We propose the name Rickettsia japonica sp. nov. (with type strain YH [= ATCC VR-13631) for a serologically specific species of spotted fever group rickettsiae that are pathogenic for humans (J. Infect. Dis. 159:1122-1126, 1989; J. Clin. Microbiol. 28:1177-1180, 1990). The biologic and genomic characteristics of the organism (G+C content, 31.2 +-0.7 mol%) are essentially the same as those of other pathogenic spotted fever group rickettsiae, although the R. japonica isolates cause a persistent infection in Vero cells for many subcultures.In this paper we formally describe Rickettsia japonica (23,25), which has been identified as the causative agent of a human disease. The first isolate, strain YHT (T = type strain), was isolated in 1985 from the blood of a patient with febrile exanthematous illness in Japan by using a tissue culture technique (20). Five strains of the causative agent, including strain YHT, have been isolated from patients that were serodiagnosed as having a spotted fever group (SFG) rickettsiosis (22).Justification for a new species. The justification for establishing a new species for this organism is based on the current standard method for comparing rickettsial taxonomic types by serologic analysis (23). Experiments involving reciprocal cross-reactions of mouse polyclonal antibodies to strains of the new species and other species of SFG rickettsiae produced by the standard method were carried out to calculate the specificity differences. By using this approach we demonstrated that all five strains belong to a single species that is distinct from all of the previously described SFG rickettsiae that are known to be pathogenic for humans (23). In addition, none of the isolates reacted with mouse monoclonal antibodies that are species specific for other pathogenic SFG rickettsiae (23). Furthermore, the results of Western immunoblotting revealed different electrophoretic mobilities and antigenic reactivities for the major immunodominant high-molecular-weight surface polypeptides of the Japanese isolates and standard pathogenic SFG rickettsial strains (23). Species-specific monoclonal antibodies to R. japonica reacted only with strains of R. japonica, supporting the conclusion that R. japonica is a new species of SFG rickettsiae (25).

A serosurvey for antibodies to Rickettsia japonica was conducted on Hainan Island of China. Serum specimens were collected from 1,030 outpatients at hospitals in different parts of the island regardless of their diagnosis. Only two among 538 serum specimens collected in Baoting and Tongshi counties, located in the southern part of the island, were demonstrated to contain antibodies reactive with R. japonica at a high dilution. The specimens also reacted with R. rickettsii at the same titer as with R. japonica. These two specimens reacted with other pathogenic spotted fever group (SFG) rickettsiae to a lesser extent. On the other hand, the specimens were shown to possess antibodies reactive with R. typhi at a significantly lower dilution or were not reactive at all. The findings suggested the occurrence of an SFG rickettsiosis on Hainan Island. More than half of the serum specimens collected from patients with suspected rickettsial infections in the southern area were found to contain IgM and IgG antibodies to R. typhi, indicating a high incidence of murine typhus.