Association of Mycobacterium avium paratuberculosis serostatus with age at first calving, calving interval and milk production in dairy cows

Abstract

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of bovine paratuberculosis, also known as Johne's disease. This infection is responsible for negative effects, ranging from reduction of milk production to reproductive compromise and increased susceptibility to other diseases such as mastitis. Contradictory information on the association between this infection and reproductive performance has been reported in dairy cows. The aim of this work was to investigate associations between individual cow MAP seropositivity and lifetime reproduction and production performance. MAP serum ELISA (IDEXX MAP Ac) results from all the 13,071 adult cows present on 191 farms and corresponding birth- and calving-date records obtained from the National Association for Genetic Improvement of Dairy Cattle were used. Cows and farms were classified as positive or negative, based on ELISA results. Outcomes assessed, for all cows and all calvings from first to fifth, were age at first calving (AFC), intercalving intervals (ICI) from first to fourth interval, and average milk production per day of productive cycle (Milk-305/ICI - a ratio between 305 d corrected milk production, for each lactation, and the number of days of the respective ICI). Multilevel mixed models were used to investigate the association of cows' MAP status with AFC, ICI and Milk-305/ICI. Three levels were considered in the models: "measurement occasion," the first level, was nested within cows and cows were nested within farms. The "measurement occasion" is the time point where all the observed measures (between 2 successive parturitions, such as milk production and SCC) were referred to. Our results indicate that MAP positive Cows have a significant 14-d lower mean AFC than MAP negative ones. The overall average ICI in our study was 432.5 d (s.d. 94,6). The average ICI, from 1st to 4th, was not significantly affected by MAP seropositivity. No significant effect of MAP positivity was found on the overall ICI. In relation to Milk-305/ICI, MAP positive cows did not produce significantly less milk than negative cows, across their productive lifetime. We observed higher but non-significant Milk-305/ICI (Kg/day) in MAP positive cows. In our study, the proportion of MAP positive Cows within lactations remained similar across all lactations suggesting that seropositivity did not increased drop-off rate.