‘Fish‐tank’ granuloma: a diagnostic dilemma

A 45-year-old man complained of a 3-month history of multiple, mildly painful, non-tender, purple-red nodules on the dorsum of his right hand (Fig. 1). They started as small superficial lesions days after he had worked on his boat, followed by swelling, reddening and nodularity. Further nodules had grown on the volar surface of the right forearm (Fig. 2). He was afebrile and routine laboratory studies were normal; there was no palpable lymphadenopathy. Biopsied tissue was sent for routine histologic analysis and staining for fungi and acid-fast bacilli, the results of both being non-diagnostic (Fig. 3a,b). Intracutaneous testing with tuberculin purified protein derivative was positive at 48–72 h. Culture from secretion of the lesion at 32°C grew a mycobacterium identified as Mycobacterium marinum. A 2-month course of oral clarithromycin (500 mg twice daily) resolved his symptoms, with no relapses. M. marinum is a free-living mycobacterium that causes disease in poikilothermic water animals and occasionally in humans through contaminated water. Approximately 50% of infections with identifiable exposures are aquarium-related with the remainder related to fish and marine occupations or activities. Infection with M. marinum most commonly manifests with erythematous, papulonodular or plaque-like lesions of the extremities, tending to occur over prominences, such as a finger, hand or knee, the incubation period varying from 2 to 6 weeks after the traumatic inoculation. It is usually limited to the skin, less commonly extending to involve deeper structures, such as joints and tendons. Dissemination is rare.