Mycobacterium marinum infection (fish tank granuloma): causes and treatment

Mycobacterium marinum is one of the so-called non-tuberculous or atypical mycobacteria, relatives of the tuberculosis bacterium that are found in the environment. It is the cause of long-term infection in fish and occasionally causes granulomatous skin and soft tissue lesions in people, according to a published case report on two aquarists.

The infection goes by several names: fish tank granuloma, aquarium granuloma and swimming pool granuloma. All describe the same skin infection. It is not common, and it is not the same as tuberculosis, although the two can look similar enough that doctors may test for tuberculosis as part of the work-up.

No UK public-health or NHS patient page on this infection was available when this guide was prepared, so it relies on MedlinePlus (a US government resource) and a published case report from the Czech Republic. These are non-UK sources, and UK clinical practice may differ in detail.

MedlinePlus lists the risks as exposing skin cuts or scrapes to swimming pools, aquariums, or fish or amphibians that are infected with the bacteria. People who fish or maintain aquariums and fish tanks are therefore at higher risk.

The case report illustrates how ordinary this can be. One patient cleaned an injured knee with a brush normally used to clean the aquarium; another sustained a finger injury while cleaning aquaria. In both, bacteria matching the patients' infections were found in fish from their own tanks.

MedlinePlus also says the disease may be harder to treat in people whose immune system is not working well. In the second case report patient, an infection that had been mistaken for rheumatoid arthritis and treated with steroids for three months spread to the bones of the hand and to the testis and epididymis.

The main symptom, says MedlinePlus, is a reddish to purple bump that slowly grows into a purplish and painful nodule. The elbows, fingers and backs of the hands are the most commonly affected places, with knees and legs also involved. Nodules may break down, ulcerate and leave an open sore.

Sometimes the infection spreads up the lymph channels of the limb, producing further bumps in a line, though more often it remains a single nodule. Tendon, joint or bone infection is a recognised complication.

• A slowly enlarging red-purple bump or nodule on a finger, hand, elbow or lower limb.

• A nodule that breaks down and forms an open sore.

• Further bumps in a line up the arm or leg.

• Swelling or stiffness of a finger or wrist joint that does not settle, which needs medical review.

MedlinePlus says the provider will examine you and ask about recent swimming or handling fish or amphibians. Tests include a skin test or blood test for tuberculosis infection, because it may look similar, a skin punch biopsy with culture, and imaging such as X-ray if infection may have reached a joint or bone.

The case report notes that conventional microbiological methods are slow and that clinicians should consider other causes, including deep fungal infection. It stresses that microbiologists should be alerted to the possibility of M. marinum if an injury originated from an aquarium or water environment, because culture takes time and delay is the main cause of serious complications.

In practice this means telling your GP or dermatologist about your fish tank, fishing or pool exposure at the first appointment, even if it seems unrelated.

MedlinePlus says antibiotics are used and are chosen on the basis of culture and biopsy results. You may need several months of treatment with more than one antibiotic, and surgery may be needed to remove dead tissue if lesions do not clear with antibiotics.

The outlook is generally good. MedlinePlus says swimming pool granuloma can be cured with antibiotics, or may heal on its own, but it often leaves a scar. In the case report, one patient had to change antibiotics after an insufficient response, which illustrates why follow-up matters.

MedlinePlus advises washing hands and arms thoroughly after cleaning aquariums, or wearing rubber gloves when cleaning. Cover any cuts before contact with tank water, and do not clean wounds with equipment used for the aquarium.

Contact your GP if you develop reddish bumps on the skin that do not clear with home treatment, particularly if you keep fish or have been in untreated water. Seek help sooner if a finger or hand becomes swollen, stiff or painful.

A GP is the usual first step on the NHS, and referral to dermatology or infectious diseases follows if the cause is unclear or the lesion does not improve. Cultures for mycobacteria take weeks, which is the same whether care is NHS or private, so the most useful thing you can do is give a clear exposure history.