Mycobacterium marinum (M. balnei) causes swimming pool granuloma. The condition was first described in Sweden and was later observed in most Western countries. It involves papules with central ulceration which heal spontaneously after a few months with the formation of a small scar. Infection occurs during bathing by rubbing the skin against the rough cement lining of a swimming pool or aquarium or by touching tropical fish. 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).
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. Cat scratch disease, leishmaniasis and sporotrichosis are to be considered in the differential diagnosis.