Five strains of Rickettsia tsutsugamushi were isolated in Ehime Prefecture during December 1987 to January 1990. Of these, two strains, the Yamazaki and Noma-3, were isolated at Noma area of Imabari city and three strains, the Kakiwara-l0, -11, -12, at Kakiwara area of Uwajima city. The Yamazaki strain was isolated from a patient of tsutsugamushi disease and the other strains from wild rodents (Apodemus speciosus). These strains showed virulence in euthymic mice. The calculated LD₅₀ of the Yamazaki and Kakiwara-l0 strains showed 10^-3.0 and 10^-1.8, respectively.
The immunofluorescent antibody test using thirty monoclonal antibodies to six representative strains, the Gilliam, Karp, Kato, Irie, Hirano and Shimokoshi, revealed that two strains isolated at Noma area, the Yamazaki and Noma-3, were identified as the Karp type and three strains at Kakiwara area, the Kakiwara-l0, -11, -12, were identified as the Kato type. It was clarified that the serotypic differences were present among the strains isolated in Ehime Prefecture. Moreover, these five strains isolated in Ehime Prefecture did not react with the serotype-specific monoclonal antibodies to the Irie, Hirano and Shimokoshi strains known as the representative strains of so-called new type of tsutsugamushi disease, showing antigenic differences.