Performance of a milk leukocyte differential test for decision-making in a selective dry cow therapy program

The objectives of this study were 1) to determine the operating characteristics of a commercial milk leukocyte differential (MLD) test to detect intramammary infections in quarters of late-lactation dairy cows as compared to bacteriological culture and 2) to evaluate the milk production and udder health parameters between cows treated following blanket vs selective dry cow therapy (DCT) using the MLD test results. In a first experiment, the MLD test was compared to the bacteriological culture results (gold standard) of 363 quarters from 94 cows. The sensitivity, specificity, and predictive values for the identification of infection using the MLD test were determined. Sensitivity ranged from 44% to 77%, and specificity from 54% to 92%. In the second experiment blanket DCT was compared to selective DCT based on the results of MLD test, and treating only positive quarters; a total of 328 cows were randomly assigned to 1 of the 2 treatment groups. The proportion of quarters positive to bacteriological culture, and the incidence rate of moderate and severe cases of clinical mastitis events, did not differ between treatment groups. Results of these experiments provide information to support decision-making in a selective DCT program in low-SCC herds using the MLD test.