What to Know About the Rare But Deadly Mosquito-Borne Virus Concerning U.S. Towns

A coastal town in Massachusetts is shutting its parks, playgrounds, and fields from dusk to dawn due to concerns about the mosquito-borne virus Eastern equine encephalitis (EEE). 

Plymouth put the early closures in place on Aug. 23, and advised the public to remain cautious and follow the tips found on the Massachusetts Department of Health’s (DPH) website. The news comes after state health officials revealed on Aug. 16 that a man in his 80s had been infected with EEE after being exposed in Worcester County—reportedly the first human case in Massachusetts since 2020.

“As Mass DPH has now elevated Plymouth’s EEE risk status to high, it is important to take extra precautions when outdoors and follow state and local health guidelines to avoid unnecessary risk of exposure to EEE,” said Plymouth’s Commissioner of Health and Human Services, Michelle Bratti, in a press release. “The health and safety of our community, residents, and visitors remain our priority.”

Worcester County in Massachusetts is also concerned about the mosquito-borne virus. On Saturday, Aug. 24, state officials announced that they plan to spray for mosquitoes in sections of Worcester and Plymouth counties due to EEE.

Here is what you need to know.

What is Eastern equine encephalitis?

Per the Massachusetts DPH, EEE is very rare, but incredibly serious. Since the virus—that is spread through the bite of an infected mosquito—was first identified in Massachusetts in 1938, just over 115 cases have occurred. Furthermore, outbreaks of EEE usually occur in Massachusetts every 10-20 years.

The press release sent out by the town of Plymouth stated that, per the Massachusetts DPH, the “EEE fatality rate in humans varies from 33% to 70%, with most deaths occurring 2–10 days after the onset of symptoms.”

Symptoms can include fever, headache, vomiting, diarrhea, seizures, behavioral changes, and drowsiness. According to the Massachusetts DPH, inflammation and swelling of the brain, called encephalitis, is the most dangerous and frequent serious complication of EEE.

EEE can exacerbate quickly and some patients may go into a coma within a week.

Humans and some mammals are what’s known as “dead-end hosts,” meaning they do not spread the virus, even though they get sick, to mosquitoes that bite them.

How is Eastern equine encephalitis tested and treated? 

EEE is diagnosed through symptoms and through testing spinal fluid or blood, which can show if the virus or viral antibodies are present in the body.

There are currently no vaccines for humans or targeted treatments for EEE. Per the Centers for Disease Control and Prevention (CDC), clinical management of the virus is advised. Patients with EEE require close monitoring by their health care provider, who may prescribe pain control or other treatments to deal with the specific symptoms of EEE.

Overall, prevention is key. In Plymouth’s Aug. 23 notice, the city recommended certain strategies for citizens to avoid mosquito bites, including draining standing water, wearing long sleeves and pants during peak mosquito hours, and installing screens in your home. They also recommended utilizing bug repellant, specifically a repellent with an EPA-registered ingredient.

Animal owners should reduce potential mosquito breeding sites on their property by eliminating standing water from containers such as buckets, tires, and wading pools—especially after heavy rains.

What U.S. towns have raised their EEE risk level to "critical" or "high"?

Four towns in Massachusetts have raised the EEE risk level to “critical”—Douglas, Oxford, Sutton, and Webster.

According to the Massachusetts Department of Public Health, the towns of Carver, Dudley, Middleborough, Northbridge, and Uxbridge also raised their EEE risk levels to “high,” joining Plymouth.

Concern is also mounting over the West Nile virus, which is primarily spread by mosquitoes. In July, multiple local health departments warned citizens after officials detected mosquitoes carrying West Nile virus in states across the U.S. As of Aug. 20, the CDC had recorded 216 cases in 33 states in 2024.

On Aug. 24, it was confirmed that Dr. Anthony Fauci, who served as the chief medical advisor to the President from 2021 to 2022, had been hospitalized after being infected with West Nile virus. He is now recovering at home.