Eastern equine encephalitis (EEE), a rare but deadly mosquito-born infection, has put towns of Massachusetts under heightened vigilance, with health officials urging the public to avoid outdoor activities such as visiting parks and fields at night.

The infection that has 30% fatality and risk of neurological issues in survivors affects around six to eight people in the U.S. every year. EEE typically spreads through mosquito bites and has no vaccines or drugs for prevention.

The cautionary advice from the Massachusetts health officials comes a week after a man in his 80s from Worcester County reported the infection, marking the first human case of EEE in the state this year and the first case since 2020. The officials raised the risk level to critical for the communities of Douglas, Oxford, Sutton, and Webster and to high for Dudley, Northbridge, and Uxbridge.

In the latest development, EEE infection was detected in a horse in Plymouth last week, prompting the officials to close all public outdoor recreation facilities in the town from dusk until dawn.

"As the Massachusetts Department of Health 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. The health and safety of our community, residents, and visitors remain our priority," Plymouth's Commissioner of Health and Human Services, Michelle Bratti, said in a news release.

"The risk from EEE is high in parts of Plymouth County and critical in parts of Worcester County. 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. The mosquitoes most likely to spread EEE are most active during the dusk to dawn hours," State Epidemiologist Dr. Catherine M. Brown said.

What are the signs of infection?

While most people with EEE may not develop signs, people who have symptoms typically develop fever, chills, body aches, and joint pain within 10 days of exposure to the virus.

In some cases, particularly in infants, the infection can soon escalate to neurological diseases such as meningitis or encephalitis with symptoms, including fever, headache, vomiting, diarrhea, seizures, behavioral changes, drowsiness, and coma. For older children and adults, the neurological disease can develop days after the disease onset.

People with a severe infection are at risk of long-term physical or mental disabilities such as personality disorders, seizures, paralysis, and cranial nerve dysfunction.

Steps for prevention:

With no effective vaccines or drugs available, preventing infection relies on controlling mosquito populations and avoiding bites. Installing screens and draining standing water can help reduce mosquito breeding, while limiting outdoor activities at night, wearing protective clothing, and using mosquito repellents can help prevent bites.

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