Clinical Characteristics and Risk Factors for Severe Japanese Spotted Fever: A Retrospective Multicenter Study in 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 Nagasaki prefecture from 2010 to 2021. Severe JSF was defined by the presence of altered mental status, low blood pressure, or low oxygen saturation. In total, 65 JSF cases were diagnosed. Common symptoms were fever (87%), rash (48%), and fatigue (48%) with eschars detected in 50 (79.4%) of patients. Thirty-eight (60.3%) patients were initially diagnosed with non-JSF conditions. Twenty-one (33.3%) cases were categorized as severe JSF, including one death. Pre-hospital 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 diagnostic challenges of JSF due to its non-organ-specific clinical presentation. Delayed initial treatment, advanced age, and male sex significantly increase the risk of severity. It is crucial to further raise awareness of JSF among clinicians and residents in endemic areas.