Prior exposure of a vaccinee to certain species of environmental mycobacteria can prime the immune system against common mycobacterial antigens, which can in turn reduce the subsequent efficacy of live attenuated mycobacterial vaccines (such as Mycobacterium bovis BCG), in both human and livestock vaccination programs. In this study, two strains of Mycobacterium avium, both isolated from New Zealand livestock, were investigated to determine their growth characteristics and effects on the immune system in murine models. Markedly different effects on the immune system were observed; an IS901-negative strain ( Members of the Mycobacterium avium-Mycobacterium intracellulare complex are ubiquitous in the environment, and they are a major confounding influence on the specificity of intradermal skin tests used to detect infection with Mycobacterium tuberculosis in humans or infection with Mycobacterium bovis in farmed livestock. Crude antigens, in the form of purified protein derivative (PPD) from cultures of M. tuberculosis or M. bovis, are used as restimulation antigens in in vivo intradermal skin tests, as well as in ex vivo blood-based diagnostic assays, to identify tuberculosis infections. False-positive results are common, and it is believed that these results are due to crossreactive immune responses induced by transient exposure to, or infection by, members of the M. avium-M. intracellulare complex (1, 21).In 2002 Buddle et al. reported that presensitization of cattle with M. avium was associated with poor vaccine responses to M. bovis bacillus Calmette-Guerin (BCG) (4). After this, an experimental study investigating the potentially deleterious effects of different field isolates of M. avium on the protective efficacy of BCG vaccination was performed using a guinea pig model (6). In this study the workers identified one strain of M. avium (designated WAg 206) which could suppress the protective effect of BCG vaccination against a virulent M. bovis challenge infection (6). The immune mechanisms which underlie these findings have not been investigated yet. Neither cattle nor guinea pigs lend themselves well to in-depth studies of the immune response. In order to elucidate the mechanisms involved, we used a mouse model to study the effects of two M. avium strains (WAg 206 and WAg 207) on immune functions both in vitro and in vivo (6).The cells of the mononuclear phagocyte (MNP) system (comprising monocyte-derived macrophages and dendritic cells [DC]) are the first point of contact between the immune system and M. avium-M. intracellulare complex species. Mycobacteria are phagocytosed and placed into a phagolysosome, where they can be destroyed if the MNP system is properly activated, or they may persist within the host cell for prolonged periods depending on the virulence of the strain. A number of mycobacterial cell wall components have been identified as components that are important in the induction of inflammatory signals from MNP (20). Signaling through Toll-like-receptor 2 appears to be essential for inducti...

Factors considered potentially capable of interfering with vaccination, including feeding dead BCG to possums prior to feeding live BCG, feeding multiple doses of BCG at one time, and changing strains of BCG, were shown not to interfere with the acquisition of protective immune responses in possums. Protection against tuberculosis was undiminished up to 29 weeks after vaccination with BCG administered orally. It is concluded that vaccination of possums by feeding pellets containing BCG is a robust and efficient approach to enhance the resistance of these animals to tuberculosis.

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Further studies are required to confirm the clinical significance of flock-based antibody responses, and to validate their use in identifying recently aborted ewes, especially where there are no aborted fetuses for examination.