A Study of Rickettsia tsutsugamushi in Russia.

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For the ecological and epidemiological study of tsutsugamushi disease in the Fdr East Russia, we attempted to isolate Rickettsia tsutsugamushi from trombiculid mites and wild rodents which were collected on the field near Vladivostok. The spleens, livers and sera of 278 rodents were frozen in liquid nitrogen immediately after dissection, and the frozen samples in a dryice jar were carried back to Japan. By immunofluorescence test, antirickettsial antibodies were found only in 4 rodents : two positive and two puedopositive.The organs from these four rodents and 21 others captured in the same areas were used for rickettsial isolation. On the other hand, 12,600 engorged mites were harvested from the rodents, and the alive mites were also used for rickettsial isolation after identification of the species. The homogenates of mites and the organs of rodents were inoculated to L or Vero cell cultures, or mice treated with or without immunosuppressive agent. In our experience, these cells and mice are adequate for isolation of both virulent and avirulent rickettsiae. The cells were kept for two months by exchanging occasionally the culture medium, and spleen-homogenates of the inoculated mice were blind-passaged twice to other mice. However, rickettsiae were isolated in neither cell cultures nor mice. Furthermore anti-rickettsial antibody did not rise in the inoculated mice. These results indicated that very few mites and rodents in the Far East Russia were infected with R.tsutsugamushi at present. However, Tarasevich, a coworker of this study, reported that the rodents and mites in this region were heavily infected in 1960s and rickettsiae were easily isolated from these animals. These lines of evidence suggest that rickettsial infection in the trombiculid mites decreased markedly during a few decades. An interesting and important question on the reason of these dramatic change is remained for further studies