Editor’s note, 8/20/19: Rocky Mountain Arsenal reopened to the public on Saturday, August 17, after being closed for two weeks. This article has been updated with new information. 

The Rocky Mountain Arsenal National Wildlife Refuge (RMA) is only closed to the public three days of the year—on Thanksgiving, Christmas, and New Year’s Day. But due to a confirmed outbreak of plague in the prairie dogs who call RMA home, the refuge has been closed to visitors for over a week.

The Tri-County Health Department confirmed on August 1 that plague-infected fleas have been killing prairie dogs at numerous sites throughout Commerce City. A fireworks show scheduled to follow the Colorado Rapids game at Dick’s Sporting Goods Park on Saturday, August 3 was canceled as a safety precaution. The Rapids’ communications office did not respond to a question about how the rest of the team’s season might be impacted by the findings.

On August 20, Phish, which has played a series of sold-out shows at the park for the past eight years, announced that parking for the concerts will be restricted to asphalt lots. That means that there will be no overnight camping allowed, as in previous years.

According to a study in the Journal of Mammalogy, the plague, which entered the United States around 1900, is the only disease known to cause extensive die-offs in prairie dogs. Reads the study: “The overall impact of plague on prairie dogs during the first one-half of the 20th century is not well documented but was probably much greater than is appreciated generally.”

David Lucas, project leader for the RMA, says he knew this day was coming. The plague, he says, “is somewhat common and every so often it rears its head.” For the past several years, he and his team have been deploying preventative tactics—administering an oral plague vaccine to prairie dogs in the form of “little peanut butter balls” laced with medicine, as well as applying Frontline flea prevention to the prairie dogs’ regular food (oats). Still, two weeks ago, staff found two prairie dogs dead, and when they sent the bodies to a laboratory to be tested, they came back positive for the plague.

That’s when Lucas and his team sprang into action, closing the RMA to the public and launching into 16-hour workdays spraying insecticide into every prairie dog den within a 13-mile area to kill the fleas, which are the agents that carry plague. They have also dusted 11 miles of public-use trails with insecticide for 15 feet on each side.

Lucas says he expects parts of the RMA will reopen to the public this week, though the trails close to the prairie dog dens will stay closed for longer. “It’s kind of an adaptive process,” he says. “There’s no blueprint. There’s no trying to figure this out.” He does know that plague fatalities in prairie dogs are correlated to especially high numbers of fleas in a given area, but how and why that occurs is a mystery. “Why are there so many fleas at this moment? Why, at some moments, are there a couple hundred, and at other moments hundreds and hundreds and hundreds?” he asks. The answer is unclear.

Because the health department and the RMA have been transparent with the public about the situation and proactive in dealing with it, it’s very unlikely this plague will spread to humans. According to Dr. Dean Biggins, a Fort Collins-based research wildlife biologist with the U.S. Geological Survey who studies the ecology of wildlife diseases, in general the plague presents low risk to the human population because we have effective antibiotics to treat it. He notes, though, that the bioterrorism potential—the threat of terrorist groups or warring nations using plague as a bio-weapon—is “fairly frightening,” as is the potential for the plague bacteria to develop a resistance to antibiotics. “If certain factors lined up right, we could still end up with worldwide problems related to plague,” he acknowledges.

When it comes to the plague, however, the more pressing concern, and the one less likely to receive attention—or research money—is the risk to wildlife. For the past 10 years Biggins says he and other researchers have been going into different natural wildlife habitats and ecosystems throughout Colorado (including in the Rocky Mountains), New Mexico, South Dakota, and Idaho, and are “running into this pretty much every place we go.”

“In a lot of places plague is chronically circulating and having pretty profound effects on rodents,” Biggins says. “Some of the larger rodents—chipmunks and various ground squirrels and wood rats—apparently are quite involved in plague cycling. It’s not everything everywhere, but quite a variety of species and states from here to California.”

He says in many of the natural ecosystems, he and other researchers found that the plague is likely responsible for killing around 20–30 percent of an animal’s colony. “That’s a high enough rate that it can really disrupt the function of an ecosystem, but not high enough where you’re going to see it by casual observation,” he says, as compared to instances when the plague kills off a full 90–95 percent of a colony. He adds that the areas are wild enough that there have been no human cases tied to them, so word hasn’t really spread (bad pun intended) to the general public.

“The problem with us humans is that we are not capable of really noticing a lot of these things because we don’t very often study chipmunks, for instance, in enough detail to document what’s happening in their communities,” Biggins says. “Plague can actually decimate them just like it does prairie dogs. These rodents are at the base of the food chain, so what happens to them really has dramatic effects throughout an entire ecosystem. There is a ripple effect into other animals in those systems that rely on those species as prey.”

This ripple effect can extend to certain apex predators that might come into contact with plague-infested prairie dogs, including mountain lions and other big cats, says Biggins. Other predators, such as raptors like hawks and eagles, are resistant to the plague. Bears, he says, are somewhat resistant. “We don’t know too much about how many bears might get really sick with it, but we know they get exposed sometimes and survive the plague,” he says.

A bigger concern for the RMA is its population of 54 black-footed ferrets, the most endangered mammal in North America. These black-footed ferrets are “totally dependent” on prairie dogs as prey, says Biggins. They are also highly susceptible to the plague, and for that reason the RMA is taking special precautions to protect them, including dusting the perimeter of the black-footed ferret habitat with insecticide. “If a large die-off of prairie dogs occurs to the point where we believe that there may be a critical reduction in the available food source for ferrets, the U.S. Fish and Wildlife Service could consider trapping and relocating ferrets to another reintroduction site,” says Michael D’Agostino, public affairs specialist for the U.S. Fish and Wildlife Service Mountain-Prairie Region. They will continue to monitor the situation throughout the next couple weeks. “Addressing the plague will be an adaptive, iterative process,” says D’Agostino.  

Lucas notes that it was RMA staff who found the dead prairie dogs and were motivated to investigate the cause of death. “It was good work,” he says. He also acknowledges that the public nature of the RMA, and its resources, is likely the only reason the infected prairie dogs were identified. “Had it been some obscure place, maybe they wouldn’t have.”