Epidemiology of Mycobacterium avium subsp. paratuberculosis fecal shedding in Johne's disease infected dairy herds.

Abstract

Johnes’s disease, also known as paratuberculosis, is a chronic enteritis of ruminants caused by Mycobacterium avium subsp. paratuberculosis (MAP). The disease has a slowly progressing detrimental effect on cow health and production. In the United States most dairy cattle farms are infected, causing an economic impact to dairy herds in the short term by the association of the disease with low milk production, higher culling rates and low reproductive performance. Control of the disease has been focused on implementation of management practices that reduce the MAP transmission from infected cows to uninfected young calves, and on culling cattle that shed large amount of MAP in feces (heavy fecal shedding cows) as soon as detected. The objectives of these studies were to evaluate the association between use of recommended management practices on Johne’s disease incidence and to improve the understanding of the most commonly used diagnostic tests to identify heavy fecal shedders. The evaluation of the association between use of a standardized control program on the incidence of Johne’s disease was conducted in a prospective longitudinal observational study that in 8 dairy herds in Minnesota. Herds were followed during a period of 5 to 10 years. We found 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 was performed in two separate experiments using MAP negative bovine fecal samples spiked with different concentrations of MAP. 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 in experiments 1 and 2 using different approaches were 107 and 102 MAP/g of feces, respectively. 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 the parallel interpretation 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. The sensitivity and specificity of the bacterial culture on pooled individual samples to detect at least one heavy fecal shedding cow in the pool was 100% and 91%, respectively, and these values did not change when pool size increased from 5 to 10 cows per pool. 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.