Mycobacterium marinum

A sign of Mycobacterium marinum infection is a non-healing red sore on the skin. Mycobacterium marinum infection of the hand is shown above.

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 the contents of 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 small reddish bumps crops 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 the 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 are often delayed because of a lack of suspicion for this atypical mycobacterium versus more common bacteria like Staphylococcus.

Some synonyms for Mycobacterium marinum skin infections include tropical fish granuloma, fish tank granuloma, and fish tank granuloma.

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 in adults, although any person, regardless of age, may become infected.

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.

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 contagious; it is not spread from person to person. It is also not transmitted in hospitals like other common bacteria.

People at the highest risk include home-aquarium hobbyists, swimmers, aquarium workers, marine life handlers, anglers, and oyster workers. Overall, anyone with frequent 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)

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 and 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 healthcare 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 a compromised immune system, M. marinum infection can be especially serious and involve disseminated (widespread) disease. If an infection is suspected under such circumstances, a healthcare provider should be promptly consulted.

Many people who have skin infections will seek care from a primary-care physician, including family practitioners or internists, or a dermatologist. For severe cases, an infectious-disease specialist may be consulted. If the patient seeks care at an emergency department or urgent care facility, he or she may be treated by a specialist in emergency medicine.

Lab tests include cultures where a swab or sample is taken and grown in the laboratory. Cultures of M. marinum are fairly difficult to grow and usually may take several weeks in the lab. The culture may be negative, even if there is an active infection. Treatment may still be considered even if the test results are negative, especially if the patient's history supports past fish or fish-tank exposure.

In the absence of positive culture results, a skin or tissue biopsy may be a helpful test for diagnosis. This may help find the microscopic bacteria.

A special test called polymerase chain reaction (PCR) of tissue may be used in difficult cases to assist in naming the exact type of bacteria or Mycobacterium species involved.

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.

Most infections are treated medically with a fairly long course of oral antibiotics. The length of the course of antibiotics varies. Medications may be required anywhere from three to six months or even up to 18 months depending on the severity of the disease and the spread of the infection. Typically, physicians may recommend continuing the medications for an additional four to six weeks even after all of the symptoms have fully cleared.

The gold standard for the treatment of infection by M. marinum is oral antibiotics. Clarithromycin with ethambutol is currently the preferred antibiotic selection. Rifampin is typically added to the antibiotics regimen if a deeper organ infection is present, such as bone infection (osteomyelitis).

Some milder infections (mainly in healthy people) have cleared on their own without any treatment. Rarely, surgical treatment and drainage of deeper tissue or skin infections may become necessary in more complicated cases. However, medical treatment remains the primary and preferred treatment for nearly all cases.

The prognosis is excellent for a complete cure with a proper, full course of oral antibiotics and good medical follow-up with your physician. There are no long-term problems after treatment.

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.

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.

Patients with an impaired immune system (immunocompromised) may be much more prone to these more serious complications.

Other conditions may mimic or be confused with M. marinum infections. Possible other diagnoses include common things like bug bites, spider bites, foreign body granuloma, bacterial infections like staph or E. coli, fungal infections, tumors, and others. Additional diagnoses include cowpox infection, leishmaniasis, leprosy, sarcoidosis, and sporotrichosis. More advanced cases may be mistaken for rheumatoid arthritis, gout, traumatic tendon injury, deep fungal infections, or cancer.

The following steps may help to protect you from contracting an infection with M. marinum:

• Avoid fresh or saltwater activities if there are open cuts, scrapes, or sores on your skin, especially in bodies of water where this bacterium is known to exist.
• If you have a weakened immune system, you can reduce the risk of infection by carefully covering cuts, scrapes, or sores during fresh or saltwater activities and while cleaning fish tanks or handling, cleaning, or processing fish.
• Wear heavy gloves (leather or heavy cotton) while cleaning or processing fish, especially fish with sharp spines that may cause cuts, scratches, or sores to the hands and skin. Wash hands thoroughly with soap and water after fish processing or use a waterless cleanser.
• Wear waterproof gloves while cleaning home aquariums or fish tanks. Wash hands and forearms thoroughly with soap and running water after cleaning the tank, even if gloves were worn.
• Ensure regular and adequate chlorination of swimming pools to kill any bacteria that may be present.