Infective spots of Leptotrombidium pallidum naturally infected with Rickettsia tsutsugamushi (Rt) were discovered by exposing Microtus montebelli as sentinel animals on the ground at a corner of Hitsujiyama Park, Chichibu City, Saitama Prefecture, in November 1985. From a wild rodent Apodemus speciosus captured around the spots, engorged larvae of L. pallidum were collected and successfully reared by feeding on fresh eggs of Collembola. Four laboratory colonies (I-IV family lines) of L. pallidum positive for Karp type of Rt were established, and vertical transmission of Rt was examined from the parental generation (P) to F_3 in these family lines by isolating Rt from individual mites through the passage into mice. From F_1 to F_3 in 4 family lines, a total of 115 unfed larvae, 53 adult females and 51 adult males were examined for Rt. Only 6 males were negative and all the others were positive for Rt. The rate of transovarial transmission of Rt from P to F_1,F_1 to F_2,and F_2 to F_3 generations were 100%, and filial infection rates of their offsprings were nearly 100%. Spermatophores from infected males were examined for Rt and all of 78 spermatophores were negative for Rt. Furthermore, each of 7 Rt-positive males was mated with each of 7 Rt-negative females, and a total of 227 unfed larvae were obtained from these females. They were examined for Rt, but Rt was not isolated or detected from any of these larvae. From these results, the transmission dynamics of Rt was considered that Rt-positive males did not play any significant role in vertical transmission of Rt, but Rt was transmitted to the next generation only through eggs of Rt-positive females.
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Mamoru Takahashi (1990) studied this question.