Multilocus Short Sequence Repeat Analysis of Mycobacterium avium subsp. paratuberculosis Isolates from Dairy Herds in Northeastern United States of a Longitudinal Study Indicates Low Shedders are Truly Infected

ABSTRACT The objectives of this study were to evaluate whether low shedders of Mycobacterium avium subsp. paratuberculosis (MAP) were pass-through animals or whether they were truly infected. We also evaluated whether these animals were possibly infected by the supershedders. The MAP isolates were obtained from a longitudinal study that involved three different herds in the northeastern US. The shedding levels of animals at each culturepositive occasion were determined. Selected isolates were collected from all animals that were culture-positive at the same time super-shedders were present in the herds and from super-shedders. Using a multilocus short sequence repeat (MLSSR) approach we found 15 different strains from a total of 142 isolates analyzed. The results indicated herd-specific infections; a clonal infection in herd C with 89% of animals sharing the same strain, different strains in herds A and B. In herd C, 100% and in herd A, 17 to 70% of cows shed the same strain as that of contemporary super-shedders at a given collection date. About 82% of available tissue samples were culture-positive indicating a true infection. Taken together the results of MAP strain-typing and shedding levels, we conclude that at least 50% of low shedders have same strain as that of a contemporary super-shedder. The results of this study indicate that very few cows had characteristics of a possible pass-through animal; many more cows were actively infected. The sharing of same strain of low shedders with the contemporary super-shedders suggests that low shedders may be infected as adults by the super-shedders