Rocky Mountain spotted fever in Clermont County, Ohio. II. Distribution of population and infected ticks in an endemic area.

The epidemiology of Rocky Mountain spotted fever (RMSF) in relation to the distribution of the human population and infected ticks has not been defined. A study was undertaken in Clermont County, Ohio, to describe the distribution of population in a highly endemic focus of RMSF and to determine the prevalence of ticks infected with rickettsiae of the spotted fever group. This area was compared to another area of similar size in the same county where the incidence of RMSF was much lower. The population in the highly endemic area was distributed diffusely throughout the area, allowing maximum interfacing of the human population and the tick vector. The population distribution in the other area differed from the endemic area in that most of the population was concentrated in one town. Of 1168 Dermacentor variabilis collected in the endemic area, 18 (1.5%) were infected, compared to four (1.2%) of 339 ticks in the other area. Past studies of infected ticks have not identified the specific strains or species of spotted fever group rickettsiae. In this study, 11 rickettsiae were serotyped by microimmunofluorescence. Four were Rickettsia rickettsii, five were R. montana, and two were unclassified rickettsiae of the spotted fever group.