Tests confirmed Thursday that a horse in Georgetown was infected with eastern equine encephalitis, prompting health officials to immediately cancel all outdoor nighttime activities and alert residents about a heightened risk of infection from the mosquito-borne disease.

Michael Farrell, administrator for the community of 8,000 north of Boston, said town leaders received a call shortly before noon from state health officials notifying them of the test results. The horse was recently euthanized.

“We recommended that all outdoor activities be cancelled, postponed or moved indoors from dusk to dawn,” Farrell said.

State health officials in a statement said the risk of infection from eastern equine has been raised to “critical” in Georgetown, and to “high” in the nearby communities of Boxford, Groveland, Newbury, Rowley, and West Newbury.

Farrell said Georgetown intends to spray pesticide Thursday evening from trucks.

Mammal-biting mosquitoes infected with the virus, commonly known as EEE, were recently found in two communities north of Boston -- Reading and Lynnfield -- but this is Massachusetts’ first reported infection of a horse with EEE this season.

State health officials also reported Thursday that an alpaca stabled in the south shore community of Halifax was infected with EEE. They raised the risk of human infections to “critical” in Halifax and to “high” in neighboring East Bridgewater, Hanson, Pembroke, and Plympton. They also strongly recommended that evening outdoor events be curtailed in all of these communities for the remainder of the mosquito season.

A man in his 60s from the Metrowest region of Middlesex County was diagnosed with the virus in late July, but state health officials said the unnamed resident apparently was infected during out-of-state travels.