Wildlife Disease Laboratories, San Diego Zoo's Institute for Conservation Research, Escondido, CA, USA
Recent work suggests that Mycobacterium avium infections are not readily transmissible from bird to bird.1,3-5 Instead, most infections are probably acquired independently from the environment. Determining which environmental compartments pose the greatest risk of infection requires a testing method that distinguishes pathogenic from nonpathogenic strains of Mycobacterium avium complex (MAC) organisms (particularly M. avium subspecies avium, hominissuis, and intracellulare). Such a test might also distinguish intestinal pass-through from true infection in birds with MAC positive fecal cultures. The insertion element IS901 has been shown to correlate with pathogenicity of M. avium isolates.2 A real-time PCR assay targeting the IS901 gene was developed as a diagnostic tool for identifying pathogenic MAC strains in tissue samples and culture isolates from birds, as well as environmental samples. We analyzed a total of 104 cultured isolates of MAC from diseased (n=24) and non-diseased (n=29) birds for the IS901 insert. All isolates from diseased birds were IS901 positive strains of M. avium subspecies avium, while all isolates from non-diseased birds were IS901 negative strains of M. avium subspecies hominissuis, indicating the potential value of this test in distinguishing intestinal pass- through from true infection. Samples of soil, water, and significant food items such as crickets and annelids were collected from 238 individual aviaries. More than 65% of all environmental samples contained IS901 positive strains of M. avium subspecies avium. This suggests that birds are regularly exposed to potentially pathogenic mycobacteria, and progression to a disease state is likely to involve multiple risk factors.
The authors would like to thank the Ellen Browning Scripps Foundation for funding this study, and the staffs of the Wildlife Disease Laboratories, Veterinary Services Departments, and Bird Departments from the San Diego Zoo and San Diego Zoo's Wild Animal Park for assistance.
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