Mycobacterium marinum Hand Infection in a Kidney Transplant Recipient: A Diagnostic Challenge.

BACKGROUND: Mycobacterium marinum is a slow-growing nontuberculous mycobacterium associated with aquatic environments and may cause chronic skin and soft-tissue infections following minor skin trauma. Diagnosis can be delayed because clinical manifestations may mimic conventional bacterial infections. We report an unusual case in a kidney transplant recipient with a positive QuantiFERON-TB Gold test and subsequent M. marinum infection of the hand. CASE PRESENTATION: A 53-year-old man with a history of kidney transplantation and long-term immunosuppressive therapy developed progressive swelling, pain, erythema, and functional impairment of the right third finger. After independently discontinuing tacrolimus, mycophenolate mofetil, and prednisone following a positive QuantiFERON-TB Gold test, he developed a progressive hand infection despite empirical ciprofloxacin therapy. He reported repeated exposure to a domestic freshwater aquarium and frequent contact with aquarium water and filtration equipment, with minor skin abrasions. Surgical exploration revealed dense, whitish, caseous-appearing material. Histopathology demonstrated chronic granulomatous inflammation, while Ziehl-Neelsen staining showed acid-fast bacilli. Culture yielded slow-growing photochromogenic colonies at 28-32 °C, and species-specific PCR confirmed M. marinum. No antimicrobial susceptibility testing was performed. Following surgical drainage and six days of empirical ciprofloxacin, no targeted antimycobacterial therapy was administered. The patient achieved complete clinical resolution, and immunosuppressive therapy was gradually reintroduced approximately four weeks after surgery. CONCLUSIONS: This case highlights the importance of considering M. marinum in persistent or treatment-refractory hand infections, particularly in patients with aquarium exposure and impaired immunity. Early tissue sampling, appropriate culture conditions, and molecular identification are essential for diagnosis. A positive QuantiFERON-TB Gold result should be interpreted cautiously because cross-reactivity with M. marinum is possible. The favorable outcome observed after surgical source control and a short empirical course of ciprofloxacin is unusual and should not be interpreted as evidence supporting short-course monotherapy for deep M. marinum infection. Because antimicrobial susceptibility testing and serial follow-up cultures were unavailable, the contribution of ciprofloxacin to microbiological clearance cannot be determined.