There are a number of species which have been isolated from fish. The two most common are M. fortuitum and M. marinum. The disease these organisms cause is piscine mycobacteriosis. It is actually one of the oldest known fish diseases - it was first described in 1897. It is a chronic disease which can affect all species of fish and has been reported in coldwater, warmwater, freshwater and marine fish. And it is seen in both wild and cultured fish. In some books it is reported that M. marinum is only seen in saltwater fish and M. fortuitum in freshwater. However, we have isolated both from saltwater fish.

This is one of the few fish pathogens which is also a potential human health hazard. M. fortuitum is probably the more serious human health hazard as it can grow at 37C. Usually when the atypical mycobacteria infect humans they cause superficial infections which do not heal, however, they can be a serious threat to immunocompromised hosts.

A presumptive diagnosis of mycobacteriosis can be made on smears of affected tissue and the presence of small, nonmotile, rod- shaped, acid fast bacteria. To speciate the organisms they must be isolated.

M. marinum - Is slow growing (2-8 weeks to see any growth at all) Is photochromogenic - if grown in the dark it produces white colonies which turn yellow when exposed to light. Grows between 21-32oC M. fortuitum - Is a rapid grower (for Myco.) -5-10 days Is nonchromogenic Grows between 18-42C

The signs of disease in fish are often only emaciation, lethargy and anorexia, with no external lesions. Occasionally there are white to bloody lesions on the skin which may extend into the muscle. Internally, the most common sign is the presence of small, white to gray round areas throughout any of the visceral organs. Histologically, these areas are usually focal granulomas. In a healthy individual this walling off is sufficient to keep the infection in check. In wild fish or healthy, unstressed cultured fish, Mycobacteria is often an incidental finding. However, in stressed or malnourished individuals, or in cases where there is a very virulent strain of the bacteria, the body is not able stop the spread. In these cases the focal granulomas enlarge and in some cases there may be a more diffuse response.

The bacteria can enter the fish in a number of ways - gills, abrasions in the skin or through the gut. This bacteria has a very high lipid content and so it tends to be associated with surfaces. Once in the body, it is phagocytized by the macrophages. However, the lipid coat makes it very resistant to destruction by macrophages. So, it lives and possibly reproduces within the macrophage. This allows for the shedding of bacteria by infected fish.

This was a major problem in salmonid culture in the 1950's. It was a common practice at this time to feed raw fish viscera and other fish byproducts to salmonids. So, the disease was spread by feeding infected viscera. Since the production of commercial feeds which use pasteurized fish byproducts it is no longer a major problem in cultured food fish. Recently, it has been identified as a problem in the hybrid bass industry. It is also a major problem in the aquarium fish industry.