This disease is caused by bacteria in the Genus Mycobacteria. It has a long incubation period and it causes fish to die a slow death; with clinical signs of weight loss, inappetance, ulcers or just found dead. Mycobacteria is widespread in most water bodies. M. marinum has been cultured from swimming pools, beaches, natural streams, tropical fish tanks and even tap water!
Generally, in closed systems, introduction of infection is via contaminated, unpasteurised food sources. In an established tank system, high levels of Mycobacteria will build up as organic levels increase. Inadequate maintenance or high stocking rates predispose to this situation. The presence of high bacterial loads combined with stress due to overcrowding, fighting, external parasitic loads will predispose fish to infection and subsequent disease.
Mycobacteria are highly resistant to many antibiotics. Multiple drug therapy is generally more successful and literature suggests a combination of doxycycline and rifampin amongst others. However, due to the potential for selection of antibiotics resistance, it is not recommended to treat cases of mycobacteriosis in fishes.
How do you know if your fish has mycobacteriosis? In a live patient, it may be possible to examine fine needle aspirates. The only definitive way you can tell if the fish has systemic mycobacteriosis is to conduct a full post-mortem analysis and to collect the relevant samples for bacteriology and histopathology. There is possibly the option to monitor the environment using quantitative PCR technology.
How do you prevent mycobacteriosis? Environmental management and removal of organic loads are very important to reduce the tank loads of Mycobacteria as they seem to increase in numbers as organic loads increase. Infected fish should be promptly removed from the system and euthanased, otherwise, they will be shedding bacteria and adding to environmental contamination.