State’s first West Nile virus-positive mosquito of the year found in Berkshires

A Massachusetts mosquito tested positive for West Nile virus at a Clarksburg laboratory, making it the first case this year, health officials said.

“With warmer weather and summer officially beginning this week, this is the time of year when we typically start detecting West Nile virus in mosquito populations,” said Public Health Commissioner Ruby Goldstein, MD, Ph.D.

While testing a group of mosquitoes on June 16 for certain diseases, researchers discovered that the insects contained West Nile virus, the Massachusetts Department of Public Health said in a news release.

West Nile virus is transmitted to humans through the bite of infected mosquitoes. In 2025, nine people became infected with the virus, the Department of Public Health said.

As of June 25, no human or animal had been diagnosed with the virus in the state, health officials said.

Anyone is susceptible to West Nile virus, but people over 50 have a higher risk of “severe illness,” the Department of Public Health said.

The majority of infected patients do not experience any symptoms. Those who aren’t so lucky tend to have a fever and flu-like symptoms, the Public Health Department said.

“With West Nile virus, it is difficult to predict how much activity we will see,” said Dr. Katherine M. Brown, state epidemiologist.

Brown advises the public to check back often Web pages on mosquito-borne diseases on Mass.gov.

The Department of Public Health (DPH) advises the public to use ingredients containing insect repellents, such as DEET, when outdoors.

Health officials wrote that DEET should not be used on infants younger than 2 months old and should be used in concentrations of 30 percent or less.

The public should also protect their homes from mosquitoes. Public health officials advise against having standing water near or in your home, as mosquitoes lay their eggs in standing water, the Department of Public Health said.

Officials said animal owners should empty buckets, tires or wading pools, especially when it rains.

“Horse owners should keep horses in indoor stalls at night to reduce their risk of exposure to mosquitoes,” health officials wrote.

Public officials also warned the public about eastern equine encephalitis (EEE). The Department of Public Health said no mosquitoes carrying the virus had been detected as of Thursday.

Health officials advise horse owners to use mosquito repellent on horses approved by veterinarians.

“If an animal is suspected of being infected with WNV or EEE, owners should report it to the Department of Agricultural Resources, Division of Animal Health by calling 617-626-1795, and to the Department of Public Health by calling 617-983-6800,” officials wrote.