Abstract

Sylvatic plague, caused by the bacterium Yersinia pestis, is primarily a disease of wild rodents that is transmitted between mammals via flea bite, direct contact, or inhalation. Since its introduction into the United States in the early 1900s, plague has become firmly established in native rodent populations throughout the west, causing frequent epizootics in prairie dogs (Cynomys spp.) and other wild rodents. The disease has had devastating effects on many prairie dog populations, often killing 90–100% of individuals in affected colonies. Black-footed ferrets (Mustela nigripes) depend primarily on prairie dogs for both food and shelter and thus may be exposed to the bacteria either by consumption of plague-infected prey or by flea bite. Once thought to be extinct, a captive breeding and recovery program was established for black-footed ferrets in 1987 after an outbreak of canine distemper nearly decimated the last known wild colony that was discovered 6 years earlier. The occurrence of plague in prairie dog populations and its potentially devastating effect on black-footed ferret re-establishment is a major impediment to the captive breeding and recovery program.

We conducted an experiment to assess the feasibility of vaccinating black-footed ferrets (BFF) against plague using a recombinant fusion protein consisting of F1 and V antigens from Y. pestis. On days 0 and 28, post-reproductive BFFs were immunized with the fusion protein by subcutaneous (SQ) injection. Control animals received a placebo by the same route. Two weeks after the second immunization, mean antibody titers to Y. pestis F1 antigen were measured and found to be significantly higher in vaccinates than their pre-immunization value (p<0.001) and significantly higher than the control value (p<0.001). Six months post-immunization, 16 vaccinates and eight controls were challenged with approximately 8,000 colony forming units of virulent Y. pestis by SQ injection. Eleven of 16 vaccinates survived challenge with no ill effects; their survival rate was significantly different (p=0.02) from the eight control animals, all of which died within 3–6 days. Two months later, the 11 surviving vaccinates were challenged again by ingestion of a plague-infected mouse. None of the animals showed any ill effects and all survived. In contrast, seven control animals fed infected mice died of plague within 2–4 days, including one animal that did not actually ingest the mouse, but likely sniffed or licked it. This study demonstrates that immunization of black-footed ferrets with the recombinant F1-V fusion protein can induce significant antibody responses and reduce their susceptibility to plague infection. Until other methods of plague control are developed, the F1-V vaccine might be useful in protecting black-footed ferrets in captive-breeding facilities and animals intended for reintroduction programs.