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State health officials say a horse in Belchertown, near Amherst, is the first this season in Massachusetts to be diagnosed with Eastern equine encephalitis, a potentially lethal mosquito-borne disease that infected seven residents last year.

The finding indicates that mosquitoes that typically bite mammals, including horses and people, are infected with the virus in the area and pose a greater risk to nearby residents, health officials said Wednesday.

The news follows Tuesday’s announcement that the summer’s first batch of mosquitoes detected with Eastern equine had been found in Amherst, an area of Massachusetts that has not typically been plagued by the virus.


Based on the findings, the health department raised the infection risk level to “critical” in Belchertown, and said it should consider canceling evening outdoor events for the remainder of the mosquito season. The risk level was raised to “moderate” in the nearby communities of Granby, Ludlow, New Salem, Palmer, Pelham, and Ware. The risk was raised to moderate Tuesday in Amherst.

“Today’s finding significantly raises our concern for the area,” state public health veterinarian Dr. Catherine Brown said in a statement. “It’s important that people in high risk areas protect themselves from getting bitten by mosquitoes. Use insect repellant, cover up exposed skin, and avoid outdoor activities at dusk and nighttime, when mosquitoes are at their most active.”

The department urged all state residents to take similar precautions against mosquito bites.

Kay Lazar can be reached at klazar@globe.com. Follow her on Twitter @GlobeKayLazar.