Plague-infected fleas were biting black-tailed prairie dogs, and officials began closing affected areas “as a precautionary measure to prioritize visitor health and safety, while also allowing staff to protect wildlife health,” according to a statement from the U.S. Fish and Wildlife Service.

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The risk posed to hikers and pets is why certain areas remain closed. Dogs are less susceptible to the plague than cats but may pick up fleas that can infect other animals and people, said Gilbert Cazier, an environmental health specialist in the Tri-County Health Department.

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“If you bring the dog home and he sleeps in your bed, those fleas can then jump and get onto you,” Cazier said.

Though the plague can be treated with antibiotics, it has a dark history. According to the Centers for Disease Control and Prevention, it was responsible for the death of 60 percent of Europe’s population during the Black Death. In 1900, rat-infested ships sailing from areas with plague problems led to epidemics in U.S. port cities. The last epidemic was in Los Angeles in the 1920s.

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Today, most plague cases are reported in the western part of the United States, with an average of seven cases reported every year in recent decades. The bacterium that causes the plague, Yersinia pestis, often infects small rodents like rats, mice and prairie dogs. Fleas can then transmit the disease to humans and other larger mammals. In addition to fleas, humans can contract plague from coming into contact with the bodily fluids of infected animals or by breathing in the coughed droplets of plague bacteria.

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Bubonic plague is responsible for 80 percent of plague cases in the United States every year, according to the CDC. A boy in Idaho contracted bubonic plague last year. In 2017, Arizona officials warned residents after discovering plague bacteria fleas. And in 2015, a star high school athlete died of the plague in Colorado.

Plague continues to afflict some communities around the world. In 2017, an outbreak of pneumonic plague in Madagascar killed 202, according to reports by the World Health Organization.

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Health officials in Colorado have been coating prairie dog burrows with an insecticide powder. As the prairie dogs enter their holes and brush up against the powder, Cazier said, it kills the fleas on them and prevents 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,” said David Lucas, from the Rocky Mountain Arsenal National Wildlife Refuge. “But then there is this secondary effort which is to try to prevent the spread of this disease across the landscape.”

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At first, workers were spending 16-hour days spraying the burrows with insecticides, but since then they have been able to reduce the amount of time they spend on it, Lucas said.

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The disease is considered endemic, meaning that it is always around, but health experts don’t fully understand how it turns into a larger outbreak. Lucas said that what makes the current incident unique is the somewhat urban area where it’s taking place.

Also, the refuge is home to the country’s second-largest wild population of the highly endangered black footed ferret, which are known to prey on prairie dogs. In 2016, The Washington Post’s Karin Brulliard reported on national efforts to vaccinate black-footed ferrets and prairie dogs against the plague:

The ATVs and drones are being tested for use in firing blue, peanut-butter-flavored pellets that are the size of a gumball. The bait is laced with plague vaccine, and Fish and Wildlife says 60 to 90 percent of the prairie dogs gobbled it up in recent trials in Montana, Colorado and South Dakota. “These tests clearly indicated that these new mechanized vaccine delivery methods are practical, efficient and affordable,” the agency said in a statement.