Rare, deadly mosquito disease is causing alarm in New England. But how risky is "triple E" virus?

For some, the high-pitched buzz of a mosquito has become even more sinister than usual. In Massachusetts, a coastal county called Plymouth has shut down its parks and fields between dusk and dawn in response to the town’s “high-risk status” of eastern equine encephalitis (EEE), a rare, but severely fatal mosquito-borne illness. It's also known as "triple E" or sleeping sickness and gets part of its name from the fact that it infects horses as well as humans.

According to a public health department press release, athletic leagues and other organizations utilizing public parks and fields will be prohibited from continuing outdoor activities beyond dusk. Signs have been placed at all public parks and fields to emphasize this message.

The move comes after health officials revealed that a man in his 80s was infected with EEE after being exposed in nearby Worcester County. Other individual cases have popped up in Vermont, Wisconsin and New Jersey, while in New Hampshire, an adult (who has not yet been identified) was hospitalized and died from EEE.

“The risk from EEE is high in parts of Plymouth County and critical in parts of Worcester County,” Dr. Catherine M. Brown, a state epidemiologist, said in a press release. “In addition to recommending that people use mosquito repellent with an EPA-registered active ingredient and clothing to reduce exposed skin, we also recommend that evening outdoor events be rescheduled to avoid the hours between dusk and dawn.”

This is because mosquitoes most likely to spread EEE are most active during those hours, state health officials said. According to the Massachusetts Department of Health, EEE is a rare disease. Since it was first identified in the state in 1938, there have been just over 115 cases. A majority of them are from Bristol, Plymouth, and Norfolk counties. Outbreaks occur every 10 to 20 years and typically last between two to three years. The most recent outbreak in Massachusetts was in 2019, which saw 12 cases and six fatalities.

And it’s not just in concentrated areas of Massachusetts. According to the Centers for Disease Control and Prevention, a few cases are reported throughout the United States each year, with most cases occurring in eastern or Gulf Coast states.

The first symptoms of EEE are a fever, stiff neck, headache and lack of energy. They usually show up three to 10 days after a bit from an infected mosquito. Encephalitis, swelling and inflammation of the brain, is the most serious complication of the disease. Unfortunately, there is no known treatment for EEE.

Dr. David Sullivan, a professor in the department of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, emphasized to Salon there are typically less than 10 cases of EEE in the United States each year. Within that, it’s estimated that one out of 10 million people will have a symptomatic case. Typically, people between the ages of 20 and 50 don’t have a symptomatic case. However, young children and older adults are more susceptible to severe symptoms.

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“The rate of it is way less than even being struck by lightning,” Sullivan told Salon. “It’s very rare, but that said, there are no treatments for the disease, and 30 percent of the people that get it die and it does affect young kids under age 18.”

For those who do survive, he added, 50 percent of them are left with long-term physical or mental impairments, running the gambit from intellectual disability, seizures, paralysis and cranial nerve dysfunction.

“We also know that for each symptomatic case, there's probably about 10 or 20 other asymptomatic cases that are never detected,” he added, emphasizing this isn’t a “brand new” disease. “But I think because of its lethality with no known treatments, the public health response is warranted — and the peak case rate is in August and September, but it rapidly goes down after that.”

Separately, the news of EEE follows a report that Anthony Fauci, former head of the National Institute of Allergy and Infectious Diseases, was hospitalized for a West Nile virus infection. Some might be wondering: is this an abnormal mosquito season? And how worried should people be outside of the Massachusetts area?

Sullivan said no. Dr. Amesh Adalja, an infectious disease and senior scholar at Johns Hopkins Center for Health Security, told Salon he agreed.

“I don't think it's abnormal,” Adalja said. “I think this is mosquito season, and just because a celebrity gets infected, it doesn't mean that it's an abnormal year.”

He added 2023 was a “way worse” year for West Nile virus, which was one of the deadliest years for since the virus was introduced to the U.S. in 1999. In 2003, 9,862 people were infected and 264 died. At least 216 cases of West Nile virus have been detected in 33 states this year, according to the CDC.

“I think that post COVID, the media treats infectious disease a little different than they did pre-COVID, so they reported a lot more aggressively,” Adalja said. “I don't think there's any evidence of anything different than an ordinary year for West Nile, and it’s an ordinary year for EEE at this point.”

For people living outside of Massachusetts, both experts told Salon the risk of illness is low, but that people in the months of August and September should make an effort to protect themselves from mosquitoes.

“I think certainly it's just good practice in August and September to minimize your interaction with mosquitoes,” Sullivan said. “In the end, probably less than one out of 1,000 mosquitoes actually carry something. It's not like all mosquitoes are carrying infectious disease.”

Adalja added that vector control agencies are sampling mosquitoes around the country to try and get ahead of outbreaks.

“There are a lot of efforts that are going on behind the scenes,” he said. “And they often will make reports about mosquito pools testing positive.”

In general, mosquito-related illness is on the rise thanks to warming climates from burning fossil fuels. The bugs, after all, like it hot. This includes illnesses like malaria, dengue, Zika and more. These diseases are sometimes described as "neglected" because many Western nations treat them like a problem for the Global South despite having pandemic potential. As mosquito ranges increase, that could change and we may see more future cases of EEE, as rare as it is. So far that isn't the case, but in the meantime, all we can do is surveil and test for the disease while treatments and vaccines are developed.