P035 When cultures fail: nodular lymphangitis secondary to suspected Mycobacterium marinum

Abstract

Mycobacterium marinum is a nontuberculous mycobacterium known to cause sporotrichoid lymphocutaneous reactions. Definitive diagnosis relies on tissue culture, but the yield can be low due to slow growth, temperature sensitivity and prior anti­biotic exposure. This case highlights the role of clinical pattern ­recognition and histopathology in guiding diagnosis in culture-negative presentations. A 62-year-old man developed violaceous, scaly plaques on his left upper limb progressing in a sporotrichoid pattern from the index finger to the dorsal forearm. He had a recent history of a pathergy reaction after cannulation and frequently swam in a saltwater pool. Empirical treatment with doxycycline improved symptoms but the rash progressed with drug withdrawal. Punch biopsy showed dermal mixed inflammation with histiocyte collections but no well-formed granulomas. Treatment with terbinafine and itraconazole had no effect. Extensive serological, autoimmune and infectious investigations were all negative, including HIV, syphilis, hepatitis, Borrelia, Leishmania, QuantiFERON, mycobacterial polymerase chain reaction (PCR), panbacterial 16S PCR and three tissue cultures. Deep biopsy revealed a lymphohistiocytic infiltrate with epithelioid granuloma formation and suppurative necrosis. No organisms were visualized. Given the characteristic sporotrichoid distribution, granulomatous histopathology and saltwater exposure, empirical therapy with ethambutol and clarithromycin was initiated. Complete resolution of lesions was observed after 12 months of treatment. Other infectious differentials considered included sporotrichosis or blastomycoses, both of which should have been treated by itraconazole. Other bacterial causes considered included nocardia and erysipelothrix, both of which should be treated with doxycycline. This case demonstrates that both a high degree of clinical suspicion and histopathology are essential for supporting the presence of M. marinum in the absence of confirmatory cultures and PCR. Awareness of this diagnostic challenge is critical to prevent delays in treatment and to achieve complete resolution in patients with culture-negative infections.