Effect of Mycobacterium paratuberculosis infection on production, reproduction, and health traits in US Holsteins

Our objective was to estimate the effect of Mycobacterium paratuberculosis infection on milk, fat, and protein yield deviations, pregnancy rate, lactation somatic cell score, and projected total months in milk (productive life). A serum ELISA and fecal culture for M. paratuberculosis were performed on 4375 Holsteins in 232 DHIA herds throughout the US. Primarily first through third lactation cows (99% of total) were assayed for infection. Trait information (except productive life) was obtained for the lactation concurrent with disease tests. Productive life was total months in milk through a cow's life, which was projected if a cow was still milking. For most analyses, case definition for M. paratuberculosis infection was defined as either an ELISA S/P ratio ≥ 0.25 or a positive fecal culture for M. paratuberculosis or both. To determine if diagnostic test affected estimates, case definition was redefined to include only cows with ELISA S/P ratios ≥ 0.25 or only fecal culture-positive cows. Linear models were used to estimate effect of M. paratuberculosis infection on traits. M. paratuberculosis-infected cows (7.89% of cows) produced 303.9 kg less milk/lactation, 11.46 kg less fat/lactation, and 9.49 kg less protein/lactation (P ≤ 0.003). Infected cows had higher pregnancy rates (1.39%) (P = 0.0385) and lower productive life (2.85 months) (P ≤ 0.0001). M. paratuberculosis infection did not affect somatic cell score. Effect of infection on milk and protein yields was larger in first lactation M. paratuberculosis-positive cows relative to cows that tested positive in later lactations. Fecal culture-positive cows had consistently larger effects on all traits than ELISA-positive cows. M. paratuberculosis infection, and not just clinical Johne's disease, decreases milk, fat, and protein yields, thus increasing the estimated cost of paratuberculosis to the US dairy industry.