(CNN)Twenty-eight communities in Massachusetts are at "critical risk" of Eastern equine encephalitis, a virus transmitted by mosquitos that can cause rare and potentially fatal illness in humans, the state's health department said Thursday.
The risk level is 'critical' for a mosquito-borne virus in dozens of Massachusetts communities
Thirty-seven more communities are at high risk and another 126 are at moderate risk, state health officials said.
Four people have had confirmed cases of the virus this year. Additionally, it was detected in seven horses and found in 366 mosquito samples, state health authorities said.
"As we head into the Labor Day weekend and the month of September people should not forget to bring and use an EPA-approved mosquito repellent for any outdoor activities," Dr. Monica Bharel, the state public health commissioner, said in a press release. "The peak time for transmission of mosquito-borne illness extends through September here in Massachusetts."
Communities at highest risk include Holliston, Medfield, Brookfield and Granby.
The virus occurs sporadically in the state. Recent outbreaks occurred in 2004-2006 and 2010-2012, causing illness in 22 people, state health officials said.
The state has conducted aerial spraying to kill off mosquitos, but health officials say the risk has not been eliminated.
Residents are encouraged to protect themselves against the virus by using mosquito repellant.