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The Avium, the fish, the aquarist, and a resilient dermatology team: an unusual approach for an unusual pathogen

The Avium, the fish, the aquarist, and a resilient dermatology team: an unusual approach for an unusual pathogen Fish tank granuloma is a rare skin infection that most commonly affects people with occupational or recreational exposure to contaminated fish. The most commonly involved pathogenic agent is Mycobacterium marinum. Infection occurs following exposure of an abrasion or cut on the skin to the causative organism. The typical presentation is that of a slow-growing, erythematous nodule at the trauma site. The lesion is usually painful and can become ulcerated. Lymphatic and local spread is not unusual, whereas dissemination of the infection is most commonly seen in immunocompromised hosts. A 69-year-old professional aquarist on treatment with adalimumab (Humira, Mabura, Exemptia) for longstanding rheumatoid arthritis presented with a large, partially ulcerated, erythematous patch on his right upper arm (Fig. 1a). This was clinically suspected to be a fish tank granuloma. Multiple skin biopsies, however, only revealed nonspecific chronic inflammation, failing to confirm any firm diagnosis. Microbiology culture failed to grow any bacterium including mycobacteria. Following discussion with the patient about the discrepancy between the clinical features and the rather inconclusive histology and negative microbiology tests, he offered to bring samples taken from the fish tanks including some dead fish (Fig. 2a). Admittedly rather unsure of how to proceed, given the unusual scenario, autopsies of fish were carried out, showing positive Ziehl–Neelsen mycobacteria (Fig. 2b). Motivated by these findings, further cuts on the previous biopsies and a new set of biopsies (this time from the patient) revealed the presence of florid granulomatous inflammation with Langhans-type giant cells (Fig. 2c). Modified Ziehl–Neelsen stain showed the presence of bacilli (Fig. 2d,e). Whereas cultures from material harvested from the fish were negative, subsequent molecular tests showed the presence of Mycobacterium avium both in the fish and in the aquarist. The patient was started on treatment combining rifampicin and ethambutol. Adalimumab was discontinued, but because symptoms of rheumatoid arthritis became quite significant, it was resumed 3 months later. By that point, however, the cutaneous lesion had improved significantly (Fig. 1b), and complete resolution was achieved in 5 months (Fig. 1c). Fish tank granuloma is a rare skin condition attributed to infection with Mycobacterium marinum. Mycobacterium marinum is a well-known cause of chronic systemic infections in fish, and it occasionally causes granulomatous skin lesions in humans. Isolation of Mycobacterium avium from fish and water is not an uncommon occurrence and is considered nonpathogenic for fish in most cases. In human pathology, so far, Mycobacterium avium has not been associated with skin lesions resulting from contact with contaminated fish. Mycobacterium Marinum, on the other hand, is responsible for granulomatous reactions in both fish and humans, the latter being the most common cause of fish tank granuloma. Previously reported cases of cutaneous Mycobacterium avium infection resulted from direct inoculation like trauma or surgery and were characterized by the development of skin lesions such as ulcer formation, abscess, or erythematous plaque, usually