Health officials believe a Penobscot County resident diagnosed with eastern equine encephalitis acquired the disease within Maine, making it the first in-state acquisition of the illness since 2015.

The adult developed severe symptoms early last month and remains hospitalized, the Maine Center for Disease Control and Prevention said in a statement Thursday afternoon.

The patient did not have any history of travel outside the state, leading local health officials to believe they were infected in Maine. Those results await confirmation by the U.S. Centers for Disease Control and Prevention.

“Even though the risk of mosquito bites is low right now, it can take up to several weeks for people who were bitten by an infected mosquito to develop symptoms,” Maine CDC Director Dr. Puthiery Va said in the statement. “We want Maine people and visitors to be on the lookout for symptoms and get medical care if they get sick.”

Symptoms of EEE include flu-like issues, such as body aches, fever and headaches. Severe symptoms include brain swelling, spinal cord inflammation and, in extreme cases, death.

This marks the first EEE infection in Maine this season, though other mosquito-borne illnesses have been found in most of Maine’s 16 counties this year. The virus cannot be spread from human to human or between humans and animals.

The state CDC also reported two human cases of West Nile virus this year. West Nile and EEE were both found in more than a dozen wild birds and domestic animals, the Maine CDC said.

Recent hard frosts have reduced the risk of mosquito-borne sickness, the CDC said.