The Ecology of Rabies in Northeastern Tennessee

The study sought to determine the ecological factors which influenced the time and place at which rabid animals occurred in northeastern Tennessee. The study region consisted of 30 counties in North Carolina, Tennessee, and Virginia. Within this region three Tennessee counties, the study area, were selected for detailed field study.

The objectives of the study were pursued by means of (1) a detailed retrospective analysis of reported rabies cases in the study region over several decades, (2) the collection of data on recent (1972-1976) reported rabies cases in the study area, (3) a serum survey to detect rabies antibodies among striped skunks, opossums, and free-roaming domestic cats in the study area using rapid fluorescent focus inhibition test (RFFIT) and serum neutralization (SN) procedures, and (4) an analysis of prominent environmental features and wildlife population characteristics within the study area in relation to the sites of reported rabies cases.

The study found that over approximately three decades the major rabies host shifted from the dog, to the fox, and finally to the skunk. These changes were associated with different spatial and temporal patterns among reported rabies cases. Reported rabies cases occurred in both macrofoci within the study region and microfoci within the study area.

Within the study area reported rabies cases were most numerous during the months of March and April. Microfoci of reported rabies cases were characterized statistically by areas of low or moderate forest/woodland cover and away from major lakes or rivers. Rabies seropositive opossums, domestic cats, and a striped skunk were found. Approximately 17 percent (101/608) of the opossums sampled were found to be rabies seropositive. The portion of rabies seropositive opossums increased as the animals matured from juveniles to adults. The salivary gland of one opossum contained rabies antigen. Seropositive opossums and cats were found in most areas where reported rabies cases occurred, but they were distributed over a greater area than reported rabies cases.

The study concluded that clinical rabies within the study region was a dynamic disease entity with changes in host species, spatial pattern, and temporal pattern of reported cases. The microfoci of reported rabies cases in the study area were considered to be real concentration of clinically rabid animals and functions of environmental influences on the host-virus relationship. The temporal focus of reported rabies cases was considered to represent a real peak in clinically rabid animals and may have resulted from the activation of latent rabies infections or newly acquired infections from sources other than clinically rabid animals. Clinical rabies among skunks appeared in an area of low skunk population density. A major decline in the skunk population appeared to precede the rise in reported skunk rabies cases. Rabies infected opossums and cats were dispersed in time and space beyond the spatial and temporal foci of reported rabies cases. These data indicate that the circulation of the rabies virus may not be dependent upon the occurrence of clinically rabid animals. Overall, the study data suggest that the rabies virus may be maintained by nonlethal infections among several species and transmitted by means other than the bite of a clinically rabid animal.