INDIANAPOLIS (AP) — Health officials are warning northern Indiana residents to take precautions to avoid being bitten by mosquitoes after detecting a rare and potentially deadly mosquito-borne virus in the region.

State public health and animal health officials issued the warning Thursday after determining that the eastern equine encephalitis virus was active in multiple northern Indiana counties, with four horses testing positive for the virus.

State veterinarian Dr. Bret Marsh said the virus “is a serious threat to both horses and people in northern Indiana” until the first hard freeze of the fall season occurs.

As of Tuesday, the Indiana State Board of Animal Health said two horses in LaGrange County and one horse in Kosciusko County have tested positive for the virus.

No human cases of the virus disease or mosquitoes infected with the virus have been reported in Indiana this year. But officials said that because northern Indiana contains suitable habitat for mosquitoes that can transmit the virus “humans and horses in all northern Indiana counties are potentially at risk.”

Residents were urged to avoid being outdoors when mosquitoes are present and take other steps, including eliminating breeding grounds for the insects.

While rare, the virus can cause serious illness and has a fatality rate of around 33% or higher, and many people who recover may still experience long-term complications, officials said. Symptoms of the virus disease include chills, fever, body aches and joint pain.