Lessons from sylvatic plague: how to assess vaccine efficacy?

Sylvatic plague, caused by Yersinia pestis, is a flea-borne disease of wild rodents that frequently decimates prairie dog (Cynomys spp.) populations and associated species. The disease is considered a major contributor to the... [ view full abstract ]

Sylvatic plague, caused by Yersinia pestis, is a flea-borne disease of wild rodents that frequently decimates prairie dog (Cynomys spp.) populations and associated species. The disease is considered a major contributor to the demise of black-footed ferrets (Mustela nigripes) and other threatened North American mammals. Vaccination against plague, via injection or through oral consumption of vaccine laden baits, is increasingly being considered as a complementary plague management strategy to insecticidal dusting of burrows for flea control. However, to confirm effectiveness in field settings and justify the costs of implementing a comprehensive and perhaps perpetual vaccination program, field efficacy trials are essential. An injectable F1-V fusion vaccine was shown to be highly efficacious in black-footed ferrets, and field evidence has demonstrated that survival of vaccinated ferrets was twice that of non-vaccinated animals. In addition, an oral sylvatic plague vaccine (SPV), developed for prairie dogs and delivered via baits, is currently being tested in field trials in numerous western states with many cooperative state, federal and tribal partners. Early results suggest that SPV effects are positive and could help mitigate plague effects on prairie dogs and possibly improve black-footed ferret recovery, although that has yet to be tested. Discussion will focus on the methods and challenges of assessing vaccine efficacy in wild populations. Wildlife managers want to know, does vaccination work, and if so, when, where, how long, and how much will it cost?