Due to an increasing threat from a potentially deadly mosquito-borne disease, a coastal town in Massachusetts has closed its parks, playgrounds and sports fields from dusk to dawn, according to town officials.

Plymouth, Mass., about 60 km south of Boston, announced the closures on Aug. 23, due to a high risk of Eastern equine encephalitis (EEE), a rare and dangerous mosquito-borne disease with a 33 per cent to 70 per cent fatality rate and severe long-term effects, such a paralysis and brain dysfunction, for survivors.

The state’s health officials recently reported Massachusetts’ first human case of EEE since 2020, involving a man in his 80s. The region, as well as neighbouring towns, has also begun mosquito spraying as a preventative measure and are advising residents to use mosquito repellent.

State and local health officials urged people in those towns to avoid the peak mosquito biting times by finishing outdoor activities by 6 p.m. until Sept. 30 and then by 5 p.m. after that, until the first hard frost.

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EEE is caused by a virus transmitted through the bite of an infected mosquito. While humans and certain mammals, such as horses and llamas, can contract the disease, they do not transmit it further. There are no human vaccines or treatments for EEE.

The virus can also cause severe illness and potentially be fatal in any age group, with those under 15 and over 50 being particularly at risk, health officials warned.

Although severe, EEE is still very rare. Since the virus was first identified in Massachusetts in 1938, just over 115 cases have occurred. During the last EEE outbreak in Massachusetts from 2019 to 2020, there were 17 human cases, resulting in seven deaths.

In Canada, human infections with EEE have also been rare.

While national data is not available, Ontario has reported three human cases of the virus, with the most recent occurring in 2022.

Outbreaks of EEE typically occur in Massachusetts every 10 to 20 years, lasting two to three years. The most recent outbreak began in 2019, resulting in 12 cases and six fatalities. It continued into 2020 with five more cases, including one fatality, state officials said.

“We have not seen an outbreak of EEE for four years in Massachusetts,” said Massachusetts’ DPH Commissioner Robbie Goldstein in a media release on Aug. 24. “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.”

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Although numbers can fluctuate, Richard Martinello, a professor of Internal Medicine and Pediatrics in Infectious Diseases at Yale School of Medicine, said the medical community is still “not really clear” on the recent upward trend.

“And so there’s some thought that we may be seeing some newer strains that come into a community. And it just spreads a little more widely because there may be the birds in that area don’t have immunity against it,” he added.

However, this remains only a hypothesis that new strains of the virus may be emerging.

The first detection of the virus in Canadian mosquitoes happened in September 2009, when an EEE-positive pool of mosquitoes was found in the First Nations Community of Wahta Mohawk, Ont., according to Public Health Ontario.

In Canada, EEE infections mainly occur in the spring and are linked to birds migrating from the southern United States to northern Canada, according to Health Canada.

On Aug. 21, Ottawa Public Health reported that a horse in Ottawa tested positive for EEE and issued a warning. The health agency recommends that residents take measures to protect themselves from mosquito bites to reduce the risk of the virus.

“Eastern equine encephalitis virus (EEV) is normally transmitted between wild birds and mosquitoes but can occasionally infect horses and, rarely, humans through infected mosquitoes’ bites, similar to the transmission of West Nile virus (WNV),” the public notice read.

“Whereas mosquitoes that carry WNV are most commonly found around homes in urbanized areas, the mosquitoes that carry EEV are usually found close to swampy hardwood forests. Horses can be protected by a vaccine against EEV but there is no vaccine for humans.”

Since human infections of EEE are so rare in Canada, Martinello reassured that Canadians don’t need to lose sleep over it.

Most people infected with the virus do not develop encephalitis and experience only mild symptoms, Martinello said.

“What they experience is often just a mild illness with fever and some achiness, not feeling well and then have self-resolved,” he said.

“What we worry about are individuals who develop encephalitis.  The reason we worry about it is because we have a very high mortality rate. About a third of people who develop encephalitis will die from it. And we do not have any treatment for it, other than providing supportive care,” he added.

When an individual develops encephalitis it can cause an inflammation of the brain that causes brain swelling, ischemia and hypoperfusion.

This can result in long-term neurological damage, such as paralysis. The disease can also be fatal, with death occurring within three to five days of infection in some cases.

While the risk of EEE infection is low, taking preventive measures is crucial, especially during peak mosquito activity from late summer to early fall.

Martinello noted that if you are in Boston, you might not need to be overly concerned, as the mosquitoes carrying the virus prefer swampy environments.

For those at risk, Massachusetts’ health unit recommends scheduling outdoor events to avoid the hours between dusk and dawn when mosquitoes are most active.

When spending time outdoors, wear long pants, a long-sleeved shirt, and socks, and use a mosquito repellent containing DEET, permethrin, picaridin, or oil of lemon eucalyptus. To keep mosquitoes out of your home, repair any holes in your screens and ensure they fit tightly on all doors and windows. Also, remove any standing water around your home to eliminate mosquito breeding sites.

— with files from the Associated Press