Susceptibility of Small Mammals to Nonvirulent Spotted Fever Group Rickettsiae1

Abstract

Studies were performed to evaluate the significance of the Golden-mantled Ground Squirrel, Spermophilus lateralis tescorum, the Columbian Ground Squirrel, Spermophilus columbianus columbianus, and the Yellow-pine Chipmunk, Eutamias amoenus, as sources of infection for the nonvirulent spotted fever group rickettsiae Rickettsia montana (M/5–6 B strain) and R. rhipicephali (3–7 ♀ 6 strain). Results indicated that none of 16 animals developed a rickettsemia following intraperitoneal (i.p.) inoculation of 200 egg-infectious doses (e.i.d.) of R. montana. Similarly, none of 12 animals developed detectable rickettsemias following i.p. inoculation of R. rhipicephali. Conversely, all 6 animals, 2 per species, responded with rickettsemias following inoculation of virulent R. rickettsii (Sawtooth ♀ 2 strain). Furthermore, none of 991 ticks that fed on animals inoculated with R. montana showed evidence of rickettsial infection, whereas low infection rates of 0.9 and 0.5% were recorded for ticks that fed on a single Golden-mantled Ground Squirrel and chipmunk, respectively, that had been inoculated with R. rhipicephali. None of 781 ticks that fed on the remaining 4 rodents showed evidence of infection. However, infection rates up to 11.0% were noted in nymphal ticks that had fed as larvae on Golden-mantled Ground Squirrels, chipmunks, and Columbian Ground Squirrels infected with R. rickettsii. These observations suggest that Golden-mantled Ground Squirrels, chipmunks, and Columbian Ground Squirrels are of little or no significance in infecting ticks with R. montana or R. rhipicephali.