U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3087

October 2008 Printed on recycled pape r

Protecting Black-Footed Ferrets and 

Prairie Dogs Against Sylvatic Plague

Scientists at the USGS 

National Wildlife Health 

Center (NWHC), in 

collaboration with 

colleagues at other 

federal agencies and the 

University of Wisconsin, 

are developing and 

testing vaccines that 

can be used to protect 

black-footed ferrets 

and prairie dogs against 

plague. The black-footed ferret is commonly regarded as the 

most endangered mammal in North America, and sylvatic 

plague is a major impediment to its recovery. The three prairie 

dog species (Gunnison’s, black-tailed, and white-tailed prairie 

dogs), upon which the ferret depends for food and whose 

burrows they use for shelter, have been drastically reduced 

from historical levels, resulting in the near extinction of the 

ferret. All three species are considered “at risk” and have been 

petitioned for listing as ‘threatened’ or ‘endangered’ by the 

U.S. Fish and Wildlife Service (FWS). Additionally, the Utah 

prairie dog is listed as threatened and the Mexican prairie dog is 

considered endangered in Mexico. Like the black-footed ferret, 

all five prairie dog species are highly susceptible to plague 

and regularly experience outbreaks with devastating losses. 

Controlling plague outbreaks in prairie dogs and ferrets is a 

vital concern for ongoing recovery programs and conservation 

efforts for both species.

Sylvatic Plague – A Continued Threat to North 

American Wildlife

Sylvatic plague, caused 

by Yersinia pestis, 

is a bacterial disease 

transmitted by fleas that 

afflicts many mammalian 

species, including humans. 

For many species of 

wildlife, plague mortality 

is a serious conservation 

issue. In fact, more than 

half of North American 

rodent species considered 

of conservation concern 

by the International Union 

for Conservation of Nature 

reside within the range of 

The Black-Footed Ferret, Once Thought 

Extinct, Returns to the Wild

The black-footed ferret 

once occurred across a 

large mid-continent area 

from southern Canada 

to northern Mexico. The 

ferret depends almost 

exclusively on prairie 

dogs for food and for 

shelter in their burrows. 

Over the past century, 

prairie dog populations, 

and ferrets by extension, 

have drastically declined 

due to habitat loss, poisoning, and devastating outbreaks of 

sylvatic plague. Prairie dogs numbers have been reduced by 

over 95%. 

Once presumed to be extinct, a wild population of ferrets 

was discovered in 1981 in Wyoming. Unfortunately, this 

last colony succumbed to disease, but not before it provided 

a few animals to start a captive breeding effort that to date 

has produced over 6,000 young. Six facilities now maintain 

separate, intensively managed, captive ferret populations 

totaling around 290 animals. 

Since 1991, over 2,000 ferrets have been reintroduced 

into the wild across the western U.S. and Mexico. Ferret 

numbers in the wild total over 1,000 individuals as of fall 

2008, with perhaps half surviving to breed each spring. 

Although highly successful so far, the ferret recovery 

program will not be complete until larger numbers of ferrets 

exist in the wild and their populations are sustainable.

One of the biggest obstacles to fulfilling this goal is sylvatic 

plague, a disease highly lethal to both prairie dogs and 

ferrets. In May 2008, plague was discovered at Conata 

Basin, South Dakota, where the largest breeding population 

of black-footed ferrets resided. To date, about a third of the 

prairie dogs at Conata Basin have succumbed to plague 

along with an unknown number of ferrets. Efforts to control 

the outbreak include pesticide application, and trapping and 

vaccination of ferrets, but these are costly, labor-intensive, 

short-term solutions. Consequently, continued research 

into the development of oral vaccines to prevent plague in 

prairie dogs is needed.

Miklos Gyuranecz, NWHC

Black-footed ferret in snow.

Jennifer Cordova, AZ Fish and Game Dept.

Prairie dog in Conata Basin, S.D.

Historical range of the blackfooted ferret.

