A 32-year-old man presents requesting a consultation for a rash on his right index finger. He first noticed the condition several months ago and notes that the rash has remained asymptomatic. He has tried to treat the rash with over-the-counter wart remedies and hydrocortisone cream without effect. The patient works as a gardener and pool cleaner. He is in good health and is taking no medications. Physical examination reveals a well-demarcated erythematous plaque of the distal affected finger with a normal-appearing nail. No other fingers are affected.
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Mycobacterium marinum is a nontuberculous waterborne mycobacterium that causes the condition referred to as fish tank or swimming pool granuloma. The organism was first isolated in 1926 from a saltwater fish that resided in a Philadelphia aquarium.1 Inoculation occurs through a break in the skin and the most common location includes the knuckle or finger. Its prevalence increases in summertime.
Following an incubation period of weeks to months, an erythematous, verrucous or crusted nodular lesion arises at the primary inoculation site. This may be followed by ‘sporotrichoid spread’ resulting in lesions along lines of lymphatic drainage. Lesions may ulcerate and become tender. Complications include tenosynovitis, arthritis, and osteomyelitis.2 The disease is not transmitted from person to person.
Other conditions mimicking M marinum include blastomycosis, tularemia, nocardia, leishmaniasis, sporotrichosis, neoplasm, and verrucae.3 Infection with M marinum should be considered when confronted with a warty plaque of the extremities, especially when accompanied by a history of recent swimming or fish tank ownership.
Early diagnosis is facilitated by histopathology, culture, and polymerase chain reaction (PCR) tests.4 Skin histopathology alone is relatively non-specific and acid-fast bacilli can be demonstrated in only 50% of cases. Culture may prove negative because growth requires low temperature (30 °C) and several weeks of incubation. Treatment is not standardized and often includes a combination of minocycline and rifampin.5 Smaller granulomas may be surgically excised.
Lawrence Schiffman, DO, is the founder and director of Miami Skin Dr in Doral, Florida. Stephen Schleicher, MD, is director of the DermDox Dermatology Centers, associate professor of medicine at Geisinger Commonwealth Medical College, and clinical instructor of dermatology at Arcadia University and Kings College.
1. Akram SM, Aboobacker S. Mycobacterium marinum infection. In: StatPearls [Internet]. StatPearls Publishing; 2023. Updated July 17, 2023, Accessed August 30, 2023.
2. Aubry A, Chosidow O, Caumes E, Robert J, Cambau E. Sixty-three cases of Mycobacterium marinum infection: clinical features, treatment, and antibiotic susceptibility of causative isolates. Arch Intern Med. 2002;162(15):1746-1752. doi:10.1001/archinte.162.15.1746
3. Bezerra GH, Honório MLP, Costa VLDC, et al. Mycobacterium marinum infection simulating chromomycosis: a case report. Rev Inst Med Trop Sao Paulo. 2020;62:e95. doi:10.1590/S1678-9946202062095
4. Hashish E, Merwad A, Elgaml S, et al. Mycobacterium marinum infection in fish and man: epidemiology, pathophysiology and management; a review. Vet Q. 2018;38(1):35-46. doi:10.1080/01652176.2018.1447171
5. Aubry A, Mougari F, Reibel F, Cambau E. Mycobacterium marinum. Microbiol Spectr. 2017;5(2). doi:10.1128/microbiolspec.TNMI7-0038-2016