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A cooperative interagency program of sampling and testing wild carnivores for plague antibody has been utilized as an important component of an integrated plague surveillance program in California since 1974. The carnivore serology program involves the California Department of Health Services, the U.S. Department of Agriculture, APHIS/Animal Damage Control, the U.S. Department of Health and Human Services, Centers for Disease Control, and various other cooperators. This paper summarizes the results of the testing of wild carnivores, opossums, and feral pigs over the past two decades from 49 of California's 58 counties, and discusses the importance of the program to the overall statewide plague surveillance and control effort. A total of 8,962 samples have yielded 1,076 plague positives from wild carnivores, opossums, and feral pigs in 32 California counties. The highest percent positives have been from pine martens, mountain lions, bobcats, and grey foxes. A propensity for certain rodent prey species may increase the chance of plague infection for individual carnivorous species. The methodology has proven cost effective, allowed for detection of plague over a broader scale and in regions with no previous disease history, and demonstrated persistence of disease in apparent geographical foci within the state. In addition, the sampling of carnivorous animals in winter/spring in specific endemic regions has proven useful as an early alert system for disease activity, preceeding summer plague epizootics among susceptible rodent populations, and when human plague cases associated with epizootics normally occur.