Within-farm strain dynamics of Mycobacterium avium subsp paratuberculosis: Evidence for limited vertical transmission

Abstract

MAP-infected dairy cattle are assumed to be a high risk for transmitting infection to their daughters. Alternatively, if both dam and daughter are genetically more susceptible to MAP, they may be both infected but not necessarily due to vertical transmission. Using strain typing techniques including multi locus short sequence repeat (MLSSR) typing allows a potential distinction between vertical transmission and genetic susceptibility. Analyzing strain diversity in longitudinal datasets provides additional insight into within-herd infection dynamics, including the transmission of MAP from dams to daughters. To investigate the importance of vertical transmission, we identified 12 pairs of dams and daughters for which both animals are known MAP infected from the Regional Dairy Quality Management Alliance (RDQMA) study herd in NY. All adult animals on the farm were tested for MAP via fecal culture semi-annually for seven years. Tissue samples were available on a subset of cull animals. Animals were considered MAP-infected if they ever cultured positive or if any of their tissues cultured positive at slaughter. Cultures were performed at University of Pennsylvania on HEYM solid media. Positive cultures were substreaked and processed for MLSSR typing. Following genotyping, isolates from each dam-daughter pair were compared to determine whether they shared the same MAP genotype. Environmental MAP burden at birth was assessed via typing of MAP-positive environmental samples (collected four times a year) and known MAP-infected animals present on the farm during the high-risk first year of life. Of the 12 infected dam-daughter pairs, 9 had identical strains shared between the dams and daughters. In addition, 2 daughters had the dam’s strain as well as another circulating strain. Overall, there were 8 strains represented in the daughters that did not come from dams (2 daughters had multiple strains which did not originate from the dam). These results lend additional importance to the impact of genetics on susceptibility, as 5 of 12 daughters carried different strains of MAP than their dams, even when concurrently infected with the dam’s strain.