Case Report: A Diagnostic Odyssey Unveiling BCGosis in an Elderly Patient

Background: Bacillus Calmette-Guérin (BCG) immunotherapy is a widely used treatment for non-muscle invasive bladder cancer (NMIBC). While generally safe, rare systemic complications such as BCGosis—disseminated infection with Mycobacterium bovis—can occur, posing significant diagnostic and therapeutic challenges. Case Presentation: We report the case of a 73-year-old male with multiple comorbidities who presented with prolonged constitutional symptoms, including fever, weight loss, and lethargy, several months after completing intravesical BCG therapy for bladder transitional cell carcinoma. Extensive workup, including infectious, oncologic, and autoimmune investigations, was inconclusive. Imaging revealed granulomatous changes and hypermetabolic lesions in the urinary tract and multiple organs. Blood cultures eventually grew M. bovis, confirming BCGosis. The patient was successfully treated with a six-month course of antimycobacterial therapy, with clinical resolution. Discussion: This case highlights the importance of considering BCGosis in patients with systemic symptoms and a history of BCG therapy, even with delayed onset. The diagnosis is often challenging due to nonspecific clinical and imaging findings and low yield of microbiological tests. Early recognition and prolonged antimycobacterial treatment are crucial to prevent serious morbidity. Conclusion: Clinicians should maintain a high index of suspicion for BCGosis in patients with unexplained systemic illness and prior BCG immunotherapy. Thorough history, clinical vigilance, and multidisciplinary collaboration are essential for timely diagnosis and management.