With increased interest in organic farming and animal welfare, less intensive production systems that employ pasture grazing have gained in popularity. The impact of such practice on the transmission of Mycobacterium avium subsp. paratuberculosis (MAP) to yearling animals is unknown. The purpose of the study was to investigate the transmission of MAP to yearling animals exposed exclusively through grazing of pasture previously occupied by lactating cattle infected with MAP.
Nine Jersey steers originating from a Johne's disease-positive herd were included. At the age of 15 months, the steers were introduced to pastures heavily contaminated with MAP and remained on those pastures for a total of 10 months. Fecal cultures and fecal RT-PCR testing were performed during pasture season and at slaughter. At slaughter, blood was obtained for MAP antibody detection using ELISA. For each steer, 28 separate samples of intestinal tissue and associated lymph node were collected along the length of the intestinal tract for mycobacterial culture. These samples were processed for MAP culturing on HEYM, using standard methods. Samples of ileum and ileocecal lymph node were examined histopathologically for lesions of paratuberculosis and presence of acid-fast staining organisms.
Fecal RT-PCR was positive in all 9 steers throughout pasture season. At slaughter, 4 steers were RT-PCR positive. Two steers had positive fecal cultures. None of the steers had detectable serum antibodies for MAP. Overall, 6 of the 9 steers had at least one tissue sample positive for MAP organisms. The number of positive samples per animal ranged from 5 (18%) samples to 23 (82%) samples. In the positive animals, the number of colony forming units (CFUs) per sample ranged from 1 CFU to 300 CFUs per sample, and the total number of CFUs per steer for all samples combined ranged from 25 CFUs to 9,337 CFUs. Six of the 9 steers had no histolopathological evidence of MAP infection. The 3 remaining steers were negative for acid-fast staining organisms but individual Langhans' type giant cells, and epithelioid macrophages found in the ileal samples were suggestive of an early infection with MAP.
It was initially postulated that these animals were positive for MAP in their feces as MAP was "passing through" following oral consumption on contaminated pasture (i.e., passive shedding). The results of this study however suggest that at least 6 of these animals became permanently infected with MAP as each of them was positive for MAP in at least one tissue. We concluded that exposure of yearling cattle to pastures contaminated with MAP can result in permanent infection with MAP, and that age resistance to infection can be overcome by pressure of infection.