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Swimming pool granuloma

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


M marinum bacteria usually live in brackish 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 appear about 2 to several weeks later.

Risks include exposure to swimming pools, aquariums, or fish or amphibians that are infected with the bacteria.


The main symptom is a reddish 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 less commonly affected.

The nodules may break down and leave an open sore. Sometimes, they spread up the limb.

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

Exams and Tests

The health care provider will perform a physical examination and ask about your symptoms. You may also be asked if you recently swam in a pool or handled fish or amphibians.

Tests to diagnose swimming pool granuloma include:

  • Skin test to check for tuberculosis infection, which may look similar
  • Skin biopsy and culture
  • X-ray or other imaging tests for infection that has spread to the joint or bone


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. This helps the wound heal.

Outlook (Prognosis)

Swimming pool granulomas can usually be cured with antibiotics. But, you may have scarring.

Possible Complications

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

When to Contact a Medical Professional

Call 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.

Alternative Names

Aquarium granuloma; Fish tank granuloma; Mycobacterium marinum infection


Brown-Elliott BA, Wallace RJ. Infections caused by Mycobacterium bovis and 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, Updated Edition. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 254.

Patterson JW. Bacterial and rickettsial infections. In: Patterson JW, ed. Weedon's Skin Pathology. 4th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2016:chap 23.

Review Date 3/11/2019

Updated by: Alireza Minagar, MD, MBA, Professor, Department of Neurology, LSU Health Sciences Center, Shreveport, LA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.