Vaccination as a potential means to prevent plague in black-footed ferrets: Progress and continuing challenges

This study was conducted to further assess the feasibility of vaccinating black-footed ferrets (Mustela nigripes) against plague (caused by the bacterium Yersinia pestis). On days 0 and 28, 17 postreproductive ferrets were immunized by subcutaneous injection with a recombinant fusion protein containing F1 and V antigens from Y. pestis. Another 17 animals received a placebo by the same route. Two weeks after the second immunization, mean antibody titers to Y. pestis F1 and V antigens were measured and found to be significantly higher in vaccinates than their preimmunization values (P < 0.0001) and significantly higher than the control values (P < 0.0001). Six months postimmunization, 16 vaccinates and eight controls were challenged with approximately 8,000 colony forming units of virulent plague by subcutaneous inoculation. Eleven of 16 vaccinates (69 percent) survived with no ill effects whereas all eight control animals died within 3a??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 ferrets fed infected mice died within 2a??4 days, including one animal that did not actually ingest the mouse but was likely exposed to it. This study demonstrates that immunization of ferrets with the recombinant F1-V fusion protein can induce significant antibody responses and reduce their susceptibility to plague infection.