What it is
Sylvatic plague is caused by the bacterium Yersinia pestis, spread mainly by fleas. It is the same disease as human plague. In Colorado it circulates in wild rodents and their fleas: rock squirrels, woodrats, other ground squirrels and chipmunks commonly carry it, and prairie dogs are particularly vulnerable.
Plague is not native to North America. Rat-infested steamships brought it to U.S. ports in the late 1890s, and the first documented human case was in San Francisco’s Chinatown in March 1900. By 1908 it had spread into native ground squirrels. By 1935 it was found in wild rodents outside California, and by the early 1950s it had reached its current range, including Colorado. From 1900 to 2012, 1,006 human cases were reported in the United States.
How it spreads
Fleas are the main carriers. Wild rodents and their fleas keep the bacterium circulating, and an outbreak can move through a prairie dog colony. Black-footed ferrets catch plague from infected prey or from flea bites.
In Colorado
- Early 1950s: Plague reached its current range in the United States, including Colorado.
- 2015: Colorado had 4 human cases. In July, plague was confirmed in a prairie dog die-off near Florence, in Fremont County.
- July 2021: A 10-year-old La Plata County resident died from causes associated with plague, and six counties had laboratory-confirmed plague in animals and fleas.
- June 2023: A human case was confirmed in Montezuma County.
- July 2024: A human case was confirmed in Pueblo County.
- August 2025: A domestic cat near Evergreen, in Jefferson County, was confirmed with plague. In August 2025, prairie dog colonies in La Plata County went quiet, and officials sampled carcasses and fleas to investigate plague.
Effects on wildlife
Prairie dogs suffer 90% or more mortality in outbreaks, often resulting in local or even regional extinctions. Prairie dogs and ferrets have little natural immunity.
The black-footed ferret is hit twice: ferrets catch plague from infected prey or flea bites, and they also lose their prey when prairie dog colonies die. The U.S. Fish and Wildlife Service calls plague outbreaks “the greatest obstacle to ferret recovery.”
Conata Basin in Badlands National Park shows what an outbreak can do. It held over 300 ferrets, the largest population in the world. In 2008, plague killed prairie dogs on over 90% of more than 14,000 untreated acres, and ferrets fell to 71 by 2009.
What’s being done
Vaccinating ferrets
Captive-raised ferrets are vaccinated against plague before release. The National Black-footed Ferret Conservation Center is in Carr, Colorado.
Killing fleas
Dusting burrows with the insecticide deltamethrin kills fleas, but after more than 10 years of use, about one third of fleas from long-treated colonies survived. A newer tool, fipronil grain bait, suppressed fleas for one to two years or more, worked two to four times faster than dusting, and may cut costs by 90% or more.
Vaccinating prairie dogs
An oral sylvatic plague vaccine bait was developed by USGS and the University of Wisconsin–Madison, and USGS licensed it to Colorado Serum Company in 2017. Field results are mixed. In a 2013–2015 trial on 29 paired plots in seven states, vaccinated prairie dogs were 1.76 times (adults) and 2.41 times (juveniles) as likely to survive plague outbreaks. But a 2013–2017 study in Montana found survival averaged 0.05 lower on vaccine plots than on placebo plots, and concluded the vaccine did not adequately protect prairie dogs.
Does it affect people?
Yes. From 1900 to 2012, 1,006 human plague cases were reported in the United States. Colorado had 4 human cases in 2015. In July 2021, a 10-year-old La Plata County resident died from causes associated with plague. Single human cases were confirmed in Montezuma County in June 2023 and in Pueblo County in July 2024.
Plague is treatable with antibiotics if caught early. Symptoms include sudden high fever and swollen lymph nodes. Cats are particularly vulnerable, and in August 2025 a domestic cat near Evergreen was confirmed with plague.
How you can help
- Treat your pets with veterinary-approved flea treatment.
- Avoid wild rodent habitat and sick or dead animals.
- Wear insect repellent.
- Clear vegetation near home foundations.
- Report prairie dog die-offs to CPW or your county health department.
- See a doctor promptly for sudden fever or swollen lymph nodes after possible exposure.