Historical range

Last known range
For addition information, please contact:

Dr. Tonie E. Rocke

U.S. Geological Survey

National Wildlife Health Center

6006 Schroeder Road, Madison WI 53711-6223

(608) 270-2451 FAX (608) 270-2415

www.nwhc.usgs.gov

Current efforts to halt the spread of plague in prairie dog 

colonies typically rely on dusting individual prairie dog 

burrows with pesticides that kill plague-infected fleas. 

However, this pesticide application is labor intensive, costly 

and difficult to sustain over time. As an alternative approach, 

scientists at NWHC, in collaboration with the University of 

NWHC

USGS scientist injects a vaccine into 

a black-footed ferret.

Wisconsin–Madison, 

have developed plague 

vaccines for prairie dogs 

that can be delivered 

via oral bait. Laboratory 

studies have shown that 

voluntary consumption 

of this vaccine-laden bait 

by prairie dogs results 

in nearly complete 

protection against a 

severe challenge of the 

plague bacterium. Additional work is in progress to develop 

suitable bait for delivering the vaccine to prairie dogs that can 

be dispersed from a plane or vehicle and to further confirm 

the safety of the vaccine in non-target animals, such as other 

rodents and domestic animals. The ultimate goal is to license 

vaccine-laden bait for field application in targeted areas where 

black-footed ferrets are living or are released as part of the 

captive-breeding program. It could also be used in National 

Parks or urban areas where the potential for human exposure 

to plague-infected rodents is high. Using vaccine-laden bait 

provides effective immunity against plague that would allow 

treatment of more prairie dogs, with less labor and expense, 

than dusting. Better protection of prairie dogs would minimize 

the risk of disease transfer to ferrets, aid in prairie dog 

conservation and protect public health. Developing effective 

vaccines for both ferrets and prairie dogs will enhance recovery 

of the endangered black-footed ferret.

Miklos Gyuranecz, NWHC

Prairie dog in Conata Basin, South Dakota.

Vaccine-Laden Bait: A New Method for Field 

Application

Successful Immunization of Black-Footed Ferrets

Scientists at NWHC have demonstrated in the laboratory 

that vaccination can protect black-footed ferrets from plague. 

Collaborative field 

studies with the 

FWS and the USGS 

Fort Collins Science 

Center have verified 

that vaccination can 

improve ferret survival 

in the wild. The plague 

vaccine used for ferrets 

is an injectable protein, 

developed for humans 

by the U.S. Army 

Medical Research 

Institute for Infectious Disease. To provide full immunity 

against plague, two injections of the vaccine, spaced about 

30 days apart, are required. Starting in 2008, all captive-born 

ferrets released into the wild as part of the recovery program 

will receive two doses of plague vaccine prior to release. This 

vaccine is also being used to protect wild-born ferrets in the 

face of the ongoing epizootic in the Conata Basin. However, 

field vaccination is difficult because of the time-consuming 

tasks of finding, capturing, handling and injecting free-ranging 

animals. Unfortunately, immunization of ferrets against plague 

will not prevent the potential loss of their prey base – prairie 

dogs. Ultimately, successful management of plague in ferrets 

will depend on managing the disease in prairie dogs.

Randy Matchett, FWS

FWS scientist sprays pesticide on 

a prairie dog burrow to kill plaguecarrying fleas.

Jennifer Cordova, AZ Fish & Game Dept.

Black-footed ferret kits awaiting release into the wild.

plague outbreaks in the western U.S. The bacterium that causes 

plague was inadvertently introduced into North America in the 

early 1900’s. Because it is foreign to the evolutionary history 

of North American mammals, most species have little or no 

immunity and succumb quickly to the disease. Prairie dogs 

are particularly susceptible to plague and suffer high mortality 

rates (90% or more) during outbreaks, often resulting in local or 

even regional extinctions. Black-footed ferrets are also highly 

susceptible to plague, contracting the disease by ingestion of 

infected prey or via infected flea-bite. But even if they manage 

to avoid plague exposure, prairie dog population declines 

that result from a severe outbreak are devastating for ferret 

populations as they rely almost exclusively on prairie dogs for 

their survival. Plague is now considered endemic throughout the 

western states, but was newly discovered in the Conata Basin, 

South Dakota, in May 2008. This site is the most successful 

black-footed ferret reintroduction site in the U.S. and was 

considered plague-free until this outbreak.