Breeding and culture of ornamental fish is a fast growing trade the world over. The culture of ornamental fish are exposed to several potential stressors such as management and environmental changes, which compromise the fish's natural defenses so that it cannot effectively protect itself from invading pathogens (Ruth, 2002). Many disease causing organisms normally occur in the same environment as the fish; they usually only become a problem when present in significant quantities and/or stress occurs. One of the most common chronic bacterial diseases affecting ornamental fish is fish mycobacteriosis, affecting both temperate and tropical species both in freshwater and the marine environment (Prearo et al., 2002). Besides causing mortality in fish, they represent a potential hazard for man, being included among zoonoses (Ghittino and Bozzetta, 1994). Mycobacteriosis is caused by ubiquitous bacteria that are highly resistant in the aquatic environment and are difficult to control. Mycobacterium fortuitum, M. marinum and M. chelonae are the most common isolated species. They are all capable of inducing human cutaneous infections, with development of nodular lesions that are generally localized in the limbs. In fish, signs appear late, are non-specific, and include slow growth, lethargy and anorexia. Typical whitish nddules in the viscera may be detected at necropsy (Ghittino, 1992). The aim of this study to determine the prevalence of mycobacteriosis among some species of ornamental fish during different seasons and study the pathogenicity and histopathological alterations induced by different species of isolated fish mycobacteria in goldfish (Carassius auratus) as an experimental model.
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