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		Mycobacteriosis in Fish
		Ruth Francis-Floyd, DVM, MS, Dipl ACZM, and Roy Yanong, VMD	May 27, 2002
		Introduction Mycobacteriosis in fish is a disease caused by certain bacterial species within the genus Mycobacterium. There are many different species of Mycobacterium that cause diseases in animals. Some of these cause important diseases in humans. However, those species that cause Mycobacteriosis in fish are referred to as non-tuberculous mycobacteria and do not cause major disease in normal, healthy people. Non-tuberculous mycobacteria are relatively common environmental bacteria. This means that they can live in the aquatic environment and do not require the presence of fish or fish tissue to survive. They have been isolated from drinking water supplies, swimming pools, coastal waters, and aquaculture facilities. Mycobacteriosis is a disease of concern in fish for several reasons. First, it causes a chronic wasting condition with continuous low, to moderate, level mortality within the affected population, and can result in significant financial loss to fish culturists. Secondly, the disease is considered non-treatable once established in the fish host. Finally, Mycobacterium-infected fish do have the potential to cause disease in humans. Species of Mycobacteria that Infect Fish The genus Mycobacterium contains many species of bacteria that cause disease in mammals, birds and reptiles, as well as fish. Three species believed to account for most incidences of mycobacterial disease in fish are M. marinum, M. fortuitum, and M. chelonae. Of these, M. marinum has been reported in fish from both freshwater and marine tropical waters, and M. fortuitum in fish from tropical or temperate waters. M. chelonae has been associated with disease in hatchery-reared Pacific salmon. Environmental Conditions Favoring Mycobacteria Many of the organisms in this group occur naturally in the aquatic environment. One report compared the prevalence of selected species of non-tuberculous mycobacteria in two different aquatic environments, acidic (boggy) swamps and upland lakes. The investigators found a much higher prevalence of mycobacteria in swamps, and correlated the increased number of organisms with environmental conditions including low levels of dissolved oxygen, low pH, high organic loads, and warm water. These conditions are similar to those commonly encountered in aquaculture systems. Another investigation associated an increased prevalence of non-tuberculous mycobacteria with low salinity, another condition found in freshwater aquaculture systems. Trace amounts of certain minerals, particularly zinc and iron, have also been correlated with increased numbers of environmental mycobacteria. more ...

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