A popular hiking area near Flagstaff, Arizona, has become ground zero of a deadly plague outbreak among prairie dogs. Fleas collected from the animals’ burrows in the Picture Canyon nature preserve recently tested positive for Yersinia pestis, the bacteria that causes bubonic plague and can be spread to humans through flea bites or through direct contact with an infected animal, the Arizona Daily Sun reported over the weekend.

Officials became aware of the potential plague outbreak last week after noticing a decline in the prairie dog population in the area. "If you normally see prairie dogs then next day they're gone, there is a good chance plague is coming," Dave Engelthaler, programming director with TGen North, a pathogen research nonprofit, told the United Press International. Health officials have begun disinfecting the animals’ burrows and have posted signs at trailheads warning hikers about the potential risk.

Plague first made its way into the U.S. in 1900 and was brought to the country aboard rat-infested steamships coming from Asia. Outbreaks of the disease have occurred most frequently in West Coast port cities but have since spread to inland urban and rural areas. Symptoms of the plague in humans include swollen, tender lymph glands, fever, headache and chills. The disease has a mortality rate of between 8 and 10 percent, according to the World Health Organization.

Between 1900 and 2010, health officials confirmed nearly 1,000 cases of human plague in the U.S. Most human infections occur in the Southwest region, including Arizona, California, southern Colorado, northern New Mexico, and parts of Nevada and Oregon, according to the Centers for Disease Control and Prevention. Most cases today occur in sub-Saharan Africa.

The Arizona Department of Health Services puts the number of plague cases in the state at 64 since 1950. Health officials have advised hikers in the Picture Canyon area to use bug repellent and to avoid handling dead animals.