Epidemiology of Mycobacterium avium subsp. paratuberculosis Fecal Shedding in Johne's Disease Infected Dairy Herds

The objectives of these studies were to evaluate the association between use of recommended management practices and Johne's disease incidence, and to improve the understanding of the most commonly used diagnostic tests to identify Mycobacterium avium subsp. paratuberculosis (MAP) heavy fecal shedders.;The evaluation of the association between use of a standardized control program on the incidence of Johne's disease showed a reduction of the incidence of bacterial culture positivity, serum ELISA positivity, heavy fecal shedding status, and clinical Johne's disease associated with higher levels of implementation of the recommended management practices.;The evaluation of the analytical sensitivity of bacterial culture of feces and direct fecal PCR showed that the analytical sensitivity of the bacterial culture of feces was 105 MAP/g of feces and the probability of a higher bacterial culture result increased with the concentration of MAP in the fecal sample. The analytical sensitivities of the direct fecal PCR using different approaches were 107 and 102 MAP/g of feces.;A latent class model using a Bayesian approach was fitted to estimate the posterior conditional probabilities that the results of the bacterial culture of feces and serum ELISA correctly identified cows as high positive, low positive or negative given that they were heavy, light and non-fecal shedders, respectively. The estimated conditional probabilities that bacterial culture of feces correctly identified heavy, light and non-fecal shedders were 70.8, 32.2 and 98.5%, respectively. The same values for the serum ELISA were 60.5, 18.8 and 99.5, respectively.;Finally, we conducted a cross-sectional study to evaluate the association between bacterial culture of cow-level and pooled environmental fecal sample results for detection of MAP in dairy herds. The sensitivity and specificity of bacterial cultures of pooled environmental fecal samples from the herd to detect at least one heavy fecal shedding cow in the herd was 98.2% and 43.5%, respectively.;In summary, these studies shown that implementation of critical management practices are associated with a reduction on the incidence of Johne's disease and diagnostic tests can be used to indentify heavy shedding cows using individual or pooled fecal samples.