Approximately 115 cases of EEE have been recorded in Massachusetts since it was first identified in the state in 1938, officials said. Outbreaks historically occurred in Massachusetts every 10 to 20 years, but that appears to be changing. The last outbreak began just five years ago, when 12 people were infected with EEE, six of whom died. That outbreak continued in 2020 with five cases, including one fatality. And before that, an outbreak had hit Massachusetts from 2010-2012.

The increased frequency is reason for concern, said Catherine Brown, the Massachusetts state epidemiologist. It’s the latest evidence of the various consequences from a changing climate, and Brown said people should expect mosquito-borne diseases that were once considered tropical tobecome more common in New England.

Brown said longer mosquito breeding periods and the climate-fueled changes in migratory bird patterns can both bring disease.

“I think it’s absolutely biologically plausible that part of the changes that we’re seeing is due to climate change,” Brown said. “This used to be something that was really quite restricted to Southeastern Massachusetts, and we’ve seen geographic spread in the state. And that is new and different.”

Mosquitoes often become carriers of the EEE virus after biting infected birds. They can then pass it on to other birds, or to humans. But humans are considered “dead end” hosts, meaning they are incapable of passing the virus on to other mosquitoes, because the concentration of virus in the human bloodstream is rarely high enough.

This year, the first case of EEE in an animal in Massachusetts was confirmed in a Plymouth County horse in July, followed by the first human infection, which was announced Aug 16. In New Hampshire, EEE has been detected in one horse and seven batches of mosquitoes, state officials said.

There’s no treatment for the disease, which gets progressively worse, officials say. Some patients slip into a coma within a week of contracting it. Individuals over 50, adolescents, and younger children are most at risk, according to the Centers for Disease Control and Prevention. Even those who survive can have lifelong neurological problems.

The New Hampshire victim had tested positive for the virus and was initially hospitalized for “severe central nervous system,” officials said Tuesday. The person’s name, age, and gender weren’t disclosed.

“The (elevated) risk for (EEE) will continue into the fall until there is a hard frost that kills the mosquitoes. Everybody should take steps to prevent mosquito bites when they are outdoors,” Dr. Benjamin Chan, New Hampshire state epidemiologist, said in a statement.

Ominously, EEE is just one of several diseases that appear to be becoming more common due to climate change.

Public health officials are also worried about dengue fever. There have been 80 cases reported in people in Massachusetts so far this year, according to federal data, a 25 percent increase over last year — which was twice the amount from 2022. What’s more, this year, 262 mosquitoes have been found to be carrying West Nile virus, according to the state Department of Public Health.

On Tuesday, state officials announced two new cases of West Nile virus in Massachusetts, both in men in their 60s. They were exposed in Suffolk and Norfolk counties, respectively, and bring the total number of individuals with the virus in Massachusetts to four.

And experts warn that as climate change makes the Northeast more welcoming to mosquitoes — and the diseases they carry — the number of infected residents is probably going to rise.

“Shorter winters, longer summers with warmer temperatures, potentially more erratic rainfall patterns with flooding leading to more standing water — all these can facilitate both more mosquitoes, but also a longer healthier life cycle for mosquitoes,” said David Hamer, professor of global health and medicine at the Boston University School of Public Health who also directs a travel clinic at Boston Medical Center.

Warmer temperatures, he adds, can also speed up the pace at which viruses can replicate inside mosquitoes, which would allow them to transmit the disease more effectively.

West Nile virus spreads similarly to EEE, while dengue fever has a different life cycle. Humans are both the primary host and source of dengue and the primary recipient of new infections.

It’s not yet clear whether there is a direct link between West Nile virus rates in Massachusetts and the changing climate, though it may have played an indirect role in its arrival. The disease was first detected in Massachusetts in 1999, driven in part by changing bird migratory patterns, which may have been influenced by warming weather, experts say.

Some climate experts believe it’s only a matter of time before new tropical diseases once foreign to the region become endemic.

“If you have people traveling in from areas where there’s an outbreak and they have a virus in their system and you’ve got more mosquitoes capable of transmission, then you can have an outbreak here,” Hamer said.

Over the last two decades, a new breed of invasive mosquito, the Asian Tiger, arrived in Massachusetts. It is capable of surviving mild winters in a near-hibernation state, Hamer said, and can carry several diseases not previously endemic to the region, including dengue, Zika, chikungunya, and malaria.

If a tiger mosquito was to bite an individual suffering from one of those diseases while they are still infectious, it could spark an outbreak.

Dengue can lead to acute illness and hospitalizations, and is fatal in around 1 percent of cases. Zika poses its biggest risk to pregnant women, and can lead to babies being born with major neurological problems. Chikungunya, which can last months to years, can lead to persistent arthritis, and joint pain and destruction.

A study published earlier this year in The Journal of Climate Change and Health found that changes in the climate don’t just alter the range where mosquitoes live; temperature and climate can dictate how long mosquitoes live, how effectively they reproduce, and how good they are at transmitting diseases.

Brown, the Massachusetts state epidemiologist, said that spread is clear across New England already for those species carrying EEE.

“This used to be something that was restricted to Southeastern Massachusetts,” she said. But now it’s spreading not just to other parts of the state, but also into Rhode Island, Connecticut, New Hampshire, Vermont, and Maine.

In Massachusetts, public health officials have listed 10 communities as at high or critical risk for EEE and Tuesday night began spraying pesticides in several Worcester and Plymouth counties communities.

Some Plymouth residents said Tuesday that they were not aware of the town’s public health restrictions, such as closing parks from dusk to dawn.

”I honestly didn’t know there was a curfew in place,” said Tony Gregor, 48, a rideshare driver. He said he was not scared of EEE and is not planning to take any precautions. “No, not really.”

Brown said the state will continue to test and monitor mosquitoes and coordinate communication with local agencies so they know as soon as cases are identified to warn residents to stay inside when mosquitoes are most active.

“It’s an unpopular recommendation,” Brown said, but one that, along with other precautions such as wearing long sleeves, pants, and socks when outdoors, using insect repellent, and draining or discarding items that hold standing water, can keep people safe.

Sabrina Shankman can be reached at [email protected]. Follow her @shankman. Adam Piore can be reached at [email protected]. Travis Andersen can be reached at [email protected].