The Maine Center for Disease Control and Prevention said a horse in York County tested positive for the mosquito-borne viral disease Eastern equine encephalitis.
The horse, which had not been vaccinated against the disease, was euthanized.
It was the first detected case of the disease in a horse in Maine since 2013.
The disease is transmitted through the bite of an infected mosquito.
It can infect humans if they are bitten by an infected mosquito.
Maine CDC Director Nirav Shah said the case of the York County horse shows the virus is present in Maine. Shah said people in Maine should take precautions to protect themselves from mosquito bites “while enjoying themselves this holiday weekend.”
The last human case of the disease in Maine was in 2005.