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

Abstract

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.