A Model of Rabies Diffusion

Southeastern Geographer Vol. XX, No. 1, May 1980, pp. 1-15 A MODEL OF RABIES DIFFUSION* Neal G. Lineback Host-borne diseases have received scant attention from geographers, perhaps because of the difficulties encountered in gathering data and mapping diffusions. Of the mapping that has been done concerning rabies , most has been on a macroscale, providing information on regional directions and rates of the disease's diffusion. (1) Although regional and national trends yield important results, microscale mapping may provide considerable knowledge about source areas and the effects of physical and cultural characteristics on the behavior of the virus, particularly in and around enzootic areas. Frequent mention of enzootic areas occurs in the literature of rabies research. Winkler states, "In other areas . . . the disease has become enzootic and appears to have established a state of equilibrium in which fox rabies persists at fairly constant levels with no appreciable shifts in population numbers or characteristics. Rabies is enzootic in fox populations of Virginia . . ." (2) The understanding of enzootic areas, their physical and cultural geography , and their possible roles in the origins and diffusions of rabies epizootics appear paramount in solving the overall occurrence of rabies. This study identifies an enzootic area, analyzes several relationships between rabies occurrence and the geographic setting, and attempts to model possible movements of rabies within the study area. RABIES DIFFUSION. The spatial diffusion of rabies is of concern to persons in numerous fields such as epidemiology, zoology, biology, speleology , conservation, veterinary medicine, agriculture, and geography. Researchers are aware that the disease is transmitted through direct and possibly indirect contact between mammals, but the actual process of transmission, or diffusion, is not fully understood. The laboratory offers controlled conditions under which any rabid animal, wild or domestic, can be studied at great length. The fox has * The field research for this paper was supported by a grant from the Research Grants Committee of the University of Alabama. Dr. Lineback is Professor of Geography at the University of Alabama in Tuscaloosa, AL 35486. 2 Southeastern Geographer been the subject of most laboratory research and a large volume of data is available documenting rabid fox behavior. The problem with many of these data, however, is that the animals have not been studied in their natural environs. Thus a clear understanding of how rabies epizootics occur in nature has not been achieved. (3) The rates of movements of waves of rabies through fox populations have been calculated in Europe, Canada, and the United States. The rates are similar, approximating 24 miles (38 km) per year. Virtually all rabies researchers have noted that the rate is modified, however, by such factors as population density, level of immunity in the population, seasonal dispersal of the young, topographic and other barriers, and disease -induced aberrant behavior in individual foxes. (4) Research has repeatedly shown a wave-like spread of the disease through animal populations, but mostly through areas where the disease has been introduced for the first time in several years. Racoon rabies in Florida and southern Georgia, skunk rabies in Oklahoma, and fox rabies in Grey County, Ontario, are just a few of the examples of this wave-like phenomenon. (5) The "wave theory" may not always explain the dispersal of the disease , however, particularly in and around enzootic areas. One study, "Rabies Diffusion in Middle Appalachia," failed to show the wave-like spread of the disease, but a spotty pattern of outbreak instead. (6) Particular source points seemed to appear and grow, then to skip about and die. The waves, when they occurred, were shortlived, seldom extending over more than three or four counties or in the same direction for more than one year. Most important, however, is that the source points of most epizootics over a five-state area appeared to be in and around recognized enzootic areas, the most prominent being in the Ridge and Valley physiographic province of Virginia and Tennessee. In the corners of these two states is a group of counties where rabies occurrence is common, with almost no year passing without a positive rabies case being confirmed. Why should enzootic rabies occur here? What are the physical and cultural interrelationships which make this area so...