Investigation of Anti‐IgG Antibody Responses to the 17‐kDa Outer Membrane Protein of Rickettsia japonica in Individuals From a High‐Risk Area for Japanese Spotted Fever

This study evaluated the seroprevalence and persistence of IgG antibodies against Rickettsia japonica 's outer membrane protein (Omp) using a recombinant 17‐kDa antigen and Western blot analysis in residents of Minamiise, Japan—a high‐incidence Japanese spotted fever (JSF) area—and in individuals from non‐endemic regions. Antibody titres against the Omp antigen declined within approximately 2 years post‐infection, and the Omp epitope was not consistently recognised among confirmed JSF cases. Unexpectedly, a considerable proportion of individuals without documented JSF—including those from non‐endemic areas—tested positive for anti‐Omp antibodies. These findings raise concerns about the specificity of Omp‐based serological assays, which may cross‐react with other spotted fever group rickettsiae, such as Rickettsia tamurae , sharing 97.48% amino acid identity with R. japonica . The results suggest possible asymptomatic or past infections with R. japonica or related species and underscore the need for cautious interpretation of anti‐Omp antibody test results in both endemic and non‐endemic settings.