Clinical Characteristics and Risk Factors for Severe Japanese Spotted Fever: A Retrospective Multicenter Study in the Nagasaki Prefecture, Western Japan.

Japanese spotted fever (JSF) is a tick-borne rickettsial disease prevalent in western Japan with an increasing incidence and geographical distribution. This retrospective study aimed to describe the clinical features of JSF and identify factors associated with its severe form. We included adult patients with laboratory-confirmed JSF in the Nagasaki Prefecture between 2010 and 2021. Severe JSF was defined as an altered mental status, low blood pressure, or low oxygen saturation. In total, 65 JSF cases were diagnosed. Common symptoms included fever (87%), rash (48%), and fatigue (48%), with eschars detected in 50 (79.4%) patients. Thirty-eight (60.3%) patients were initially diagnosed with non-JSF conditions. Twenty-one (33.3%) patients were categorized as having severe JSF, including one death. Prehospital factors associated with severe JSF included age ≥75 (adjusted odds ratio [aOR] 37.53, 95% confidence interval [CI] 3.03-465.38), male sex (aOR 26.5, 95% CI 4.23-166.00), and a treatment delay ≥4 days from onset (aOR 5.96, 95% CI 1.13-31.60). This study highlights the diagnostic challenges of JSF owing to its non-organ-specific clinical presentation. Delayed initial treatment, advanced age, and male sex significantly increase the risk of disease severity. It is crucial to raise awareness of JSF among clinicians and residents in endemic areas.