Japanese Spotted Fever in an Elderly Patient Without Eschar and Negative Acute-Phase Serology: Diagnostic Value of Clinical Assessment and Histopathology

A 72-year-old woman presented with fever, generalized rash, including on the palms and soles, arthralgia, facial edema, and hepatic dysfunction. She lived in a rural area and had recent outdoor exposure to farming but no travel history. Physical examination revealed a small erythematous lesion on her right thigh, suspected to be a tick bite, though no eschar was present. Initial serological testing for Rickettsia (R.) japonica using the indirect immunofluorescence assay (IFA) was negative. Due to strong clinical suspicion of Japanese spotted fever, empirical treatment with oral minocycline was started, resulting in rapid resolution of fever and gradual improvement of liver function. A skin biopsy of a rash lesion revealed perivascular lymphohistiocytic infiltration and endothelial swelling, consistent with small-vessel vasculitis. Convalescent-phase serology later confirmed R. japonica infection with a fourfold rise in immunoglobulin G (IgG) titers. The patient fully recovered following a five-day course of minocycline. This case highlights the importance of early clinical diagnosis and empirical treatment in suspected Japanese spotted fever, especially in elderly patients with characteristic systemic symptoms and rural exposure, even when serologic confirmation is initially unavailable. Histopathological findings may support the diagnosis when serology is inconclusive. Prompt recognition and early intervention are essential to avoid complications and improve outcomes.