Diagnosis of Mycobacterium bovis Infection in Calves Sensitized by Mycobacteria of the avium/intracellulare Group

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Abstract
Because of the frequent exposure of cattle to mycobacteria of the avium/intracellulare group, an investigation was carried out into the possible repercussions thereof on the diagnosis of bovine tuberculosis. Three calves from a bovine tuberculosis‐free herd, scored avian reactors in the γ‐interferon assay for bovine tuberculosis, were sedated and inoculated endotracheally with a virulent Mycobacterium bovis strain. Then, three other avian reactors were housed with the above donor calves. Mycobacterium bovis was isolated from the nasal swabs of the three endotracheally infected, donor calves. On these samples, TB complex‐specific polymerase chain reaction (PCR) tests for IS6110 were also positive, albeit with a different time kinetics. The three contact‐infected calves showed clear immunological signs of infection; however, their nasal swabs were always PCR‐negative and only Mycobacterium avium was isolated. In the endotracheally infected donor calves there was a rise of the γ‐interferon responses to avian and bovine purified protein derivative (PPD) tuberculins, which reached the same stable plateau levels over the whole experiment. The above effect was also observed in the contact‐infected calves, even though the response to avian PPD tuberculin always remained at a higher level. By using conventional bovine and avian PPD tuberculins, the comparative intradermal test was generally positive in endotracheally infected, as opposed to contact‐infected calves; a positive intradermal test for M. bovis was obtained in two contact‐infected calves by different bovine PPD tuberculins based on M. bovis bacillus Calmette‐Guerin (BCG) secreted or somatic antigens. It was concluded that M. bovis infection may be concealed for some time in cattle sensitized by mycobacteria of the avium/intracellulare group and that different diagnostic procedures should be adopted for such animals.