Heritability estimates for antibody response to Mycobacterium avium subspecies paratuberculosis in German Holstein...

The objective of this study was to estimate heritability of antibody response to Mycobacterium avium ssp. paratuberculosis (MAP) in 4,524 German Holstein cows by applying linear and threshold models. Data were collected within a paratuberculosis voluntary control program in Thuringia, Germany, in 2005. The MAP-positive prevalence of the 12 farms in the data set varied between 5 and 36.9%. A nearly linear increase in prevalence was observed from 2- to 3-yr-old cows, whereas prevalence declined in cows older than 5 yr. This could be explained by greater culling rates associated with increasing age. Classification as MAP positive, questionable, and negative was available for all cows, and the optical density values of the Svanovir ELISA test existed for 2,084 of the animals originating from 6 farms. The heritability estimates of linear and threshold animal and sire models were compared. For the available data sets with an average of 8 progeny per sire, animal models were more robust and yielded more reliable results than did sire models. Heritability estimates from sire models led to overestimation of genetic variances because of a low number of progeny per sire and average relationship within sire progeny of greater than one-fourth, as expected between half-sibs. For all animal models, a heritability of about 0.1 was estimated for antibody response to MAP. Furthermore, it can be concluded that for the estimation of breeding values for antibody response to MAP optical density values of the ELISA test as a normally distributed trait (log-transformed) should be used rather than MAP status (positive or negative) as a binary trait because of the greater heritability and more robust parameter estimates when sire or animal models are used.