In the last several years, the number of tsutsugamushi disease (scrub typhus) patients began to increase throughout the mainland of Japan, not excepting Aomori Prefecture in the northern district. By analysing the data on distribution of chiggers, the outbreak of cases, and the isolation of Rickettsia in Aomori Prefecture, the author discusses the epidemiological aspects of the new type (spring and autumn type) tsutsugamushi disease, in which species other than Leptotrombidium akamushi (Brumpt) play the role of vector. Including the data from Sasa (1956) to the author (1979), 3 sub-families, 9 genera and 28 species of chiggers have been recorded in Aomori Prefecture. Of these, L. pallidum (Nagayo et al.) is one of the dominant species, and it is distributed widely and shows the peaks of occurrence both in spring and autumn in this district. After finding a suspected case reported in 1961,15 cases were found in Aomori Prefecture during the years 1971-1981. These cases were limited in spring and autumn, and some regions (Noheji, Shichinohe, Sannohe and Namioka). This led us to a new understanding of the "noxious areas" in this district. Strains of Rickettsia tsutsugamushi (Rt) were isolated from rodents trapped at the restricted locations in the Noheji and Namioka regions where patients had been infected. Especially in Namioka, where a lethal case occurred, the Rt positive rate in rodents was as high as about 57%, and one series of L. pallidum recovered from a Rt positive rodent was positive for Rt. The above observations suggest firmly that L. pallidum plays the role of vector of the new type tsutsugamushi disease in the noxious areas in Aomori Prefecture.
