Influenza B-Triggered Secondary Hemophagocytic Lymphohistiocytosis in an Adult Male: A Diagnostic Challenge

Hemophagocytic lymphohistiocytosis (HLH) is a rare, life-threatening hyperinflammatory syndrome characterized by excessive immune activation and multiorgan dysfunction. We report the case of a 39-year-old Syrian male residing in the United Kingdom who presented with high-grade fever, abdominal pain, and polyarthritis. Investigations revealed bicytopenia, hyperferritinemia, and an H-score of 213, consistent with HLH. Influenza B infection was identified as the precipitating factor after extensive evaluation excluded malignancy, tuberculosis, and autoimmune disease. The patient was treated with high-dose corticosteroids and oseltamivir, leading to rapid clinical improvement and normalization of ferritin levels. He subsequently developed a left calf deep vein thrombosis, considered a thromboembolic complication related to the hyperinflammatory and hypercoagulable state of HLH, and was managed successfully with anticoagulation. This case underscores the diagnostic complexity of adult HLH and highlights the importance of recognizing viral triggers such as influenza B in atypical inflammatory presentations.