Public health officials say the virus that causes Eastern equine encephalitis has been found in mosquitoes collected in Bolton.
James Garreffi, director of the Nashoba Associated Boards of Health, says the state Department of Public Health confirmed the discovery on Wednesday.
A horse in nearby Lancaster died from EEE earlier this month.
The potentially fatal disease is spread to humans through the bites of infected mosquitoes.
A Rhode Island man has been confirmed as having the virus, which officials say he probably contracted in southeastern Massachusetts, an area of high risk from mosquitoes. He was hospitalized in critical condition.
Massachusetts had last reported a human case in 2008 and Rhode Island in 1998.