Antigenic and genotypic variation in Orientia tsutsugamushi and its relation to virulence.

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Various antigenic variants and diversity of virulence to mice are recognized among the pathogenic agents of scrub typhus, Orientiatsutsugamushi. To clarify the variety of this agent, and its relation to pathogenicity, sero-and geno-typing of many newly isolated strains in Japan and Taiwan was performed by testing reactivity of the isolates with 15 type-specific monoclonal antibodies and PCR-RFLP patterns of their 56KDa type-specific antigenic protein gene. The results obtained indicated that 34 isolates in Japan were classified into 4types of Gilliam, Karp, Kawasaki and Kuroki, but the isolates of each type were further divided into 3,3,4, and 2 subtypes of Gilliam, Karp, Kawasaki and Kuroki, respectively. Ten isolates in Taiwan were clasified into 6 subtypes of Karp and 1 Gilliam type. Additionally, all Taiwan isolates could be distinguished from Japanese isolated. These results indicated that many variants in type and subtype levels exist among the isolates of this pathogenic agent. On pathogenicity to mice, most isolates off Gilliam and Karp types showed high virulence but all isolates of Kawasaki and Kuroki types were avirulent, suggesting the existence of a certain relationship between the pathogenicity and their serotypes.During the cause of these studies, we succeeded the cloning and sequencing of 16SrRNA gene of this microbe. Till quite recently, this microbe has been considered as a member of the genus Rickettsia and designated as Rickettsia tsutsugamushi. However, our many previous studies on this microorganism and the results of 16SrRNA gene homology analysis in the present study indicated that this microbe should be separated from the genus Rickettsia. Therefore we proposed to classify this microbe into Orientia gen.nov., as Orientia tsutsugamushi comb.nov.This proposal was accepted in the International Journal of Systematic Bacteriology.