Eastern equine encephalitis (EEE) is a rare but serious disease caused by a virus that spreads through the bite of an infected mosquito. Only a few human cases are reported in the United States each year. 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: use insect repellent, wear long-sleeved shirts and long pants, and control mosquitos indoors and outdoors. 

EEE Virus in Vermont

From July through mid-October, mosquitos are collected from sites around the state and tested for EEE at the Health Department Laboratory. In 2023, EEE virus was detected in mosquito pools collected in Alburgh, Swanton, and Highgate, and an unvaccinated horse in Swanton died after being infected with the EEE virus. These were the first detections of EEE virus in Vermont since 2015. 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 was first detected in Vermont in 2011. There have been no human cases of EEE in Vermont since 2012 and EEE has been reported in 6 animals in Vermont. 

Eastern equine encephalitis (EEE) Risk Map
  • 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 Response Plan.

See the weekly surveillance results

In the Region

In the Northeast, most human cases have occurred in Massachusetts, and outbreaks have historically occurred in the southeastern part of that state every 10 to 20 years. New Hampshire has had a recent increase in reported cases. Since 2005, New Hampshire has recorded nine human cases.

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.


Most people with EEE virus do not develop symptoms. For those who do, they typically occur four to 10 days after a bite from an infected mosquito. 

Most people who do get sick experience a flu-like illness with fever, chills, body aches, and joint pain. This illness can last one to two weeks, and most people recover completely when there is no central nervous system involvement.

In rare cases, infection of the brain and spinal cord occurs, causing sudden high fever (103° to 106°F), stiff neck, and a headache that keeps getting worse. Inflammation and swelling of the brain, called encephalitis, can occur. The disease can sometimes worsen quickly. About one-third of people who develop encephalitis from an infection with the EEE virus will die. Among those who survive, many are left with mild to severe disabilities.

People over age 50 or younger than age 15 are at greatest risk for developing severe disease.


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.

Read more about symptoms, diagnosis, and treatment

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.

Read more about EEE transmission

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.

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.


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