Health officials in northern Arizona said that fleas in a Flagstaff park tested positive for the plague.
The Arizona Daily Sun reported Wednesday that the Coconino County Public Health Department said the fleas had been feasting on prairie dogs and that the animals were being watched for signs of the highly infectious disease.
Officials said the plague has been found in the county before, though its the first instance this year. It may be present in other areas of the county.
The bubonic plague, also known as the Black Death, ravaged Europe in the 14th century, killing 25 million people in five years. Symptoms include fever, bloody coughing and seizures.
The disease is carried by rodents and then spread animal to animal through flea bites. Human can contract the plague either through a flea bite or contact with an infected animal. Outdoor cats are especially vulnerable.
Residents were instructed to not touch sick or dead animals, de-flea pets, use insect repellent, wear gloves when cleaning wild animals and to avoid sleeping on the ground.
According to the World Health Organization, the plague is survivable with early detection and treatment.