Interspecific differences between small mammals as hosts of immature Dermacentor variabilis (Acari: Ixodidae) and a model for detection of high risk areas of Rocky Mountain spotted fever.

Fourteen species of small mammals were captured from July 1990 through August 1991 in Tennessee, from which 1,217 immature Dermacentor variabilis and 1 Ixodes dentatus were collected. Mammal species were given scores of importance (TS) as hosts to immature D. variabilis based on mean intensity and prevalence. The rice rat ranked the highest, with a TS = 5, followed by the golden mouse TS = 4, white-footed mouse TS = 3, pine vole TS = 2, cotton rat TS = 1, with the Norway rat, house mouse, and short-tailed shrew all having a TS = 0. Assigning a TS allows a quantitative method for differentiating and ranking small mammals as hosts for immature D. variabilis. Relative abundance of a species can also be important in determining D. variabilis populations, even with a low TS. The potential of Rocky Mountain spotted fever (RMSFP) to occur in an area was estimated using the total score of small mammal hosts in an area and multiplying the relative abundance of important host species. The RMSFP of a site, based only upon small mammal species composition and relative abundance of important host species, was an accurate estimate of adult D. variabilis infesting raccoons and opossums at that trap site (P < or = 0.001). A RMSFP of 1.61 is needed to produce an estimated 252 adults per ha (RMSF threshold) at 98% survival of engorged immature ticks (P < 0.001).