Epidemiological Studies on the Background of the Endemic Occurrence of Tsutsugamushi Disease in Toyama Prefecture

Abstract

In order to clarify the epidemiological background of the endemic occurrence of tsutsugamushi disease in Toyama Prefecture, Japan, since 1978, comparative surveys have been carried out between endemic and nonendemic areas. Rickettsia tsutsugamushi (Rt) was isolated at a rate of about 36% (158/439) from field rodents in the endemic area while it was not isolated from any of 280 in nonendemic areas. In all of six stations in the endemic area, a significantly high proportion of rodents were found to be Rt carriers. However, no Rt was isolated from rodents captured from July to September. The organism was isolated from rodents captured in the other months, especially in a high proportion in November when infestation of rodents with Leptotrombidium pallidum was at its peak. When the rodents were examined by indirect immunofluorescence staining, the rate of anti-Rt antibodypositive animals was about 55% (157/287) and about 17% (62/368) in endemic and nonendemic areas, respectively. Larvae of mites collected from the rodents were found to belong to four genera and 11 species. Among them L. pallidum was the only mite that had been known to be a vector of Rt. L. pallidum was found most frequently and in abundance from rodents in the endemic area, whereas it was present in very small numbers in rodents in nonendemic areas. The infestation of rodents with L. pallidum showed a seasonal variation, i.e. two peaks per year, in spring and autumn, and the number of mites detected was markedly greater in November than in spring. Rt was isolated from L. pallidum on rodents captured in the endemic area.

Scrub typhus ecology: a systematic review of Orientia in vectors and hosts

“…The differences between endemic and non-endemic areas have also been examined in a few cases. In Japan, 50% L. pallidum tested positive for O. tsutsugamushi in an endemic area (of human disease) and only 3.8% in a non-endemic area [219]. At the site of an intense outbreak of disease in plantation workers on the Goodyear Estate, Deli, Sumatra, 50.5% of rats were infested with mites with a chigger index of 104, whereas on two nearby estates without outbreaks, 6.3% and 2.3% of rats were infested with indices of 7 [192].…”

“…At the site of an intense outbreak of disease in plantation workers on the Goodyear Estate, Deli, Sumatra, 50.5% of rats were infested with mites with a chigger index of 104, whereas on two nearby estates without outbreaks, 6.3% and 2.3% of rats were infested with indices of 7 [192]. The study by Ishikura et al [219] certainly suggests that even areas thought to be non-endemic for scrub typhus, but occurring within an endemic region, may not be completely free of risk. Our understanding of this remains poor.…”