A swimming pool granuloma is a skin infection that may become long-term (chronic). It is caused by the bacterium Mycobacterium marinum (M marinum).

Aquarium granuloma; Fish tank granuloma; Mycobacterium marinum infection

M marinum bacteria can live in fresh and salt water, unchlorinated swimming pools, and aquarium tanks. The bacteria can enter the body through a break in the skin, such as a cut, when you come into contact with water that contains this bacteria.

Signs of a skin infection usually appear about 2 to 3 weeks after exposure, but symptoms may appear later, up to a month or more.

Risks include exposure of skin cuts or scrapes to swimming pools, aquariums, or fish or amphibians that are infected with the bacteria. As a result, people who fish or maintain aquariums and fish tanks are at higher risk.

The main symptom is a reddish to purple bump (papule) that slowly grows into a purplish and painful nodule.

The elbows, fingers, and back of the hands are the most commonly affected body parts. The knees and legs are other areas affected.

The nodules may break down, ulcerate, and leave an open sore. Sometimes, they spread up through the lymph nodes of the limb, creating additional bumps in a line, but more often remain as a solitary nodule.

Since the bacteria cannot survive at the warmer temperature of the internal organs, they usually stay in the skin, causing the nodules.

Antibiotics are used to treat this infection. They are chosen based on the results of the culture and skin biopsy.

You may need several months of treatment with more than one antibiotic. Surgery may also be needed to remove dead tissue if the lesions do not clear with antibiotics. This helps the wound heal.

Swimming pool granulomas can be cured with antibiotics, or they may heal on their own. They often leave a scar.

Tendon, joint, or bone infections sometimes occur. The disease may be harder to treat in people whose immune system is not working well.

Contact your provider if you develop reddish bumps on your skin that do not clear with home treatment.

Wash hands and arms thoroughly after cleaning aquariums. Or, wear rubber gloves when cleaning.

Published Date: February 06, 2025
Published By: Michael Lehrer, MD, Clinical Associate Professor, Department of Dermatology, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

Brown-Elliott BA, Wallace RJ. Infections caused by nontuberculous mycobacteria other than mycobacterium avium complex. In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 252.

Patterson JW. Bacterial and rickettsial infections. In: Patterson JW, ed. Weedon's Skin Pathology. 6th ed. Philadelphia, PA: Elsevier; 2025:chap 24.