Mycobacterium marinum was first isolated in 1926 by Aronson from salt water fish in an aquarium. It was discovered that this germ caused a kind of tuberculosis in fresh water fish and it was subsequently called Mycobacterium platypoecilus. In 1951, it was recognised as a human pathogen and was named Mycobacterium balnei. Later it was found the two are identical and the name Mycobacterium marinum was retained. This mycobacterium causes swimming pool granuloma. The condition was first described in Sweden and was later observed in most Western countries. The incubation period ranges from 2 to 6 weeks. It involves papules with central ulceration which often heal spontaneously after a few months with the formation of a small scar, but chronic infections lasting years do occur. In 20% of cases there is a sporotrichoid distribution due to proximal spread of the infection up the lymphatics. Deep infections such as tendinitis or osteomyelitis are rare, but in immunosuppressed people, dissemination can occur. Infection occurs during bathing by rubbing the skin (fingers, hands) against the rough cement lining of a swimming pool or aquarium or by touching tropical fish. Snails, shellfish, dolphins and water fleas were described as vectors. The diagnosis can be suspected clinically (finger or hand papule / nodule / ulcer in fish handler). Histology will show infective granulomata with lymphocytes, histiocytes, epithelioid cells and giant cells. Identifying acid-fast baccili on a smear has a low sensitivity (± 10%) in humans with a normal immune system, but numerous bacilli tend to be found in immunosuppressed hosts. M. marinum is a photo-chromogen, i.e. the colonies form yellow pigment when exposed to light. Culture has a sensitivty of ± 75%. The incubation temperature for optimal growth of M. tuberculosis is 37°C, but M. marinum grows optimally at 30-32°C. Therefore, clinical specimens should be set up in duplicate (30° and 37°C). Cultures are important to assess antimicrobial susceptibility. PCR can be performed in some laboratories.

For treatment, a combination of rifampicin (600 mg/day on an empty stomach) with minocycline or doxycycline (100-200 mg per day) is used, together with clarithromycin (500 mg twice daily), cotrimoxazole (twice 800/160) or ethambutol (max. 2.5 g/day).

Differential diagnosis includes other mycobacterial infections, sporotrichosis, deep fungal infections, cat scratch disease, leishmaniasis, tularaemia, sarcoidosis, tumors and foreign body reactions. The disease must not be confused with erysipeloid (Rosenbach’s disease), an infection caused by the Gram-positive bacterium Erysipelothrix rhusiopathiae. Infections with this organism also occur frequently in fishermen and people who handle crabs. Pig slaughterers represent another risk group.