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Positive West Nile virus samples turned up this month in mosquitoes trapped in Woburn and Stoneham, prompting Winchester’s Health Department to remind residents to guard against bites as the state’s mosquito season pushes toward its historical peak.
The town said the overall risk of West Nile virus, or WNV, to Winchester residents remains low. Still, officials urged people to use insect repellents with an active ingredient registered by the Environmental Protection Agency, or EPA, such as DEET, and to limit outdoor exposure at dusk and dawn, when mosquitoes are most active.
The Massachusetts Department of Public Health, or DPH, assigns every city and town in the state a WNV risk level — low, moderate or high — based on mosquito surveillance testing conducted from June through October.
Levels typically climb as infected samples accumulate in and around a community and fall after the season’s first hard frost.
Beyond repellent and timing, officials recommend draining standing water around homes, since mosquitoes lay eggs in birdbaths, clogged gutters and unused containers, and repairing torn window and door screens.
Winchester has faced this warning before. Middlesex County communities, including Winchester, saw their WNV risk level raised to high in September 2021 after the state confirmed that year’s first animal case, an alpaca exposed to the virus in the county. A similar advisory went out in August 2020, when two more human cases were confirmed statewide with exposure tied to Middlesex County.
Statewide, this year’s WNV activity began further from Boston than usual. The season’s first positive mosquito sample was collected June 16 in Clarksburg, in Berkshire County, and confirmed by the Massachusetts State Public Health Laboratory on June 25 — a notable shift from recent seasons, which more often opened in communities closer to the city.
This year’s activity arrives against a starker national backdrop. The Centers for Disease Control and Prevention, or CDC, reported the earliest and most intense start to West Nile season in more than two decades, with 48 cases confirmed nationwide through the end of June — 38 of them, or 79%, involving severe neuroinvasive disease.
The last time the season started this early, in 2004, the year ended with more than 2,500 cases and 100 deaths nationwide.
Massachusetts, by contrast, has yet to confirm a single human or animal case this year, according to DPH. As of July 28, only mosquito samples have tested positive statewide, a pattern echoed in other counties this month, including Franklin County, where samples in Deerfield and Northfield turned up positive in June and July.
“It is difficult to predict the amount of activity we will see,” State Epidemiologist Catherine M. Brown said.
Statewide case counts have swung widely year to year. Massachusetts recorded 19 human WNV cases in 2024, its worst season since 2018, when 49 people were infected, along with four cases of Eastern equine encephalitis, or EEE, one of them fatal. Nine WNV cases followed in 2025, with no EEE activity confirmed.
People older than 50 face the highest risk for severe illness from WNV, according to DPH. Most infected people show no symptoms; those who do typically develop fever and body aches, with encephalitis or meningitis occurring in rare cases.
Winchester’s notice pointed residents to the state’s regularly updated risk maps and additional prevention guidance, noting that active surveillance in the region will continue into fall.
Massachusetts’ peak season for human WNV cases historically falls in August and September, according to DPH.
Will Dowd is a Massachusetts journalist who covers municipal government and community life for Winchester News. He is also the founder and editor of The Marblehead Independent, a reader-funded digital newsroom.
