S5100 MAC Attack: Intestinal Mycobacterium avium Complex Infection in an Immunocompetent Host Mimicking Tuberculosis

Introduction: Mycobacterium avium complex (MAC), a nontuberculous mycobacterium, is a rare cause of intestinal infection, particularly in immunocompetent individuals. In tuberculosis (TB)-endemic regions, MAC infection may be misdiagnosed as intestinal TB due to overlapping clinical and histological features. Case Description/Methods: A 22-year-old immunocompetent man presented with a 2-month history of intermittent fever, upper abdominal pain, constipation, and unintentional weight loss. He initially underwent exploratory laparotomy for acute abdomen and was empirically started on anti-tuberculous therapy (ATT). Six months later, persistent symptoms led to a second laparotomy with right hemicolectomy and ileostomy. Histopathology showed granulomatous inflammation, consistent with intestinal TB, and ATT was restarted. Despite completion of therapy, the patient’s symptoms worsened. Repeat imaging revealed small bowel dilation, multiple calcified mesenteric lymph nodes, and bilateral pulmonary micronodules. Colonoscopy with random biopsies was performed to evaluate for Crohn’s disease. Histopathology, culture, and polymerase chain reaction (PCR) of intestinal biopsies confirmed Mycobacterium avium complex, with negative results for Mycobacterium tuberculosis. AFB staining and GeneXpert supported the diagnosis of NTM infection. RIPE therapy was discontinued due to hepatotoxicity and clinical non-response. A targeted MAC regimen (clarithromycin, rifampin, and ethambutol) was initiated, resulting in significant clinical and biochemical improvement within 6 weeks. The patient completed a 12-month treatment course with normalization of liver enzymes and no relapse on follow-up. Discussion: This case emphasises the importance of including MAC in the differential diagnosis of granulomatous intestinal disease, even in immunocompetent patients, particularly in TB-endemic areas. Misdiagnosis can lead to inappropriate and potentially harmful therapy. Early use of tissue biopsy, PCR, and mycobacterial cultures is essential to differentiate MAC from TB. Clinicians should maintain a high index of suspicion for NTM when facing refractory or atypical presentations of intestinal granulomatous disease.