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
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.
University of Minnesota Ph.D. dissertation. February 2012. Major: Veterinary Medicine. Advisor: Scott J. Wells. 1 computer file (PDF); xi, 141 pages.
Espejo, L. A..
Epidemiology of Mycobacterium avium subsp. paratuberculosis fecal shedding in Johne's disease infected dairy herds..
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