Abstract
RATIONALE: Japanese spotted fever (JSF) is a rare tick-borne disease caused by Rickettsia japonica. Atypical manifestations and a lack of standardized diagnostic assays often result in delayed diagnosis and treatment, potentially leading to life-threatening complications. PATIENT CONCERNS: A 57-year-old immunocompetent female from a region with no previously reported JSF cases presented with acute-onset high-grade fever (39.5°C), a generalized maculopapular rash, and systemic symptoms after participating in agricultural activities. The patient denied having any history of tick exposure and presented without eschar, leading to the initial misdiagnosis of respiratory infection. DIAGNOSES: Metagenomic sequencing (MetaCAP) technology enabled a definitive diagnosis by identifying Rickettsia japonica-specific DNA sequences in the patient's blood. The genomic results completely aligned with the clinical presentation. INTERVENTIONS: The patient was treated with doxycycline, which achieved rapid clinical resolution. OUTCOMES: The patient achieved full recovery with only residual lower-limb hyperpigmentation at the month follow-up, without disease recurrence. LESSONS: This case demonstrates the diagnostic value of metagenomic testing for fevers of unknown origin. JSF should be a key consideration for agricultural and forestry workers presenting with compatible symptoms, even in nonendemic areas without documented insect bites. The optimal diagnostic approach combines clinical evaluation with advanced molecular testing to ensure the accurate identification and proper management of tropical febrile illnesses.