Case-control Study:

Mycobacterium avium paratuberculosis (MAP), the cause of Johne's disease (JD), is common in US dairy herds with over 68% of herds containing infected cattle. As there is no approved or effective treatment for JD, control depends on minimizing transmission of MAP to uninfected cattle. Early identification and removal of infected cattle would help minimize JD transmission. Unfortunately, because of the slow progression of JD, early identification of infected cattle is problematic due to poor sensitivity and specificity of currently available diagnostic tests in young, subclinically infected heifers. Moreover, it is unclear whether testing young heifers is cost-effective, given the constraints and costs of current tests, as well as the indefinite effects of subclinical MAP infection on individual cow productivity reported in the literature. Repeated exposure to high levels of MAP accelerates JD progression such that clinical signs are exhibited in relatively young cattle. Accordingly, these cattle should test positive for MAP at a younger age. Multiple studies have documented fecal shedding of MAP in naturally infected calves less than 24 months of age, but none have followed these calves into adulthood to see how they perform as mature cows. The objective of this study was to compare herd longevity, production performance, and adult MAP test status of cows testing positive for MAP <24 months of age to age-matched herdmates.