A cluster of seven towns including Hopkinton have been deemed to be at critical risk for Eastern equine encephalitis after the latest round of mosquito pool testing in the area.

In addition to Hopkinton, the critical warning has been issued for Grafton, Northbridge, Shrewsbury, Southborough, Upton, and Westborough. Many outdoor events have been canceled, and residents are cautioned to use repellent containing DEET, avoid being outside between dusk and dawn, wear long-sleeve shirts and long pants, and dump any standing water.

According to state officials, EEE virus-positive mosquitoes have been detected this year in 310 samples. Included are mosquitoes known to bite humans.

Positive samples also have been detected in Bolton, Harvard, and Shirley.

On Aug. 16, the Grafton Board of Health confirmed a human case of EEE, a man between 19 and 30, according to state offficials. The state on Aug. 10 had announced that a man over 60 from southern Plymouth County became the first Massachusetts resident to test positive for the virus since 2013.

EEE, which can cause swelling of the brain, is fatal for three out of 10 people who contract it, according to the state. Most of the positive tests have come from south of Boston in Bristol and Plymouth counties.

The state also has recorded 53 samples of mosquitoes testing positive for West Nile virus, including in Brookline, Framingham, Newton, Stow, and Upton.

State officials announced Aug. 21 plans to conduct aerial spraying in parts of Worcester and Middlesex counties. The spraying is anticipated to begin Sunday, Aug. 25, and continue over several evenings. The 17 communities in the spray zone are Ashland, Berlin, Framingham, Hopkinton, Marlborough, Milford, Millbury, Northbridge, Northborough, Shrewsbury, Sudbury, Sutton, Worcester, Upton, Grafton, Southborough, and Westborough.