Eastern equine encephalitis, also known as EEE or triple E, is a rare mosquito-borne viral disease that has killed one person and infected at least five others in the United States this year. The deadly virus, which is circulating in the Northeast, is stoking fears about outbreaks and putting some communities on high alert.
Just days after New Hampshire officials announced the death of a healthy 41-year-old man from the virus, a second human case of EEE was confirmed in Massachusetts. Cases have also been reported in at least three other states.
Eastern equine encephalitis is a rare but serious infection caused by a virus spread to humans by mosquitoes. The EEE virus can attack the central nervous system and cause brain damage or death.
Here's what to know:
EEE outbreak 2024
At least five states in the U.S. have confirmed human cases of eastern equine encephalitis so far this year. Massachusetts in particular is taking precautions to reduce the spread of the triple E virus, which has also been reported in Vermont, Wisconsin, New Jersey and New Hampshire.
On Thursday, the Massachusetts Department of Public Health confirmed the state's second human case of eastern equine encephalitis this year. The patient is a woman in her 30s who was exposed in Plymouth County, officials said in a press release.
The first EEE case reported in Massachusetts this year was a man in his 80s, who was exposed to the virus in Worcester County, officialssaid on Aug. 16. The state has also reported two horse cases.
Ten communities in Massachusetts are now considered to be at critical or high risk for EEE.
"We have not seen an outbreak of EEE for four years in Massachusetts,” Massachusetts' health department commissioner Dr. Robbie Goldstein said in a press release. “This year’s outbreak and activity raise the risk for communities in parts of the state. We need to use all our available tools to reduce risk and protect our communities. We are asking everyone to do their part.”
Massachusetts officials are spraying for mosquitos by plane and truck in high-risk areas to control eastern equine encephalitis. Several towns have announced nighttime park closures and voluntary curfews to limit exposure from dusk to dawn, when mosquitoes are most active.
Eastern equine encephalitis is spread to people through the bite of an infected mosquito, according to the U.S. Centers for Disease Control and Prevention. Infection can cause swelling of the brain and spinal cord, and about one-third of people with EEE will die.
The EEE news comes as another virus spread by mosquitoes, West Nile, circulates in the U.S. In early August, a woman in Dallas, Texas, died after contracting West Nile virus. Last week, Dr. Anthony Fauci, the former chief medical advisor to Joe Biden, was hospitalized with a case of West Nile virus.
“We’re at a critical time of the year for mosquito-borne illnesses of all kinds, including triple E, which has a higher risk of leading to hospitalizations and potentially being fatal,” Dr. Kavita Patel, primary care physician and NBC News medical contributor, said on TODAY in a segment aired Aug. 26.
What is the triple E virus?
EEE stands for eastern equine encephalitis, a rare but life-threatening infection caused by the eastern equine encephalitis virus, which can infect mosquitoes, humans and a range of animals. The virus is found in North America and the Caribbean, according to the CDC.
In humans, the virus can affect the central nervous system, namely the brain and spinal cord, and cause severe disease or death.
There are typically only a handful of human cases of eastern equine encephalitis in the U.S. each year, but annual case count can vary, according to the CDC.
“You’ll see a couple of cases each year, then some years have explosive outbreaks, where everything comes together — the mosquitoes, the birds, the migration, the bird immunity — and you’ll see dozens and dozens of cases,” Dr. Daniel Pastula, chief of neuro-infectious diseases and global neurology at the University of Colorado School of Medicine, tells TODAY.com.
There were seven cases the U.S. in 2023, one in 2021, and 13 cases in 2020. In 2019, the U.S. saw 38 cases, the highest annual count in over a decade.
Most cases are reported from July through October, says Pastula. Infections peak during the summer months during mosquito season, but cases can continue into the fall depending on the region and climate, per the Cleveland Clinic.
What is the EEE death rate?
About 30% of people with eastern equine encephalitis will die, as the infection can cause inflammation and swelling of the brain, Pastula says. Survivors often suffer from lasting neurological problems, per the CDC.
How does eastern equine encephalitis spread to humans?
In nature, the eastern equine encephalitis virus is maintained through a transmission cycle between mosquitoes — specifically, the species Culiseta melanura or the "black-legged mosquito" — living freshwater hardwood swamps and wild birds, which are reservoirs for the virus, Pastula notes.
Because this species of mosquito primarily feeds on birds, they are not a main vector of the virus to humans, per the CDC.
