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Mycobacteriosis in Fish
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Mycobacteria-infected Fish Populations
Mycobacteriosis causes a chronic to subacute disease in many different species of fish, including those that inhabit fresh, brackish or marine environments. When present in a population, infection rates can vary from 10% to 100%. Unfortunately, there is no non-lethal method available to identify infected individuals, especially those in early to mid stages of disease. Fish with late-stage disease often develop ulcerations and other external lesions that may have some diagnostic value. Therefore, if one fish in a population is diagnosed with the condition, then the entire population must be considered exposed and potentially infected. Methods for detection of infected individuals have yet to be developed. In addition, factors that promote the establishment of mycobacteria within a given aquaculture system also need to be identified to decrease chance of exposure. Finally, factors that promote infection need to be determined to avoid development of disease in exposed individuals.

Unlike most other bacterial diseases, there is no cure for mycobacteriosis. The infection will smolder in infected fish, resulting in chronic health problems and mortality in the population. This situation is not consistent with maintenance of "Good Management Practices" for aquaculturists. Efforts to eliminate infection in affected populations with prolonged use of antibiotics have not been successful medically and are prohibitively expensive.


Transmission
Epidemiology, which is the study of the prevalence and spread of the disease in a population, is poorly understood for mycobacteriosis in fish. The most common method of transmission is assumed to be ingestion of infected material. This can occur if fish are fed fish products that have not been thoroughly cooked, or if an infected fish dies and is consumed by other fish in the population. Fish may also contract the infection through open wounds in their skin if the number of bacteria in the environment is high enough or if the fish has a poorly operating immune system. It is suspected that vertical transmission (transmission from parent to offspring) may occur through egg or sperm products. If this is the case, maintenance of mycobacteria-infected broodstock would be totally unacceptable for fish culturists.


Human Health Concerns
Although transmission from infected fish to humans is rare, it is important to be aware of the potential occurrence.

Mycobacterium spp. that cause disease in fish occasionally cause a skin infection in people that is referred to as "fish handler's disease" or "fish tank granuloma." In most cases, the infected individual has been in contact with high numbers of infective particles, with a break in the skin at the original infection site. The infection is characterized by pustules that resemble a rash and occur on body parts exposed to contaminated water or fish, generally the hands, arms, or legs. The pustules exude a thick white to yellow fluid. In healthy individuals the disease is usually self-limiting and often heals on its own. However, there are reported cases of more severe localized infections resulting from deep penetrating wounds in fishermen. Also, immunologically compromised individuals are at greater risk of developing systemic infections. Regardless, anyone who suspects that he or she may have contracted mycobacteriosis from fish or an aquarium should see their physician.


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