system involvement.

Neurologic Disease

Neurologic disease may include meningitis (inflammation of the membranes that surround the brain and spinal cord) or encephalitis (inflammation of the brain).

Signs and symptoms of neurologic disease may include:

Fever
Headache
Vomiting
Diarrhea
Seizures
Behavioral changes
Drowsiness
Coma

In infants, neurologic disease often occurs soon after onset. In older children and adults, encephalitis might occur after several days of systemic illness.

About a third of all people who develop severe EEE die. Death usually occurs two to 10 days after onset of symptoms but can occur much later.
Many people who recover are left with long-term physical or mental problems. These can range from mild to severe intellectual disability, personality disorders, seizures, paralysis, and cranial nerve dysfunction.
People with severe disease and ongoing disabilities often require long-term care and die within a few years.

Treatment

There are no medications to prevent or treat EEE virus infections. Antibiotics are not effective against viruses. Rest, fluids, and over-the-counter pain medications may relieve some symptoms.

For severe disease, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

How it Spreads

EEE spreads to humans through the bite of an infected mosquito. It does not spread from person to person. EEE virus typically infects birds, and in most years it remains in bird populations, posing little danger to humans and horses. Mosquitoes become infected by feeding on infected birds. If an infected mosquito bites a horse, human or other susceptible animal, the animal or person can become sick. The virus that causes EEE is only spread by mosquitoes. People, horses and deer do not spread the disease.

Information for Health Care Professionals

Eastern equine encephalitis illness is required to be reported to the Vermont Department of Health within 24 hours. Report cases by calling 802-863-7240 (24/7).

EEE should be considered in any person with a febrile or acute neurologic illness who has had recent exposure to mosquitoes—especially during late summer months.

In addition to other more common causes of encephalitis and aseptic meningitis (e.g., herpes simplex virus and enteroviruses), other arboviruses (e.g., West Nile and Powassan virus) should also be considered in the differential etiology of suspected EEE illness.

A rapid and accurate diagnosis of acute arboviral disease can be made by the detection of virus-specific IgM antibody in serum or cerebrospinal fluid (CSF). The detection of only IgG antibody is not suggestive of an acute infection. PCR on a CSF sample can also be diagnostic when testing is done early in the course of illness.

WNV and EEE virus antibody tests are available commercially. However, a positive IgM test result should be confirmed by neutralizing antibody testing through the Health Department Laboratory. At least 0.5 mL of serum or 1.0 mL of CSF is required for confirmatory testing.

Read the Health Advisory (August 29, 2024)

Find information for health care providers about EEE from CDC:

Surveillance and Response Plan

The Vermont Arbovirus Surveillance and Response Plan includes information about education, surveillance and guidance about the state’s response to EEEV and West Nile Virus (WNV) detections. This plan is based upon the best available scientific information and incorporates guidelines from CDC and the recommendations of the Vermont Agency of Agriculture, Food and Markets and the Vermont Department of Health.