Bubonic plague may seem like a disease that’s been relegated to the history books, but that’s not the case. The disease that struck terror in people in the Middle Ages is alive and well in the modern world, and it's most recently appeared in prairie dog towns in the suburbs of Denver.

Morgan Krakow at The Washington Post reports that in late July, the United States Fish and Wildlife Service shut down the 15,000-acre Rocky Mountain Arsenal National Wildlife Refuge north of the city when fleas infected with the plague bacterium Yersinia pestis were found in the refuge's black-tailed prairie dog colonies. Last weekend, parts of the refuge reopened, but certain areas will remain closed through Labor Day. According to a press release from Colorado’s Tri-County Health Department, the Prairie Gateway Open Space in Commerce City is also closed to the public as well as First Creek at DEN Open Space, a nature preserve near Denver International Airport. So far, there are no reports of any humans contracting plague in the area.

“The prairie dog colonies are being monitored and burrows are being treated with insecticide, but there is still evidence of fleas in the hiking and camping areas, which could put people and pets at risk, so those areas will remain closed,” John M. Douglas, Jr., Executive Director of the Health Department, tells CNN’s Eric Levenson.

The Post’s Krakow reports that health department workers have been coating the prairie dog burrows with powdered insecticide. As the little mammals run into their burrows, they brush up against the powder, hopefully killing off the fleas and preventing the spread to other animals.

“We are closing trails and spraying an insecticide to kill fleas in plague-affected areas where there might be humans,” David Lucas of the Rocky Mountain Arsenal National Wildlife Refuge tells Krakow. “But then there is this secondary effort which is to try to prevent the spread of this disease across the landscape.”

So how did prairie dogs get a virulent infection that plagued the Byzantine Empire and killed 60 percent of Europeans in the 1300s? During the last half of the 19th century, plague spread across China. When it hit the port of Hong Kong around 1894, the disease-carrying fleas began to spread to port cities around the world, eventually killing about 10 million people. Ester Inglis-Arkell reports i09 that bubonic plague came to the U.S. via Chinatown in San Francisco around 1900, though local officials refused to acknowledge the disease, worried about driving away tourists. In 1906, however, when an earthquake leveled large parts of the city, rats carrying plague fleas proliferated in the rubble, leading to an outbreak of the disease.

The bacteria was also transmitted to San Francisco area squirrels, and from there, spread to the small rodent population of the American West. Now, the disease is endemic, meaning it’s always present at low levels, though researchers don’t completely understand why larger outbreaks occur during certain years. On average, between one and 17 cases of plague are reported annually in humans, with hotspots located in the high deserts of northern New Mexico and Arizona as well as southern Colorado, according to the CDC.

But it’s not just humans that suffer from Yersinia pestis. Outbreaks of the plague, which is called sylvatic plague when it infects small mammals, can kill over 90 percent of prairie dogs infected with the disease.

“It all depends on the species of prairie dog and the level of die off. A number of prairie dog colonies see complete loss,” explains Paul Marinari, a senior curator at Smithsonian Conservation and Biology Institute.



These prairie dog die-offs are one of the reasons the black-footed ferret, which relies on the rodents as prey, is endangered. Teams have already developed a safe and effective plague vaccine for black-footed ferrets, and an improved vaccine for prairie dogs is in the works, says Marinari. Nidhi Sherman at LiveScience reports that over the last five years, a vaccine has been given to prairie dogs living near ferret populations—sometimes using drones to airdrop vaccine-laced peanut butter pellets—and so far it appears to be working.

“Wildlife managers have struggled to recover ferrets and manage prairie dog colonies due to the devastating effects of plague,” Dan Tripp, a researcher with Colorado Parks and Wildlife says in a press release. “It is our hope that use of the sylvatic plague vaccine in select areas, with the support of willing landowners, will help to limit the impact of plague to wildlife.