Sequential Onset and Prognostic Impact of Japanese Spotted Fever in a Family Cluster: A Case Report and Brief Literature Review.

Japanese spotted fever (JSF) is an increasingly reported tick-borne disease in Japan. Although person-to-person transmission has not been documented, familial clusters have rarely been reported. We report two cases of JSF in a married couple in their 70s who developed symptoms one week apart after shared weeding activities. The wife presented with the classic triad of fever, rash, and eschar, whereas the husband presented with fever and rash but no identifiable eschar, creating a diagnostic challenge. Following confirmation of JSF in the wife, a skin biopsy specimen and blood samples were obtained from the husband, and the diagnosis was confirmed by polymerase chain reaction. We reviewed all previously reported familial JSF clusters in Japan and identified 11 patients from five families. Including the present two patients, a total of 13 familial cases were analyzed. Fever was observed in all 13 patients (100%), rash in 12 (92.3%), and eschar in seven (53.8%). The median interval between symptom onset among family members was five days (range = 0-10 days). Among five index cases, four experienced treatment initiation ≥6 days after symptom onset or received no effective JSF-directed therapy, whereas five of six subsequent cases received treatment within five days. Both fatal cases occurred among index patients. In familial JSF clusters, symptom onset may be staggered despite shared environmental exposure, and eschars may be absent in nearly half of patients. Recognition of an index case may facilitate earlier diagnosis and treatment of subsequently affected family members. Clinicians should consider JSF in patients presenting with unexplained fever and rash, even in the absence of an eschar, and obtain a detailed history regarding family illness, outdoor activities, and potential tick exposure.