When Influenza Masquerades as Gastroenteritis: Acute Diarrhea in an Adult With Influenza B.

Influenza is a common viral respiratory illness caused by influenza A and B viruses, typically presenting with fever, cough, sore throat, myalgia, and malaise. Gastrointestinal symptoms are less common, particularly in adults, but may occasionally predominate. Diarrhea as the initial manifestation of influenza is rare and may mimic acute gastroenteritis, leading to diagnostic delay and inappropriate therapy. This is a case report of a 46-year-old male with influenza B presenting primarily with acute watery diarrhea and fever, without initial respiratory symptoms. The atypical presentation initially led to a misdiagnosis of acute gastroenteritis. Confirmation of influenza B was obtained via rapid antigen testing, and the patient was successfully treated with oseltamivir, achieving full recovery. This case underscores the importance of considering influenza in the differential diagnosis of febrile diarrheal illness, especially during seasonal epidemics and in travelers. Recognition of such atypical presentations enables early laboratory confirmation and timely antiviral therapy, preventing unnecessary antibiotic use, and contributes to improved clinical and public health outcomes.