A dead bat found in a Tisbury home tested positive for rabies last week, the town board of health has confirmed.

It marks the first documented case of rabies on Martha’s Vineyard since testing began in 1985, but public health officials said there is little cause for concern.

The bat was tested at the Massachusetts state public health laboratory last week, and the town was notified of the results on Thursday, Tisbury health agent Maura Valley said.

Ms. Valley said the bat was found in a Tisbury home after the residents had been out of town.

“They just weren’t sure how long the bat had been in the house and they wanted to have it tested,” she said.

She said she typically sends about three bats to the state lab for testing each year. On average, about three per cent of bats tested for rabies in the state are positive.

“This positive test is a reminder that, although the incidence of rabies in bats is extremely low, bats, like any mammal, can carry rabies,” a board of health press release said in part.

Rabies is a viral disease that spreads through the saliva of infected animals. It is not harmful with immediate treatment, but can be fatal otherwise. All mammals can carry rabies, but most cases found in humans since 1990 have come from exposure to bats, according to the release. It is possible to contract the disease without being aware of exposure, for example if a bat comes into contact during the night while a person is sleeping.

Vineyard wildlife biologist Luanne Johnson said the infected bat was a member of the species known as the big brown bat, the most common variety found on the Island. She said the bat is about the size of a tufted titmouse and is found year-round in human environments. Unlike the rarer northern long-eared and little brown bat, which go into cool damp places to hibernate, big brown bats are often found hibernating near chimneys and in attics during the winter.

Ms. Johnson said though this is the first documented case, it is possible rabid bats have been present before and gone undetected. She said rabid bats remain mobile, enabling them to migrate to and from the Island.

“We don’t know if this bat had been wintering on the Island. We don’t know if it just arrived here or came over during one of the warm spells in past month,” she said. She added, “I don’t think this is any indication that there is a rabies outbreak on the Island.”

She said bats can live up to 20 years, and unlike rodents, they lack gnawing teeth. She said they usually enter houses through gaps and holes made by other animals.

Ms. Johnson said all pets should be vaccinated, and warned that though rabies has generally been absent from the Island, rabid animal corpses from coyotes to fishers could wash up on beaches. She added while contact with bats should be avoided, the animals are beneficial to humans overall because of their steady diet of moths, beetles, and mosquitoes.

Exposure to bats can be prevented through sealing of small holes in attics, installing window screens and properly capping chimneys. Ms. Valley said if exposure is suspected, people should contact a doctor immediately and, if it can be done safely, capture the bat for testing.