Eastern Equine Encephalitis (EEE)

Fact sheet about Eastern Equine Encephalitis (EEE)

What is Eastern Equine Encephalitis?

Eastern equine encephalitis (EEE) is a rare but serious disease caused by a virus.

How is EEE spread?

The virus that causes EEE is primarily spread through the bite of an infected mosquito. Mosquitoes become infected when they bite infected birds. Although humans and several other types of mammals, particularly horses and llamas, can become infected, they do not spread disease. Rarely, the virus can also be transmitted through organ transplants.

How common is EEE in Massachusetts?

EEE is a very rare disease. Since the virus was first identified in Massachusetts in 1938, just over 122 cases have occurred. The majority of cases typically have been from Bristol, Plymouth, and Norfolk counties. However, in an active year human cases can occur throughout the state.

Outbreaks of EEE usually occur in Massachusetts every 10-20 years. These outbreaks will typically last two to three years. The most recent outbreak of EEE in Massachusetts began in 2019 and included twelve cases with six fatalities. The outbreak continued in 2020 with five cases including one fatality. A small outbreak occurred in 2024 with four cases and one fatality. No cases occurred in 2025.

What are the symptoms of EEE?

Symptoms can include high fever (103-106º F), stiff neck, headache, change in mental status, seizures, vomiting, diarrhea, and fatigue. Encephalitis (inflammation and swelling of the brain), is the most dangerous and frequent serious complication. The disease gets worse quickly and some patients may go into a coma within a week.

How soon do symptoms of EEE appear?

Symptoms can begin within 4-10 days of being bitten by an infected mosquito.

How is EEE diagnosed?

Serum (a component of blood) or cerebrospinal fluid (CSF) can be collected and tested for antibodies against the virus or for viral genetic material. 

How is EEE treated?

There is no specific treatment once infection with EEE virus has occurred. Treatment consists of supportive care, rest, and fluids to prevent dehydration. In Massachusetts, the fatality rate is 30-50%.  Individuals who survive may be permanently disabled.  Few people recover completely.   

How can you prevent EEE?

Preventing mosquito bites is the best way to prevent EEE infection. Here are some things you can do to reduce your chances of being bitten: 

• Use EPA registered mosquito repellents.
• Products with DEET may be used on skin. Repellents containing DEET should not be used on children less than 2 months of age and should be used in concentrations no higher than 30% on older children.
• Use permethrin products on items such as clothing, shoes, bed nets and camping gear. Permethrin should not be applied to skin.
• Other repellents, such as picaridin, oil of lemon eucalyptus and IR 3535, have also been found to provide protection against mosquitos. More information on choosing a repellent and how to use repellents safely is included on the DPH Public Health Fact Sheet on Mosquito Repellents at mass.gov/DPH/Mosquito.
  • Oil of lemon eucalyptus should not be used on children under three years of age.
• When outdoors, wear long pants, a long-sleeved shirt and socks. This will help keep mosquitos away from your skin.
• Schedule outdoor events to avoid the hours between dusk and dawn, when mosquitoes are most active.
• Keep mosquitoes out of your house by repairing any holes in your screens and making sure they are tightly attached to all your doors and windows.
• Remove areas of standing water around your home:
  • Look around outside your house for containers and other things that might collect water and turn them over, regularly empty them, or dispose of them.
  • Drill holes in the bottom of recycling containers that are left outdoors so that water can drain out. 
  • Clean clogged roof gutters; remove leaves and debris that may prevent drainage of rainwater.
  • Turn over plastic wading pools and wheelbarrows when not in use.
  • Change the water in birdbaths every few days; aerate ornamental ponds or stock them with fish.
  • Keep swimming pools clean and properly chlorinated; remove standing water from pool covers.
  • Use landscaping to eliminate standing water that collects on your property.

Where can I get more information on EEE?

• For questions about your own health, contact your health care provider.
• For questions about diseases spread by mosquitos, contact DPH at [phone] or online at mass.gov/DPH/mosquito. You may also contact your local Board of Health.
  • Mass.gov | Mosquito-borne Diseases
  • Mass.gov | Arbovirus Dashboard
• Health effects of pesticides, DPH, Bureau of Climate and Environmental Health at [phone].
• Mosquito control in your city or town: Mosquito control in Massachusetts is conducted through eleven mosquito control districts.  The State Reclamation and Mosquito Control Board (SRMCB) oversees all nine districts. Contact information for each district can be found online at State Reclamation and Mosquito Control Board (SRMCB) webpage. You may also contact the SRMCB within the Massachusetts Department of Agricultural Resources at 617-626-1700 or your local board of health.
• CDC.gov | Eastern Equine Encephalitis