What we know so far about the mosquito-borne disease EEE in Mass. this year

"We want to make sure we catch it when it first emerges."

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The mosquito-borne virus, Eastern Equine Encephalitis, better known as EEE, is expected to make a return to Massachusetts this year.

Last week, crews across the state began their annual mosquito trapping and testing program on their way to sussing out the first carriers of the disease, state epidemiologist Dr. Catherine Brown told on Thursday.

No carriers have been detected yet. When exactly that will happen can obviously vary, although state officials usually see a confirmed EEE-carrying mosquito sometime before mid-July, according to Brown.

“We always start in the middle of June because that is before we expect there to be any either EEE or West Nile Virus activity,” Brown said. “We want to make sure we catch it when it first emerges.”


EEE outbreaks typically last three years, making this summer the last of this approximately once-in-a-decade-or-two cycle that started in 2019. In Massachusetts, the previous outbreak cycle occurred in 2012.

EEE is a potentially fatal infection that has no cure. The illness can trigger brain swelling, fever, and coma, and kills approximately a third of the people who contract it. Those who survive often face severe neurological disabilities and complications.

In 2019, 12 human cases were reported and six people died — the most intense outbreak in Massachusetts since the 1950s. Last year, the state recorded five human cases.

“2019 was unprecedented, but 2020 was sort of an average outbreak year,” Brown said.

This year, there are some early signals that the Bay State could see a more mild outbreak.

According to Brown, the drought the region experienced last fall and this past spring may result in a smaller population of the species of mosquito that carries EEE. (The mosquitoes primarily lay their eggs in water pockets underneath tree roots in red maple and white cedar swamps. Rainy seasons generally bring favorable conditions for mosquito breeding.)

A smaller number of mosquitoes that spread the virus — after contracting it from the bloodstreams of birds, who serve as reservoir hosts of the virus — can slow the amplification of EEE, Brown said.


“Because that population looks like it might be smaller, you know, that may benefit us,” she said.

However, population sizes don’t necessarily dictate the severity or scope of an outbreak, Brown said. Temperature and the virus’s prevalence among the bird population are among several other ecological factors, she said.

“Generally, there are no hard and fast rules,” Brown said, when asked about when the first cases of EEE typically emerge. “I will say the earlier we find evidence of EEE, the more likely it is to be a bad year.”

State officials each year also track West Nile Virus cases. Most people who become infected with the virus do not experience symptoms, although 1 in 5 develop a fever and can experience headaches, body aches, vomiting, diarrhea, or rash, according to the U.S. Centers for Disease Control and Prevention.

In rare instances, infected persons can experience brain inflammation and other serious nervous system complications. Last year, Massachusetts recorded eight human cases.

On Cape Cod, anecdotal reports have also indicated a possible increase of salt marsh mosquitoes, largely on the outer Cape.

Brown said those mosquitoes, which she described as “aggressive human biters,” are generally more of a nuisance than a serious health concern, however.


“Certainly they can make being outdoors unpleasant, but they are not particularly associated with either the West Nile Virus or the EEE cycle,” she said.

As for EEE, Massachusetts residents don’t have to be alarmed this early in the season, according to Brown.

The Massachusetts Department of Public Health will make an announcement if and when EEE is first detected, she said. She also encouraged the public to stay informed this summer through the information released by the state, especially through the department’s online case tracker.

For West Nile Virus prevention, this is a good time of year to take some precautions, such as checking window screens for holes where insects can enter and regularly draining and replacing standing water, such as in bird baths, Brown said.

“All I want right now is for people to start kind of remembering, ‘Alright, mosquito-borne disease is going to happen. Here are the things I will need to do, and here’s how I stay up to date on the information,'” she said.


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