Influence of parity at time of detection of serologic antibodies to Mycobacterium avium subspecies paratuberculosis on reduction in daily and lifetime milk production in Holstein cows.

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of bovine paratuberculosis, also known as Johne's disease. Two production variables (daily and lifetime milk) were compared in this longitudinal study of cows classified based upon serum ELISA results for MAP. The sample included adult Holstein cows that had completed their productive life (n = 2,808 animals) since testing began and that had been raised by and were located on 2 commercial dairies in Texas. These animals were tested at the conclusion of each of their lactations using a commercial ELISA. The serological response was interpreted based on the calculated sample-to-positive value, a quantifiable measure of antibodies, and categorized on both a dichotomous scale [positive (≥0.25) vs. negative] and into 5 classes (negative: 0 to 0.09, suspect: 0.10 to 0.24, low positive: 0.25 to 0.39, positive: 0.40 to 0.99, or strong positive: ≥1.00). All models developed were adjusted for herd, birth (year) cohort, and parity at first MAP test. Based on adjusted comparisons using the dichotomous scale: 1) cows detected as positive during their first parity produced less (P < 0.0001) milk per day of life since first parturition (-1.64 kg/d) and during their lifetime in the herd (-8,704 kg); 2) cows testing positive for the first time during second parity did not produce less per day of life (P > 0.05); however, cows did produce less (P = 0.024) milk overall during their lifetime in the herd, but only when tested for the first time during their second lactation (-4,058 kg); 3) cows testing positive for the first time during third parity did not differ (P > 0.05); and 4) cows testing positive for the first time during their fourth or later parity produced more milk during their lifetime, but only when tested for the first time at third or greater lactation (e.g., 10,602 kg for fourth parity; P = 0.005) compared with their always-negative herd mates tested for the first time in their first lactation. In the adjusted model developed to evaluate milk production losses throughout the lifetime of the animal, when categorizing animals into 5 MAP classes at their first test, suspect animals numerically have reduced production (-1,419 kg; P = 0.162), whereas significant losses occurred in the low positive class (-4,397 kg; P = 0.034) and highly significant losses in the positive (-7,724 kg; P < 0.0001) and strong positive classes (-10,972 kg; P < 0.0001) compared with herd mates testing negative.