Sequential Onset of Japanese Spotted Fever in a Parent-Child Household: A Familial Cluster of Two Cases

Japanese spotted fever (JSF) is a tick-borne rickettsial infection caused by Rickettsia japonica. Although most cases occur sporadically, familial clusters may arise when multiple individuals are exposed to infected ticks in the same environment. In this report, we describe two sequential cases of JSF, representing, to our knowledge, the first reported parent-child familial cluster. Case 1 involved a 61‑year‑old man who developed fever, rash, and multiple eschars one day after a tick bite. Laboratory testing revealed thrombocytopenia and elevated inflammatory markers. Intravenous minocycline, followed by oral therapy, resulted in clinical improvement. Polymerase chain reaction (PCR) testing of blood and eschar samples confirmed R. japonica. Case 2 involved his 86‑year‑old mother, who developed fever and fatigue four days after the onset of her son's symptoms. She had participated in the same outdoor activities and had continued to work in the field behind their home shortly before presentation. Physical examination revealed faint erythema, including the palms. Laboratory findings revealed mild hepatic dysfunction and elevated inflammatory marker levels. Oral minocycline was initiated, and PCR testing confirmed R. japonica. Both patients recovered without recurrence. Given the feeding behavior of ticks, sequential bites from a single tick are rare. This case suggests exposure to multiple infected ticks, either inside the home or in the adjacent field. Clinicians should consider JSF in patients presenting with fever and rash when a cohabiting family member is diagnosed with JSF, even in the absence of clear outdoor exposure. Early antimicrobial therapy is essential to prevent severe complications.