Secondary hemophagocytic lymphohistiocytosis associated with Rocky Mountain spotted fever in a toddler: A case report.

A three-year-old boy presented with fever, maculopapular rash involving palms and soles, and hyponatremia two weeks following a tick bite. Empiric doxycycline that he was on was discontinued following negative initial rickettsial serology based on the non-endemicity of Rocky Mountain spotted fever (RMSF) in Northeast Ohio. He demonstrated high inflammatory markers and met the criteria for hemophagocytic lymphohistiocytosis (HLH). With a working diagnosis of macrophage activation syndrome secondary to presumed systemic-onset juvenile idiopathic arthritis (soJIA), he received HLH-directed therapy. Rising antibody titers in convalescent sera established the diagnosis of RMSF. The patient recovered completely with HLH directed therapy and re-institution of doxycycline. This is the first pediatric case report of induced HLH demonstrating a favorable outcome despite modified therapy.