State health officials Wednesday said residents of in the Merrimack Valley community of Tyngsborough face a critical risk of being infected with Eastern equine encephalitis, an often lethal mosquito-borne disease, after discovering a horse there infected with the virus.

Communities designated at critical risk are urged to cancel outdoor evening activities until after the first hard frost.

The risk level for infection in the surrounding towns of Chelmsford, Dracut, Dunstable, Groton, Lowell, and Westford has been raised to moderate.

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“Mosquitoes remain present in our environment until the first hard frost, so people need to continue to take precautions to avoid getting bitten,” state public health veterinarian Dr. Catherine Brown said in a statement.

“The most effective tools are available to all of us,” she said. “Use insect repellant, cover up exposed skin, and avoid being outdoors after dusk in the highest risk areas, when mosquitoes are at their most active.”

State health officials reported last month that a Norfolk County woman in her 80s died of Eastern equine, the only person to die of EEE in Massachusetts this season.

Last year, seven residents were infected with the virus.