Human Metapneumovirus-Associated Secondary Hemophagocytic Syndrome in an Immunocompetent Adult: A Rapid-Response Case Report

Background Human metapneumovirus (HMPV) commonly causes severe lower respiratory tract infection in adults, but HMPV-triggered secondary hemophagocytic lymphohistiocytosis (HLH) is exceedingly rare. Case presentation: A previously healthy 44-year-old woman presented with 4 days of fever and cough. On admission she had pancytopenia (white blood cells 2.9×10⁹/L, hemoglobin 64 g/L, platelets 39×10⁹/L). Bone-marrow smears showed hemophagocytosis and NK-cell cytotoxicity was reduced (8.6%). Metagenomic next-generation sequencing (mNGS) of bronchoalveolar-lavage fluid (BALF) identified HMPV (38,433 reads) as the predominant pathogen; cerebrospinal fluid (CSF) mNGS was negative. She met 7/8 HLH-2004 criteria (persistent fever, ≥ 2-line cytopenias, hemophagocytosis, low NK activity, hypofibrinogenemia, elevated ferritin, and elevated sCD25) and was diagnosed with HMPV-associated secondary HLH. Management included ganciclovir for antiviral coverage, a short taper of methylprednisolone for cytokine control, empiric voriconazole under steroid exposure (Day 6–12), high-flow oxygen, and osmotherapy for intracranial hypertension. Defervescence occurred by Day 5 with rapid hematologic and neurologic recovery; she was discharged on Day 11 and remained well at follow-up. Conclusions HMPV can precipitate HLH in immunocompetent adults. In severe viral pneumonia with otherwise unexplained cytopenias, early bone-marrow evaluation and HLH assessment are essential. Pathogen-directed therapy guided by respiratory-sample mNGS, combined with measured immunomodulation, may improve outcomes.