Mycobacterium marinum, a nontuberculous mycobacterial species that is widely found in aquatic environments, can cause cutaneous infections in humans. The chlorination of pool water has virtually eliminated “swimming pool granulomas,” but fish tank exposure remains a vector for these infections. Lewis and associates report on a series of eight patients treated for cutaneous infections caused by M. marinum.

All eight patients in their report had had contact with fish tanks at home or at work. Cutaneous infections began on the hands in all subjects, and two patients subsequently developed deep tissue infections. The infections began at sites of skin breakdown in several cases, such as a wart removal site in one patient and at bilateral hand lesions in a patient with psoriasis. Spread of infection occurred in a slow, skipping fashion, similar to skin infection with sporotrichosis.

Mycobacterial culture of skin lesions, performed on both solid and broth media, was positive for M. marinum in six patients. Tuberculin skin testing was performed on seven patients with purified protein derivative (PPD) antigen, and all had a reaction of at least 10 mm.

The authors recommended a two-drug regimen using clarithromycin in combination with another antimycobacterial agent such as ethambutol or rifampin. Treatment typically is continued for one to two months after resolution of skin lesions. One patient in their series required 14 months of treatment, and another patient with concomitant malignant melanoma had persistent infection despite cutaneous dèbridement and more than two years of treatment with three agents.

The authors conclude that most current cases of M. marinum are caused by fish tank exposure. A positive PPD reaction supports the diagnosis, which may be confirmed by mycobacterial culture. Treatment with at least two antimycobacterial agents is recommended.