Mycobacterium marinum, the cause of chronic systemic infections in fish, occasionally causes granulomatous skin and soft tissue lesions in humans. Cutaneous mycobacterial infection in two patients owing to unusual circumstances is presented in this report. The first patient was infected through improper hygienic behavior, while infection in the second patient was previously misdiagnosed as rheumatoid arthritis and treated with methylprednisolone for a period of three months, which resulted in a rare systemic spread of M. marinum into the bones of the hand, testis, and epididymis. Simultaneously, screening for possible sources of M. marinum infection in patients' aquaria revealed positive fish harboring VNTR profiles identical to those obtained for clinical isolates from patients.

A survey was carried out on occurrence of Mycobacterium marinum in fish kept in aquaria and those living in their natural environment. Species-specific qPCR targeting the erp and IS2404 genes together with the conventional culture method were used. The analysis of 72 ornamental fish (n = 216 samples: gills, muscle and intestine) collected from aquaria revealed the presence of M. marinum in 30 individuals (41.7%) of whom 17 (23.6%) were later culture positive. Culture-independent detection revealed the presence of M. marinum in 16 of 83 environmental samples (19.3%) collected in aquaria. The presence of viable M. marinum cells was later confirmed in 5 samples (6.0%). No qPCR or culture positivity was observed when 123 groundwater fish and their corresponding environmental samples (n = 142) were analysed.

Mycobacterium marinum is the most frequent non-tuberculous Mycobacterium in humans. We report the first ever described case of epididymoorchitis resulting from hematogenous spread of M. marinum from hand oligoarthritis. This was initially mistaken for rheumatoid disease and methylprednisolone-induced immunosuppression led to hematogenous spread of infection to the testis and epididymis.