A 61-year-old man presented with skin lesions on his arm. The first lesion was a cat bite wound on his right thumb (fig 1A). Over 1 month the erythematous, scratching, seeping nodular lesions gradually spread from his hand to his axilla (fig 1B). The patient did not have fever or other symptoms. He had recently bought a fish aquarium and cleaned it twice a week without wearing gloves. This was suggestive of a sporotrichoid form of infection by Mycobacterium marinum, a non-tuberculous waterborne mycobacterium that commonly infects fish from cold or warm, fresh or salt water.1^–3 M marinum was detected by polymerase chain reaction (PCR) from a skin biopsy fragment.

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Figure 1

(A) Cat bite wound on the right thumb in a 61-year-old man, who regularly cleans his fish aquarium without wearing gloves. (B) Sporotrichoid form of infection by Mycobacterium marinum. The gateway of infection was the cat bite lesion and contact with water from the aquarium.

M marinum infection in humans occurs rarely (0.27 cases per 100 000 inhabitants in the USA). The most frequent site of infection is the skin on the hands, with aquarium owners affected most commonly. The incubation period is usually around 21 days but may be prolonged for >30 days. Single lesions consist of papulo-nodular, verrucous, or ulcerated granulomatous inflammation with minimal purulent secretions. Sporotrichoid lesions display lymphogenous spread from the primary lesion, and chains of granulomatous, inflamed, pustular lesions occur centripetally, as observed in our patient. Immunosuppression may cause pulmonary or visceral lesions. Humans are often infected with M marinum following contact with aquarium water (half of the reported cases). The gateway for our patient was the cat bite lesion. Infection may also occur following fish or shellfish injuries, and less frequently following contact with saltwater or brackish water. There is no association with a specific fish species. Prolonged antibiotic treatment (a minimum of 3 months) is needed in cases of M marinum infection. Doxycyclin, clarithromycin or trimethoprim-sulfamethoxazole, as monotherapy, can be used in superficial cutaneous infections, but drug resistance varies and combination therapy of two drugs may be required, which may also include ciprofloxacin. A combination of rifampicin and ethambutol can also be used in cases of severe infection, including those with a sporotrichoid distribution pattern.1^–3