More insights about the interfering effect of Mycobacterium avium subsp. paratuberculosis (MAP) infection on Mycobacterium bovis (M. bovis) detection in dairy cattle.

The estimated herd and within herd Mycobacterium bovis (M. bovis) infection prevalence in the southern Chile regions are 0.3 and 0.67%, respectively. However, higher rates of infection still remain in some herds. In parallel, it is well established that a big proportion of cattle herds are infected with Mycobacterium avium subsp. paratuberculosis (MAP), which has been also associated with a clear interference effect on M. bovis diagnosis. The present study aims to provide more insights about the diagnostic interference for Mycobacterium bovis detection due to co-infection with MAP. To better understand the dynamics of this identified interference, the effect of MAP genotype present, as well as MAP faecal shedding values (as proxy of the infection progression), for each of the CFT results was compared. No relationship was observed between MAP genotype with any type of differential response to the diagnostic tests of M. bovis infection. However, MAP shedding values in animals with positive CFT diagnostic results for M. bovis infection was significantly lower than animals with a negative CFT result, observing that as the MAP shedding load raises, the response to the bovine tuberculin test tends to be negative. The findings reported in this study allows to interpret that one of the causes of the prolonged elimination of M. bovis infection from some cattle herds may be due in part to the advanced MAP infection status in co-infected individuals affecting the outcome of screening in-vivo diagnostic techniques such as CFT. These false negative animals that show negative results to M. bovis detection tests, may maintain the infection at herd level and spread the pathogen to healthy individuals.