Eastern equine encephalitis virus transmits to people when another mosquito species which feeds on humans and birds (such as the Culex mosquito, which also spreads West Nile) becomes infected and creates a "bridge" between the infected birds and uninfected people or animals, says Pastula.
Humans are considered "dead-end hosts." Eastern equine encephalitis cannot spread from human to human or from horse to human.
There have been rare human cases of eastern equine encephalitis transmitted through organ transplantation, Pastula notes.
Can dogs get EEE?
Yes, dogs can get EEE, but it's rare, according to the Massachusetts DPH. The cases in dogs have been reported in young dogs who lived only outside and were in the Southeast U.S.
Symptoms of EEE mosquito virus
Most people who become infected with eastern equine encephalitis develop no symptoms.
For those who do, symptoms of triple E include:
- Fever
- Chills
- Muscle aches
- Joint pain
These typically last for one to two weeks, and most people will recover completely if the central nervous system is not affected by the virus, according to the CDC.
“In about 5% of infections, people can get a very severe encephalitis," says Pastula.
A severe infection with eastern equine encephalitis can cause neurological disease, which may lead to inflammation of the brain (encephalitis) or the tissues around the brain and spinal cord (meningitis).
According to the CDC, symptoms of severe eastern equine encephalitis include:
- High fever
- Behavioral changes
- Seizures
- Vomiting
- Diarrhea
- Coma
These complications can become life-threatening, and about one in three people with eastern equine encephalitis will die. Many people who recover are left with long-term mental and physical problems, including intellectual disabilities, seizures or personality disorders, per the CDC.
The age groups most affected by severe infections are children under the age of 15 and adults over the age of 50, says Pastula.
Incubation period for EEE
If EEE symptoms do occur, these typically begin within four to 10 days after being bitten by an infected mosquito, per the CDC. According to the Massachusetts health department, they can also start as early as three days after getting bitten.
Where is EEE found in the U.S.?
In the U.S., eastern equine encephalitis cases most often occur in states on the Eastern seaboard, the Gulf Coast, and near the Great Lakes, says Pastula.
Per CDC dataas of Aug. 27, as well as the New Hampshire and Massachusetts health departments, there have been six human cases of eastern equine encephalitis reported in the U.S. this year. At least five cases were the more severe (neuroinvasive) form of the disease. These cases have been reported in five states:
- Massachusetts
- New Hampshire
- New Jersey
- Vermont
- Wisconsin
Triple EEE virus map of US
Cases are most common in areas near bodies of freshwater, including swamps and the coast, according to the Cleveland Clinic.
In Massachusetts, the communities considered to be at higher risk are located in Worcester County and Plymouth County, according to health officials. Those being sprayed to reduce risk of EEE are:
- Carver
- Halifax
- Kingston
- Middleborough
- Plymouth
- Plympton
- Rochester
- Wareham
- Douglas
- Dudley
- Oxford
- Sutton
- Uxbridge
Although Massachusetts has not reported a case of eastern equine encephalitis since 2020, the state has experienced prior outbreaks. "In 2019, when there were dozens of cases, these were primarily from Massachusetts and Michigan," says Pastula.
In 2019, Massachusetts saw 12 human cases and six deaths, DPH health officials said in a press release.
Cases may increase as mosquito season continues and CDC data is updated with reports from state or local health departments.
EEE virus treatment
There is no cure or specific treatment for eastern equine encephalitis. Antibiotics are not effective, as the infection is caused by a virus.
Treatment is often focused on alleviating symptoms, and such as rest, fluids to prevent dehydration and over-the-counter pain relievers, per the Cleveland Clinic.
Severe cases of eastern equine encephalitis may be managed in the hospital and require additional monitoring and supportive care.
How to prevent eastern equine encephalitis
There is no vaccine to protect against eastern equine encephalitis. Protecting yourself from mosquito bites is the best way to lower your risk of eastern equine encephalitis and other diseases spread by mosquitoes, Pastula notes.
You can take the following actions to prevent mosquito bites, according to the CDC:
- Apply an EPA-registered insect repellent
- Wear long sleeve shirts and pants outside
- Limit outdoor activities from dusk to dawn, when mosquitoes are most active
- Use window screens to keep out insects
- Dump or drain any standing water around the home, where mosquitoes can breed
Avoid using homemade mosquito repellents, which are ineffective, according to experts.
Caroline Kee
Caroline Kee is a health reporter at TODAY based in New York City.
