Abstract
Regardless of the presence or absence of nest holes or other living signs of field rodents, soil samples (each with a volume of ca. 2,000(cm)^3) were dug out with a quadrate steel frame (20cm×20cm×5cm) at the selected sites in Nagano Prefecture. These samples were placed in the modified Tullgren's funnels for separating chigger mites. Chiggers obtained from the soil differed to some extent in the species composition from those found on the field mouse, Apodemus speciosus, livetrapped concurrently in the respective survey sites. At Matsumoto City, for example, Leptotrombidium intermedium was predominant among the chigger species in the soil, followed by L. pallidum, L. palpale, and Gahrliepia saduski; whereas, L. fuji was noticeably numerous among the chiggers found on A. speciosus. Species composition in the soil samples, however, varied according to the areas surveyed. Numbers of chiggers in the soil samples were usually less than those found on the wild rodents; but this alone could not be a serious drawback of the soil sampling. Rather, soil samples seem to give the better epidemiological information, because the samples taken from the sites haunted by the residents such as foot-paths between cultivated fields or grasslands around the dwelling quarters in Miasa Village and Iida City, where we could hardly trap the wild rodents, yielded a number of L. pallidum larvae, the most probable vector in this prefecture, besides several other chigger species. Such infiltrating distribution of L. Pallidum into the residential areas as evidenced in this report may give a positive explanation for the risk of contracting tsutsugamushi disease in such areas.
