Fish tank granuloma: A peculiar case in a fish tank dealer

Dear Editors, A 53-year-old male patient presented to our department complaining of a purple lesion localised at the dorsal region of his right hand, which appeared approximately 12 months before. On clinical examination, a reddish to violaceous patch with sharply defined borders was present on the dorsal region of the right hand. Furthermore, an eczema-like lesion on the fingertip of the third finger was evident (Figure 1). The lesions were itchy, with an associated burning sensation. The patient denied any systemic symptoms. Previously, the patient was treated with oral amoxicillinclavulanic acid and topical injection of steroids, with no improvement. When asked about any occupational or hobby contact, he reported working in a pet shop and managed fish tanks. A skin swab was negative for common pathogens. A cutaneous incisional biopsy was performed. The histological examination demonstrated an intense neutrophilic and lymphocytic infiltration, with giant cell and noncaseating granulomas. Polymerase chain reaction (PCR) showed an infection of Mycobacterium marinum. Considering the clinical aspect, the patient's occupational hobby, and the pathology results, a diagnosis of fish tank granuloma was made. Fish tank granuloma is a cutaneous disease caused by M. marinum. This non-sporogenous, aerobic bacterium is widespread, with a predilection of fresh and salt water. Among non-tuberculous mycobacterial infections, M. marinum is the most common (about 45% of cases). The infection is caused by contact with infected water or fishes through minimal skin trauma. The incubation period is approximately 2 to 3 weeks, and the causative agent of human infection is lethal for animals; in fact, some patients report the onset of the lesion concurrently with the death of infected fishes. Based on the clinical aspect and the infiltration of deep tissues, skin lesions are classified into four types: (a) papules/nodules and patches with erosion an ulceration, (b) more than one lesion with a sporotrichoid pattern, (c) deep tissue infection, and (d) systemic disease. Some authors report an eczematous-like presentation with the involvement of fingertips. In our case, two clinical patterns were simultaneously present: a typical one with nodules and an atypical with eczema-like lesions. Fish tank granuloma is often asymptomatic, even if some patients may complain of itch and burning sensation, as described in our patient. Differential diagnosis includes cutaneous leishmaniosis; cutaneous tuberculosis; sarcoidosis; and neutrophilic dermatosis, such as erythema elevatum diutinum and Sweet syndrome. The absence of systemic symptoms, normal laboratory test, and skin biopsies are important to rule out the cutaneous disease mentioned above. Biopsy examination often shows an intense inflammatory infiltrate, with non-tuberculous granulomas. Ziehl-Nielsen colouration demonstrates acid-positive Mycobacterium. FIGURE 1 A reddish to violaceous patch with sharply defined borders of the dorsal region of the right hand. An eczema-like lesion on the fingertip of the third finger is also present Received: 27 April 2019 Accepted: 28 April 2019