Evaluation on the occurrence of Tsutsugamushi disease in Tottori Prefecture

In Tottori Prefecture, we gathered the data of all patients known to have been infected with tsutsugamushi disease (TD) over a span of 49 years, from 1950 to 1998. The total number of patients reported so far has been 19, 16 of which occurred after 1988. Recently, it has been observed that the incidence tended to increase. Of the 13 laboratory confirmed patients, 7 came from Saji Village and Chizu Town, in the Yazu district, a place whereby, a body of water known as the Sendai River partly passes through, and is located in the eastern region of Tottori Prefecture. The remaining 6 came from Nichinan Town in the Hino district, a locality whereby the Hino River partly passes through; in the western part of the same prefecture. Both are mountainous areas, where corniferous trees such as Japanese cedars and pines grow. These results indicate that the occurrence or prevalence of TD in Tottori Prefecture was restricted only to particular places. Seasonal occurrence was during April and May in Nichinan, while October and November in Yazu, particularly in Saji, with one exceptional case in Chizu in which TD occurred during spring. It is likely that the causative chigger mite is the Leptotrombidium pallidum (L. pallidum). The causative agent in Yazu was the Gilliam strain only of Orientia tsutsugamushi (O. tsutsugamushi), while in the latter, it was the Karp, Kato or some unknown strains with antigen(s) common to the Gilliam, Karp and Kato strains. The seasonal difference in the occurrence among the three places may be caused by the difference in temperature because the average autumn/spring temperature difference in the 2 districts was 2-3 degrees C lower in Nichinan than in the Saji-Chizu areas. As a result, the activity of larval chigger mites may be temporarily halted or stopped by the relatively lower temperature in Nichinan during autumn.