Rodents, both commensal and wild, serve as reservoirs for a variety of diseases in nature communicable to man. Forty-six percent of the 30 human plague cases in the past two decades in California are associated with campgrounds and public use recreational sites. In addition, human cases of tularemia, giardiasis, relapsing fever, Rocky Mountain spotted fever, Colorado tick fever, and Lyme disease have been associated with recreational activities. Commingling of humans, wild rodents and ectoparasites in disease endemic areas places the public at risk from the above-mentioned diseases. Construction of recreational sites without knowledge of wild rodent and disease ecology may enhance rodent and ectoparasite densities and increase disease potential. Cooperation between vertebrate pest specialists, vector ecologists, and public landuse and recreation specialists is needed in the development and management of campground and public use recreational areas. Knowledge of disease awareness and management technology would result in improved disease prevention and better public health protection.

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, preceding summer plague epizootics among susceptible rodent populations, and when human plague cases associated with epizootics normally occur.

The occurrence of plague (Yersinia pestis) at Ice House Reservoir in 1994 and 1995 was characteristic of widespread epizootics in high use recreational areas of California. Staff of the Vector-Borne Disease Section investigated these epizootics and found dense populations of plague susceptible California ground squirrels (Spermophilus beecheyi) with high numbers of fleas, primarily Diamanus montanus, the most important plague vector. This combination indicated a high risk of plague exposure to campground users. A non-fatal human case of plague, contracted at Mountain Camp 11, was reported after the initial epizootic investigation. The patient's exposure occurred prior to the reporting of the epizootic die-off among the California ground squirrels. The plague investigation included direct observations, animal trapping, and laboratory testing of rodent carcasses, sera, and fleas. Plague management and prevention included flea control with 2% Diazinon dust and rodent population reduction using 1% zinc phosphide treated grain. Evaluation of the 1994 applications indicated a successful reduction of rodents and fleas. However, the need for an ongoing management program was emphasized in 1995 when the plague epizootic continued. Intrusion of plague infected rodents and their fleas necessitated a 1995 treatment in the four campgrounds involved.

Carnivores obtain plague infection through ingestion of infected rodents or rabbits or via flea bite. Most are resistant to infection, show little or no symptoms, and produce antibodies to Yersinia pestis which may persist for several months. Consequently, carnivores can be used as serological plague sentinels using the specific passive or indirect hemagglutination test in the laboratory. A carnivore serology program for plague detection began in California in 1974. The program is a cooperative effort incorporating state and federal vector-borne disease units and state and federal predator animal control personnel. The program has proven to be an important tool in the overall statewide plague surveillance program of the California Department of Health Services, and has been effective in detecting plague over broad geographical areas of the state, as well as in areas heretofore unknown to experience plague activity. Carnivore serology has also proven useful in helping to delineate the extent of plague epizootics, and has given insight into the patterns of enzootic plague among wild rodent populations.