Wildlife and nature areas near Denver have been shut down as officials continue efforts to stem the spread of the disease.
Rocky Mountain Arsenal National Wildlife Refuge, a 15,000 acre nature area northeast of Denver, was able to partially reopen on Sunday.
Plague-infected fleas were biting the prairie dogs, and officials began closing affected areas "as a precautionary measure to prioritise visitor health and safety, while also allowing staff to protect wildlife health," according to a statement from the US Fish and Wildlife Service.
Certain areas remain closed because of the risk posed by hiking through them and taking pets. Dogs are less susceptible to plague than cats, but may pick up fleas that can infect other animals and people, Gilbert Cazier, an environmental health specialist in the Tri-County Health Department, said.
"If you bring the dog home and he sleeps in your bed, those fleas can then jump and get onto you," Mr Cazier said.
In 1900, rat-infested ships sailing from areas with plague problems led to epidemics in US port cities, but the last epidemic was in Los Angeles in the 1920s.
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 plague, Yersinia pestis, often infects small rodents like rats, mice, and in the most recent incident in Colorado, 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 body fluids of infected animals, or by breathing in the coughed droplets of plague bacteria.
Bubonic plague is responsible for 80 per cent 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 sadly, in 2015, a star high school athlete died from plague in Colorado.
Health officials in Colorado have been coating prairie dog holes with an insecticide powder. As the prairie dogs enter their holes and brush up against the powder, Mr Cazier said, it kills the fleas on them and prevents the spread to other animals.
Some parts of the Rocky Mountain Arsenal National Wildlife Refuge and nearby areas will remain closed through Labor Day Weekend, officials announced on Friday. The areas include parts of the wildlife refuge and other open spaces in Commerce City, a suburb outside of Denver.
Officials did clarify that Dick's Sporting Goods Park, an event venue nearby would continue to host all events, but that parking was restricted to asphalt parking only, since nearby grassy areas had prairie dog populations.
The Washington Post