1. Forty strains of a rickettsia, referred to as the EM rickettsia, which was avirulent for guinea pigs and which retained this property after serial passage in chick embryos were isolated from Dermacentor variabilis collected in two counties of eastern Montana. The EM rickettsia possessed properties common to agents of the spotted fever group. However, although it appeared to be a variant of Rickettsia rickettsii, it could not be identified with any of the four previously distinguished type strains of this species.

  2. The EM rickettsia was also isolated from Dermacentor andersoni collected in one of these localities.

  3. EM isolates were compared with an avirulent U strain of R. rickettsii which had been recovered from D. andersoni from the Bitterroot Valley, Montana. They differed primarily in their respective capacities to produce cross-reacting antitoxin against the virulent R strain of R. rickettsii.

  4. Guinea pigs injected with 104 doses of EM rickettsias developed high titers of CF antibody but little, if any, antitoxin, against the R strain. In contrast, the U strain produced high titers of both antitoxin and CF antibody for the R strain.

  5. Guinea pig hyperimmune serum against the EM rickettsia neutralized toxins of three strains of R. rickettsii representing the R, S, and T types, respectively, but did not neutralize toxins of Rickettsia conori, Rickettsia siberica, or the rickettsia from the tick Amblyomma maculatum.

  6. A high rate of infection with EM strains was found in the tick populations examined. This rickettsia was recovered from about 6 to 8% of D. variabilis from each of five localities studied.

  7. With one exception all strains isolated in eastern Montana were of the EM type. One isolate which possessed the characteristics of strains carried by the tick Haemaphysalis leporis palustris was isolated from a lot of ten D. variabilis from the eastern Montana area. The T strain commonly isolated from D. variabilis from eastern United States was not encountered in the study area.

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