Mycobacterium marinum

  • Medical Author:

    Dr. Alai is an actively practicing medical and surgical dermatologist in south Orange County, California. She has been a professor of dermatology and family medicine at the University of California, Irvine since 2000. She is U.S. board-certified in dermatology, a 10-year-certified fellow of the American Academy of Dermatology, and Fellow of the American Society of Mohs Surgery.

  • Medical Editor: Frederick Hecht, MD, FAAP, FACMG
    Frederick Hecht, MD, FAAP, FACMG

    Frederick Hecht, MD, FAAP, FACMG

    Frederick Hecht, MD, lives in Scottsdale, Arizona. Dr. Hecht is a Pediatrician and Medical Geneticist and is certified by both the American Boards of Pediatrics and Medical Genetics. Dr. Hecht was born and raised in Baltimore and attended Dartmouth College in Hanover, N.H. and the Sorbonne at the University of Paris receiving his BA degree cum laude with distinction from Dartmouth.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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How does a person get infected with Mycobacterium marinum?

Human infections with M. marinum under normal circumstances are rare. People are prone to this infection when there is minor trauma to an extremity like the forearm before or during contact with marine animals like fish or turtles, or just an aquarium, saltwater or freshwater.

However, people who have minor breaks in the skin such as small cuts or scrapes are at increased risk

  • when in contact with water from an aquarium or fish tank,
  • when handling, cleaning, or processing fish,
  • while swimming or working in fresh or salt water, or
  • while standing in contaminated water.

One form of the infection, known as "swimming pool granuloma," can occur when there is inadequate chlorination of swimming pools. However, in the U.S., most human infections with this bacteria have been associated with contact with fish tanks.

M. marinum infection is not spread from person to person. It is also not transmitted in hospitals like other common bacteria.

Who is at risk for Mycobacterium marinum infection?

People at highest risk include home-aquarium hobbyists, swimmers, aquarium workers, marine-life handlers, anglers, and oyster workers. Overall, anyone with frequent or persistent saltwater or freshwater exposure is at potential risk. Here is a list of at risk people:

  • personal home-aquarium owners
  • professionals who clean aquariums
  • marine biologists
  • fishermen and workers exposed to saltwater fish
  • immunocompromised patients (HIV/AIDS)

What are the symptoms of Mycobacterium marinum infection?

Typically, patients may initially notice a small red bump or non-healing red sore on their skin a few weeks after a history of exposure to non-chlorinated water. Ninety percent of the cases involve the arms (upper extremities). They may remember getting a scratch, scrape, or puncture wound several weeks before while in the water. Many people may easily overlook the early signs and try over-the-counter antibiotic creams and disinfectants on their own in an attempt to make the bump or sore go away. Often, patients may not decide to go to their physician until they can't get rid of the bump for weeks or months, when they see more bumps, or when they see spreading bumps in a "line" pattern up their arm or leg.

Some patients may feel no pain or itch while others commonly have some localized pain and firmness at the site of the infection. Most otherwise healthy people overall feel well during the infection and do not have fever or chills.

Patients in poor health or those with other health issues like an impaired immune system or other serious illnesses may experience fever, enlarged localized lymph nodes, and systemic infection.

When M. marinum infects the skin, it causes localized microscopic nodules to form. These nodules are called granulomas. They occur at sites of skin trauma where there are scratches, cuts, and the like.

The granulomas slowly increase in size usually become visible within two to three weeks of exposure. Some reported cases have developed two to four months or more after exposure to M. marinum because of the very slow-growing nature of this bacterium.

The most frequent sign is a slowly developing nodule (raised bump) at the site the bacteria entered the body. Frequently, the nodule is on the hand or upper arm. Later the nodule can become an enlarging sore (an ulcer). Swelling of nearby lymph nodes occurs. Multiple granulomas may form in a line along the lymphatic vessel that drains the site. These lesions will usually spontaneously heal in several months. This infection can also involve the joints (septic arthritis) and bones (osteomyelitis).

A health-care provider should be consulted if a skin nodule or reddened sore (ulcer) develops following direct skin contact with fresh or saltwater or after handling or processing fish.

For people with compromise of the immune system, M. marinum infection can be especially serious and involve disseminated (widespread) disease. If an infection is suspected under such circumstances, a health-care provider should be promptly consulted.

Medically Reviewed by a Doctor on 4/15/2015

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