Japanese Spotted Fever With Marked Serological Cross‐Reactivity to Rickettsia typhi Confirmed by Sequence‐Based Molecular Analysis: A Case Report

We report a case of Japanese spotted fever (JSF) showing marked serological cross-reactivity with Rickettsia typhi, which made serological differentiation difficult. An 80-year-old man presented with fever and disseminated erythematous macules. Laboratory finding suggested a rickettsial infection. Minocycline treatment led to clinical improvement. Real-time polymerase chain reaction screening was positive for spotted fever group rickettsiae. Serological testing using paired sera demonstrated comparable increases in antibody titers against R. japonica and R. typhi in the convalescent phase, precluding differentiation based on serology alone. Sequence-based molecular analysis using DNA extracted from the patient's serum revealed 100% identity with R. japonica strain YH, leading to a definitive diagnosis of JSF. This case highlights the diagnostic challenges posed by serological cross-reactivity and underscores the clinical value of molecular diagnostic approaches when serological findings are inconclusive.