Mycobacterium marinum (M. marinum) is a slow-growing atypical mycobacterium that is commonly found in bodies of fresh or saltwater in many parts of the world. Skin infections with Mycobacterium marinum in humans are relatively uncommon and are usually acquired from contact with aquariums or fish.
Most infections occur following skin exposure to the bacteria through a small cut or skin scrape. The first signs of infection with M. marinum include a reddish or tan skin bump called a granuloma.
Less commonly, a string or batch of the small reddish bumps crop up on the exposed body area in a classic pattern called sporotrichotic lymphangitis.
It is somewhat rare to acquire this infection from well-maintained swimming pools because of protection afforded by proper chlorination. Mycobacterium marinum does not typically grow at normal body temperature, which is why it remains localized to the cooler skin surface.
Overall, diagnosis and treatment of this unusual skin infection is often delayed because of a lack of suspicion for this atypical mycobacterium versus more common bacteria like Staphylococcus.
M marinum is water-borne atypical Mycobacterium species that commonly infects fish and amphibians. It was first recognized to cause human disease in 1951. M marinum infection commonly develops as a complication of skin and soft-tissue injuries exposed to aquatic equipment such as fish lines and fishhooks, among others.
Domestic exposures involved in infection commonly involve fish tank manipulations. M marinum infection was once called swimming pool granuloma, but that term is now rather obsolete because of the widespread use of chlorination in swimming pools. Chlorinated swimming pools are not considered an exposure risk.
M marinum grows best at 32°C; therefore, cooler extremities, particularly hands, are affected more often than central areas. This feature is also important for optimal growth in the microbiology laboratory.
M marinum can disseminate in severely immunosuppressed individuals (eg, transplant recipients).
What are other names for Mycobacterium marinum infections?
Some synonyms for Mycobacterium marinum skin infections include tropical fish granuloma, fish tank granuloma and fish tank granuloma.
How common is Mycobacterium marinum?
Although rare, infections can occur worldwide, most commonly in individuals with occupational and recreational exposure to fresh or saltwater. In the United States, infections caused by M. marinum are rare.
The infection is very rare in children and is typically a disease of adults.
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
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, they see more bumps, or 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 usually appear 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.
The criterion standard for diagnosis is culture from the tissue biopsy. Cultures have been reported as positive in 70%-80% of cases. At least a 4-mm skin punch biopsy or multiple biopsy specimens are recommended.
M marinum is a nonmotile acid-fast bacillus (AFB) that grows in 2-3 weeks, with optimum growth on Lowenstein-Jensen medium at 30°C. Laboratory personnel should be notified in advance since AFB cultures are mostly carried at higher temperatures. Cultures should be observed for 6-12 weeks.
M marinum is a photochromogen (Runyon group 1), producing a yellow pigment when exposed to light.
M marinum produces urease and catalase (weakly) but does not produce niacin or nitrate.
Polymerase chain reaction (PCR) amplification techniques using Mycobacterium genus-specific primers can be used to diagnose M marinum infection directly in the biopsy sample.
Tuberculin skin test using purified protein derivative is positive in 67%-100% of cases.
Quantiferon-TB Gold and enzyme-linked immunospot assay may be positive in M marinum infections.
Positive blood culture findings have
How do I find a specialist?
Specialists including dermatologists, infectious disease physicians, and rheumatologists may have additional expertise in the diagnosis and treatment of this rare disorder.
What are possible complications from Mycobacterium marinum?
M. marinum infections are usually localized and typically do not spread past the skin in healthy people. Most patients with a normal immune system don’t experience other complications. However, undetected or untreated, the infection may progress and cause deeper and more longstanding infections.
Patients with an impaired immune system (immunocompromised) may be much more prone to serious complications such as spread of infections to involve the bone marrow and internal organs.
Some rare potential problems include infection of the underlying bone called osteomyelitis, infection of the deep muscle tendons called tenosynovitis, inflammation of the joints called arthritis, and widespread bodily infections called disseminated disease.
Do fish get infected with Mycobacterium marinum?
Yes. There are probably two different types of M. marinum. One type only causes a longstanding (chronic) progressive disease in fish without affecting humans. The second type, which can infect humans, seems to cause a deadly sudden illness in fish.
The mainstay of treatment in M marinum infection is antimicrobial therapy. The duration of therapy is not well defined, but treatment of skin and soft-tissue infections should be continued for 1-2 months after resolution of symptoms and lesions.
Therefore, the treatment duration is typically 3-4 months, longer if deeper structures are involved. Some authors have suggested a minimum duration of 6 months. In some cases, a treatment duration of up to 2 years has been reported.
Combination treatment with 2 active agents is preferred, although success has been obtained with single-agent approach. Spontaneous resolution of M marinum infection has been reported.