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Mycobacterium avium subsp. avium and M. intracellulare are ubiquitous organisms in the environment. The reservoir of M. avium subsp. avium is generally accepted to be environmental, in particular, water and soil are sources of the organism. In contrast to M. avium infection in wild and domestic birds, M. avium infection in mammals occurs only sporadically and is rarely transmissible. Generalised disease is usually uncommon, owing to the non-progressive, chronic character of the infection. However, some cases of disseminated disease have been reported, e.g. in captive non-domestic hoofed animals as well as in immunosuppressed dogs and cats. The majority of M. avium and M. intracellulare infections in livestock are detected at slaughter and the diagnosis is confirmed by bacteriological procedures. Condemnation of affected portions of the carcass can result in significant economic losses, although gross lesions are mostly restricted to lymph nodes close to the alimentary tract. Successful treatment with antibiotics in combination with surgery has been reported in some affected domestic cats, but is not considered to be effective or economical in other species. In the past, differentiation of M. avium bacteria from the closely related M. avium subsp. paratuberculosis was based on the mycobactin dependence and prolonged incubation period of the latter. More recently, amplification of the genomic insertion sequence IS900 has proved to be a powerful tool for identification of M. avium subsp. paratuberculosis. The potential zoonotic importance of M. avium infections has been indicated, but requires clarification.