Babesiosis-associated HLH in an immunocompetent adult: Host-driven hyperinflammation at low parasitemia.

Hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening hyperinflammatory syndrome triggered by viral infections and malignancies, as well as tick-borne pathogens such as Babesia microti. Babesiosis-associated HLH is uncommon and can be difficult to recognize, particularly in immunocompetent individuals, due to clinical overlap with sepsis. We report a case of a 32-year-old previously healthy male presenting with fever, cytopenias, transaminitis, and hyperferritinemia, found to have babesiosis with low-level parasitemia (0.01%). Despite appropriate antimicrobial therapy with atovaquone and azithromycin, the patient developed worsening cytopenias, hemolysis, hypertriglyceridemia, and elevated soluble interleukin-2 receptor levels, fulfilling HLH-2004 diagnostic criteria. Pulse-dose intravenous methylprednisolone was initiated, resulting in rapid clinical and laboratory improvement. The patient was discharged on antimicrobial therapy with a prednisone taper and remained clinically stable. This case highlights that babesiosis-associated HLH can occur even in immunocompetent hosts and may present with severe hyperinflammation despite minimal parasitemia. Early recognition of persistent cytopenias and hyperferritinemia, along with timely initiation of immunosuppressive therapy in addition to antimicrobial treatment, is critical to improving outcomes.