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This information was reviewed and approved by Shannon H. Kasperbauer, MD (7/1/2017).

Mycobacterium marinum (M. marinum) is a slow-growing bacterium that can be found in both fresh and salt water around the world.

The M. marinum is a nontuberculous mycobacterium (NTM), a species that may cause infections in humans, but that does not cause tuberculosis (TB). M. marinum can also cause infections in fish.



Human infections are seen in low, but consistent numbers. The risk of infection increases for people who have a scrape, cut or other skin abrasion that can serve as an entry site for the bacteria and come into direct contact with a contaminated water source.

Common sources of exposure in the United States include coastal areas, particularly involving surfers or those who work in brackish waters, such as the Chesapeake Bay. People can also be exposed by handling or cleaning fish tanks or fish, and swimming or working in fresh or salt water. There is no evidence that a M. marinum infection can be spread from person to person.



Symptoms of Mycobacterium marinum typically occur within 2 – 4 weeks of exposure; however, some cases have developed 2 – 4 months or longer after exposure due to the slow-growing nature of this bacterium.

Symptoms may include:

  • Slowly developed single or multiple localized skin lesions at the site where bacteria entered the body typically appear around two weeks after exposure. The lesions may be ulcerated, crusted or wart-like (verrucous) in appearance, and are initially not painful. Over weeks or months, they may spread proximally up the involved limb.

  • Swelling of the nearby lymph nodes

  • Infection progression in more advanced infections from superficial involvement of the skin to invade deeper structures including tendons and the sheaths through which the tendons pass

  • Invasion of the bones, which is less common.


Diagnosis & Treatment

If you develop symptoms following direct contact with fresh or salt water or after handling fish, you should consult a health care provider. Additionally, a Mycobacterium marinum infection can become severe for people with weakened immune systems, such as those with emphysema, cystic fibrosis or previous tuberculosis infection. People with compromised immune systems, or those who are receiving immunosuppressant medications such as prednisone or REMICADE®, should consult a health care professional as soon as possible.

As with other potential nontuberculous mycobacteria (NTM) infections, a health care provider may perform a variety of tests to diagnose an M. marinum infection.

Treatment of M. marinum has not been systematically studied. Some cases demonstrate spontaneous healing. In others, healing may occur with the administration of antimicrobial agents including minocycline, clarithromycin, rifampin or ethambutol. In less extensive cases, single drugs have been prescribed; however, for more extensive disease, drug combinations are preferred. Depending on the severity of the infection, treatment can be a lengthy process, lasting from several months up to two years.

Infrequently, rapidly progressing or deeply invasive disease may result in surgery. Surgery should always be accompanied by antimicrobial therapy.



You can take the following precautions to prevent or reduce the risk of Mycobacterium marinum infection:

  • Minimize or eliminate exposure to fresh or salt water when open cuts, scrapes or abrasions exist on the skin.

  • Be sure to cover any open cuts, scrapes or abrasions, and wear heavy or waterproof gloves when cleaning fish, fish tanks or equipment.

  • Wash hands and forearms after direct contact with fish, fresh or salt water.

  • Maintain swimming pools with adequate chlorination.

There has been no evidence that drinking fresh or salt water will cause infection. Additionally, there has not been any evidence that eating infected fish will cause M. marinum infection. However, it is not recommended to eat any fish that appears to be diseased.

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