adult and juvenile prairie dogs kissing

Plague in Denver? Infected prairie dog colonies reflect changes in Denver-area ecosystem

September 4, 2019

For those readers wondering if the plague recently found in the greater Denver metro area is the plague—the same disease that wiped out roughly half of the European population during the Middle Ages—the answer is yes. Given the connotation of the word plague, which incites fear in most mortals, many Denver residents are concerned. The current outbreak, however, is not particularly dangerous to humans, as long as they take reasonable precautions.

Discovering the outbreak

Tri-County Health Department issued a press release on Aug. 1, 2019 announcing plague had been confirmed in prairie dogs at numerous sites in Commerce City, including Rocky Mountain Arsenal National Wildlife Refuge, Dick’s Sporting Goods Park and Commerce City open space areas. (Commerce City is a northern suburb of Denver.) Dr. John M. Douglas, Jr., Executive Director of Tri-County Health Department, said, “Plague in prairie dog colonies is common in Colorado and can be managed safely with insecticides to kill the fleas that spread the disease.” Douglas also stated that while plague can be a serious illness in humans, it is treated effectively with antibiotics.

A view of downtown Denver from Rocky Mountain Arsenal National Wildlife Refuge. Photo by Rich Keen, DPRA.

Rocky Mountain Arsenal closed in July “as a precautionary measure to prioritize visitor health and safety,” according to the U.S. Fish & Wildlife Service’s Rocky Mountain Arsenal website. What many people don’t know is that the national wildlife refuge responded swiftly to identify and treat plague. “We were alerted of approximately 10 dead prairie dogs on July 22nd,” states David Lucas, Project Leader of the Colorado Front Range National Wildlife Refuge Complex. Two of the animals were submitted to the CSU Veterinary Diagnostic Lab, which confirmed they had plague on Jul. 31, 2019. “This triggered the response,” Lucas explains, stressing that partnerships between the refuge, the State of Colorado, the cities of Commerce City and Denver, and the Tri-County Health Department “allowed us to quickly convene and attack this issue.”

Prairie dogs at Rocky Mountain National Wildlife Refuge. Photo by John Carr.

The story goes viral

The story remained local Colorado news until mid-August when national newspapers and online news sites ran a series of articles about the plague-infected prairie dogs. Lucas credits the local and national media with “getting accurate information to the public.” Yet some of the stories did use alarming headlines like “Fleas can spread it to pets and humans,” “Blame the plague-infected prairie dogs,” and “Plague continues to strike Denver.”

So we decided to consult Laurel Hartley, PhD, an ecologist in CU Denver’s Department of Integrative Biology who studies the impact of introduced bubonic plague on both urban and rural black-tailed prairie dog (Cynomys ludovicianus) communities in and around Denver. Hartley explains that plague is caused by the Yersinia pestis bacterium. She also says it’s not uncommon: “Plague outbreaks in Colorado are periodic. Plague is always around.” The outbreaks in Colorado prairie dog colonies occur approximately every five to seven years in colonies on the plains and less frequently in suburban colonies.

Professor Laurel Hartley studies the effects of bubonic plague on rural and urban prairie dog colonies.

Real risks

If plague is common in prairie dogs, then why is Denver afraid? Hartley did not want to generate fear. “Municipalities in and around Denver monitor for plague,” she said, “and most people are not in contact with the plague.” On the other hand, Hartley advises, “Don’t be foolish about it.” She recommends keeping your animals and yourself away from plague areas. The Centers for Disease Control and the U.S. Fish & Wildlife Service also suggest ways to reduce a person’s risk of exposure. People living near plague-infected areas should avoid handling dead animals, wear insect repellent, use flea-control products on pets, and avoid feeding wildlife, especially prairie dogs and other rodents. If pets get sick, take them to the veterinarian as soon as possible.

The sad truth about plague pertains to the prairie dogs. A 2011 scientific article by Hartley and other authors published in the journal Ecological Applications cites these research statistics: “Plague epizootics [disease outbreak in animals] within prairie dog towns result in >95% reduction of populations.” 

Hartley puts it simply, “The prairie dogs will die.”

The sad reality

Lucas confirms these statistics at Rocky Mountain Arsenal: “This outbreak has caused a complete die-off of approximately 150 acres of prairie dog colonies.” What happens to all the dead prairie dogs? “Most of the prairie dogs will die in their burrows. The remaining dead animals will be consumed by nature,” explains Lucas. The refuge employees do not handle dead animals unless they are in areas where visitors might encounter them.

A view of the mountains taken from Rocky Mountain National Wildlife Refuge. Photo by Elisa Dahlberg, USFWS.

For animal lovers everywhere, especially those who enjoy the cute burrowing creatures, take heart—the colonies will come back in a year or two. In the meantime, enjoy the healthy prairie dog colonies all over Denver. “If you’re paying attention, you’ll see prairie dogs everywhere,” Hartley says. There are also healthy prairie dogs at Rocky Mountain Arsenal. Lucas wants people to know the refuge “manages for approximately 3,000 acres of black-tailed prairie dogs…[and] the vast majority of our prairie dog colonies remain intact.”

That’s great news, because the species is fun to watch. They stretch, they kiss, they bark, they sun themselves—they’re adorable. They are also a keystone species. They aerate and improve soil, provide nesting areas for other animals, and serve as food for predators, including the highly endangered black-footed ferret. Because the refuge is home to one of the largest wild black-footed ferret populations, “the refuge has invested many years into plague management,” Lucas states.

Adult and juvenile prairie dogs eating grass shoots
Prairie dogs aerate the soil and provide nesting areas for other animals.

A changing ecosystem

Perhaps what is more interesting than the plague outbreak itself is what it says about the Denver ecosystem. In researching plague and its effects on prairie dog communities, Dr. Hartley has analyzed the weather conditions that create a disease outbreak. She hypothesizes that a wet and cooler summer caused the current plague because fleas “thrive in moist burrow conditions.” Lucas agrees: “There is good evidence that it [plague epizootic] is climatic and associated with El Niño (the wetter and cooler spring and early summer that occurred in 2019).”

Hartley will continue to research how plague in prairie dog communities impacts the surrounding ecosystem. She may get some help from CU Denver biology students as they complete an Urban Wildlife Monitoring Project that Hartley oversees. Her students, including Jesse Credit, set up and take down cameras all over Denver that capture wildlife moving in the urban landscape. “This project has opened my eyes to the many diverse animals actually surviving here,” explains Jesse, who has seen everything from racoons and foxes to deer and elk, as well as some big cats. The students organize all photos and submit them to a national database. All of that wildlife data “can contribute to understanding plague,” Hartley said. The project cameras include locations that capture prairie dog activity. “I did actually see a large bird of prey swoop down in front of one of our cameras where prairie dogs are abundant,” Jesse said. “Maybe it was looking for a snack.”

Denver residents should remain calm, if cautious. Plague is rare in humans—the Centers for Disease Control cites an average of 7 cases per year in the United States—and it is treatable. The Denver denizens who should be worried are overwhelmingly of the four-legged variety.