Serological cross-reactivity between spotted fever and typhus groups of rickettsia infection in Japan.

We examined the frequency of cross-reactions to Rickettsia typhi in patients with Japanese spotted fever (JSF) and evaluated the differences between two rickettsiae using antibody endpoint titers. Patients' immunoglobulin (Ig)M and IgG titers against Rickettsia japonica and Rickettsia typhi in two phases were measured using an indirect immunoperoxidase assay at two reference centers for rickettsiosis in Japan. Cross-reaction was defined as a higher titer against R. typhi in convalescent sera than in acute sera among patients fulfilling the criteria for JSF diagnosis. The frequencies of IgM and IgG were also evaluated. Approximately 20% of cases showed positive cross-reactions. A comparison of antibody titers revealed the difficulty in identifying some positive cases. Cross-reactions of 20% in serodiagnosis may lead to the misclassification of rickettsial diseases. However, with the exception of some cases, we were able to successfully differentiate JSF from murine typhus using each endpoint titer.

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Declaration of competing interest The authors have no conflict of interest to declare.