SOMERSET — Bristol County was put on an increased alert yesterday after new cases of mosquitoes carrying serious illness were found, Health Agent Christina Wordell told the Board of Selectmen last night.

Not only have insects in the region tested positive for both West Nile virus and Eastern equine encephalitis (EEE), she said, health officials have uncovered evidence of a disease-harboring species that is so hardy it could potentially thrive into November, unless felled by a killer frost. The species was found in Raynham.

So far, EEE has been found in insects in Seekonk, Rehoboth, Raynham and Easton. West Nile has been found in mosquitoes from Berkley, Easton and Raynham.

No human cases have been reported.

Because infected mosquitoes were found in the area last year, the region has been on a Level 3 alert all summer.

“People need to take personal protective measures, especially being outdoors in the hours of dusk and early morning,” she said. “You should be wearing protective clothing if you have to be out and using insect repellent.”

She said the Bristol County Mosquito Control Program is planning ground spraying to keep the insect population down, but it hasn’t been done because weather conditions haven’t been right.

The insects typically become more active in late August, but the lack of rainfall has helped keep populations down.

But residents must be particularly vigilant, she said, and check around their homes for any standing water that might serve as breeding grounds, including rain gutters and drains, unused flower pots, birdbaths and wading pools. Birdbath water should be changed frequently.

“Any standing water that has been there for four or more days can breed mosquitoes,” she said.

The Bay State had five human cases of EEE last year; two of those people died. There were also three cases of West Nile virus, but no fatalities.

gemery@projo.com