Plague, a highly lethal disease caused by a flea-borne bacterium (Yersinia pestis), can infect >200 species of mammals. Its virulence and catholic preference for varied hosts gave Y. pestis the potential to become highly disruptive as it invaded western North America ca 1900. Prairie dogs (Cynomys), colonial rodents of North American grasslands that supported endangered black-footed ferrets (Mustela nigripes), were known to be highly plague-susceptible, with nearly 100% mortality during epizootics. Like many carnivores, ferrets were thought resistant, with plague’s impacts limited to loss of prey. That resistance assumption proved false, and the common detection of plague antibodies in other carnivores raised concerns that ferrets themselves might also be vulnerable to background levels of enzootic plague.
Results/Conclusions
Enzootic plague is difficult to detect in small mammal populations. In a Montana study, an experimental plague vaccine or insecticides used in burrows to control fleas increased annual ferret survival rates by 240% despite no noticeable changes in prairie dog numbers and failure to find evidence of Y. pestis in fleas or serum of small mammals. In a concurrent and nearby study, survival of adult black-tailed prairie dogs (C. ludovicianus) was improved by about 44% with vector control, again without other evidence of plague in rodents. Similar white-tailed prairie dog (C. leucurus) /ferret studies in Colorado and Utah confirmed the substantial hazard of enzootic plague to ferrets. Although details of Y. pestislife history in sylvatic form remain elusive, the manipulative trials above suggest the effects of enzootic plague can be demographically transformative.
Is the ferret/prairie dog/plague scenario described above an anomaly, or does it exemplify a broader phenomenon? Additional investigations were done in 5 western states at sites selected without consideration of plague presence. Plague vaccines or vector control improved survival 25-150% for 7 other species of murid and sciurid rodents, including the federally-listed Utah prairie dog (C. parvidens). Enzootic plague seems to be maintained in a wide variety of ecosystems. The rate of mortality, even due to background levels of plague, may substantially alter population and community attributes, indirectly and directly affecting ecosystem function and structure over the long term. Plague should be considered as a potential cause for a wide variety of conservation challenges in the western U.S. Combinations of vector control, vaccines, and genetic manipulations are being considered and tested for integrated plague management on moderate scales at core conservation sites.
(This information is preliminary and subject to revision)