Atypical waves: nonsporotrichoid <i>Mycobacterium marinum</i> skin infection in an immunocompromised scuba diver

A 58-year-old man presented with an enlarging painful ulcer on his left medial knee. He had a history of psoriatic arthritis, treated with methotrexate and adalimumab for the last 5 years. He had travelled to over 20 countries in the past 2 years, and his hobbies included scuba diving and seawater/freshwater swimming. He recalled freshwater swimming in a cave lake in Brazil 1 year prior. Subsequently, he developed small erythematous papules on each knee with rapid resolution on the right knee. The left knee lesion slowly enlarged, and he sought medical attention in Ireland 9 months later. He received antibiotics and topical steroids, and was switched from adalimumab to infliximab for a presumed diagnosis of pyoderma gangrenosum. The lesion's growth accelerated over 8 weeks, leading to an acute presentation to the Emergency Department with severe pain and limited mobility. On examination, he had a 20 × 10 cm erosive plaque on his left knee with an exudative base and a ragged erythematous edge. He also had a 2.5 × 2 cm ulcerative lesion on his right flank and a hyperpigmented patch on his right knee. He denied fevers or systemic symptoms. Interferon-γ release assay was positive. A skin biopsy from the left leg demonstrated ulceration, mixed inflammation and underlying dermal necrosis. Necrotizing granulomas were seen, and both auramine and Ziehl-Neelsen stains were positive for abundant acid-fast bacilli. Skin biopsy from the right flank showed similar findings. A mycobacterial culture was incubated at 30°C and after 4 weeks, <i>Mycobacterium marinum</i> was identified. <i>Mycobacterium marinum</i> is a free-living, slow-growing, nontuberculous mycobacterium found in cold and warm fresh and saltwater environments. It can cause a tuberculosis-like disease in fish and can opportunistically infect humans when injured skin is exposed to a contaminated aqueous environment. The typical presentation is a 'fish tank granuloma': nodules, plaques or ulcerated lesions that spread in a sporotrichoid pattern along lines of lymphatics, commonly on the upper limb. This case represents a highly unusual cutaneous <i>M. marinum</i> presentation, with noncontiguous ulcerative lesions in an immunocompromised host. The diagnostic delay highlights the challenges of recognizing atypical nontuberculous mycobacterial infections, particularly in patients who are immunosuppressed.