Sylvatic Plague Immunization in Black-footed Ferrets and Prairie Dogs

Black-footed ferret
in Conata Basin,
South Dakota.

Sylvatic Plague is a bacterial disease transmitted by fleas that can afflict numerous species of mammals, including humans. Prairie dogs are one of the most important reservoirs of this disease and are the primary food source of the highly endangered black-footed ferret, which is also susceptible to the disease. Sylvatic plague can decimate prairie dog colonies, with mortality rates of 90 percent or more, resulting in local extinctions and population reductions. Because of the susceptibility of prairie dogs to sylvatic plague, coupled with the potentially devastating effect the disease can have on black-footed ferrets, it is a vital concern for ferret recovery programs and conservation efforts.

Scientists at the USGS National Wildlife Health Center (NWHC), in collaboration with other Federal agencies, have tested the feasibility of vaccinating black-footed ferrets and prairie dogs against sylvatic plague infections. The plague vaccine was developed for humans by the U.S. Army Medical Research Institute for Infectious Disease and is being tested for use on animals at the USGS National Wildlife Health Center. Vaccination of ferrets by injection was shown to be highly successful both in the laboratory and in the field. However, injectable vaccines are not practical for field use in free-ranging animals. Ultimately, management of the disease in ferrets will depend on managing the disease in prairie dogs.

Immunizing entire populations of free-ranging prairie dogs and other rodents is highly challenging, but preliminary NWHC studies indicate prairie dogs can be successfully immunized by voluntarily feeding on vaccine-laden baits. These studies suggest that plague could be managed through oral immunization, which would be especially useful in areas where captive-reared black-footed ferrets are to be released and in National Parks or urban areas, where the potential for human exposure is high.

Other efforts have been made to halt the spread of plague by dusting individual prairie dog burrows with pesticides that kill plague-infected fleas, but pesticide application is labor intensive, costly and difficult to sustain over time. Black-footed ferrets can also be vaccinated against plague to prevent infection, but protection from the disease will not prevent the potential loss of their prey base.

Sylvatic Plague Outbreak in Conata Basin, South Dakota

On May 15, 2008, sylvatic plague, a bacterial disease transmitted by fleas, was confirmed in prairie dog colonies in the Conata Basin Area. The U.S. Fish and Wildlife Service indicated that about 9,000 acres of prairie dog habitat have been affected as of June 19, and that some of the affected areas include colonies occupied by black-footed ferrets. To help increase ferret survival during this outbreak, biologists are vaccinating wild ferrets to provide immunity if they become exposed to plague. The plague vaccine was developed for humans by the U.S. Army Medical Research Institute for Infectious Disease and is being tested for animals at the USGS NWHC in Madison, Wisconsin.

Biologists from Prairie Wildlife Research working with federal agencies have captured and vaccinated 40 black-footed ferrets since the outbreak began, said Scott Larson with the U.S. Fish and Wildlife Service, who is coordinating measures to conserve ferrets among the federal agencies. About 75 prairie dogs were experimentally vaccinated in 2007 in South Dakota, and vaccination is continuing in 2008.

This is the first time the vaccine has been used during a major plague epizootic—an animal version of a human epidemic. Sylvatic plague is an infectious bacterial disease usually transmitted from animal to animal by fleas. This exotic disease is usually deadly for black-footed ferrets and their primary prey, prairie dogs.

Strategies to control the outbreak include applying insecticide to over 300,000 burrows to reduce the flea populations in prairie dog colonies that support black-footed ferrets, but that have not yet experienced plague die-offs. Another strategy is vaccinating some of the ferrets. As of July 23, over 60 ferrets had been captured and given the vaccine, which was originally developed by the U.S. Army Medical Research Institute for Infectious Disease (USAMRIID). USGS is currently working on an oral vaccine that could be incorporated into bait or food pellet that prairie dogs would readily consume. NWHC staff have been participating in conference calls regarding this outbreak and are closely following events.