Swimming pool granuloma

Alternative names 
Aquarium granuloma

A swimming pool granuloma is a chronic skin infection caused by the bacteria Mycobacterium marinum.

Causes, incidence, and risk factors
A swimming pool granuloma results when water containing an infectious organism, Mycobacterium marinum, enters a break in the skin. A localized infection called a granuloma results after about 3 weeks. The lesions appear as reddish bumps (papules) that slowly enlarge into purplish nodules. The elbows, fingers, back of the hands, and knees are the most common sites affected. The nodules may break down and leave an open sore, or spread up the limb.


  • history of exposure to swimming pools, salt water aquarium or ocean fish  
  • single or multiple red to reddish-purple bumps  
  • occur frequently on the hands, elbows, knees, legs, and areas of trauma  
  • may be present for months

Signs and tests

Though swimming pool granulomas have a characteristic appearance, a skin biopsy and culture is usually necessary to confirm the diagnosis. A PPD tuberculin skin test will usually be positive as well.


Oral antibiotics are needed to clear this infection. Choices include minocycline, Bactrim, clarithromycin, and levofloxacin. Several months of treatment are often required.

Expectations (prognosis)

Swimming pool granulomas can usually be treated completely with antibiotics.


Occasionally, joint or bone infections occur. Patients with defective immune systems may also have longer or more complicated cases.

Calling your health care provider
Call your health care provider if you develop reddish bumps on your skin which do not clear with home treatment.


Avoid contact with contaminated water. Wear gloves or was thoroughly when cleaning aquariums.

Johns Hopkins patient information

Last revised: December 5, 2012
by David A. Scott, M.D.

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