Computer Simulated Dynamics of a Rabies-Controlled Fox Population

Abstract

Computer simulation was used to examine the epizootiology of rabies in the red fox (Vulpes fulva). A hypothetical fox population was defined by age, sex, and location of its members. Reproduction and the quality and quantity of animal movements were simulated in accordance with available information. Rabies was the sole mortality source. The annual rabies cycle produced by the computer model was similar to that observed in the U. S. and Ontario, Canada. Its temporal dynamics were primarily determined by the relative number of susceptibles available at different times of year. Population mixing associated with juvenile dispersal had only a nominal influence on the rabies incidence pattern. The simulations suggest that the 3-year rabies cycle reported in Europe and Ontario, Canada could be an inherent consequence of interactions between red fox life history and the rabies transmission mechanism. This, however, is probably not true for the 4to 5-year cycles reported from areas where fox populations are primarily limited by factors other than rabies. The bite transmission mechanism appears to be well adapted to fox populations having low interindividual contact rates. In the simulations, each infective rabid fox needed to contact slightly more than 1.25 percent of the susceptible individuals in the vicinity of its travel path for rabies to become enzootic. J. WILDL. MANAGE. 37(4):501-512 The fox is one of the major vectors of wildlife rabies in North America (Johnson 1969, Johnston and Beauregard 1969, U.S. DHEW 1972). The dynamics of the foxrabies system results from the interacting life history processes of the host and disease. Though many of these processes are reasonably well documented, their holistic behavior does not lend itself to experimental analysis. Since the human health hazard makes most field experiments unfeasible, most rabies research has been limited to clinical studies and field surveillance. Computer simulation provides a useful alternative analytic method. In this study, a computer simulation model was constructed to examine the epizootiology of rabies in the red fox. The model constitutes one of the many hypothetical frameworks which could be proposed to describe the major interactions in a rabies controlled fox population. By comparing the behavior of the model with that observed in natural populations, the credibility of this hypothetical framework can be assessed. This research was supported by NIH Training Grant No. TO1 GM01779 from the National Institute of General Medical Sciences and U.S. Atomic Energy Commission (COO-1332-91). D. H. Johnston, A. B. Sargeant, and G. L. Storm read the manuscript in earlier forms and offered many valuable suggestions. H. Archibald and D. B. Siniff were consulted concerning computer programming. M. Gravdahl drew the figures.