The effect of subclinical Mycobacterium paratuberculosis infection on milkproduction in Michigan dairy cows

The objective of this study was to determine the effect of subclinical Myco bacterium paratuberculosis infection on mature equivalent milk, protein, an d fat production in a sample of Michigan dairy herds with a history of cows positive for M. paratuberculosis diagnosed by fecal culture. A prospective two-group cohort study was conducted. Participating herds were tested, and productivity and reproduction records were monitored for 18 mo. All cows a ged 24 mo and greater were tested for M. paratuberculosis infection using t he ELISA and radiometric fecal culture (RFC) techniques. Using both tests i n parallel, the overall sample apparent prevalence for M. paratuberculosis infection was 41.8%. Adjusting for diagnostic sensitivity and specificity r esulted in a calculated sample true prevalence of 59.9%. Subclinical paratu berculosis test-positive status had no statistically Significant effect on mature equivalent milk, fat, or protein production. The results of this stu dy concur with the findings of other studies, reporting that the magnitude and direction of the association between subclinical paratuberculosis infec tion and milk production depends upon the parity of the animal, stage of di sease, and the stage in lactation being monitored. Assessment of the impact of subclinical paratuberculosis on milk production must consider the avera ge parity of the sample population. In herds that have an average parity of 2 or less, subclinical paratuberculosis infection may have little impact o n milk production.