Multilocus Short Sequence Repeat technique to identify Mycobacterium avium subspecies paratuberculosis strains in supershedders and contemporary Pass through or Active passive-shedders in three Northeastern US dairy herds

Paratuberculosis in cows is caused by Mycobacterium avium subsp. paratuberculosis. (MAP) Infected cows spread MAP in the environment. Recently Supershedder (SS) cows were recognized. These cows are spreading extremely high numbers of MAP in the environment. These MAP in the environment can be ingested by herd-mates causing positive fecal samples in these herd-mates. We hypothesized that these fecal positive cows become Pass through cows, ingesting the MAP and shedding MAP in the feces without becoming positive on tissue rather than Active passive-shedders, ingesting the MAP, shedding MAP in the feces and becoming positive on tissue. By using the Multilocus Short Sequence Repeats (MLSSR) method for 3 different loci (1, 2 and 8) on SS cows and cows that were fecal positive at the same time as the SS, in three different herds in the North East of the US, we aimed to evaluate if low shedders should be considered Pass through cows or Active passive-shedders and if Active passive-shedders were infected by the SS. We observed that a large percentage of the cows that were positive on fecal samples at the same date as the SS, were shedding the same strain as the SS. When cows were found positive on fecal culture, the majority also were eventually positive in tissue at slaughter. Of these tissue positive cows several were found having the same strain in their tissue as the SS. We therefore conclude can be that a large number of cows become Active passive-shedders rather than being Pass through cows.