Somatic cell count threshold in dairy sheep affected by the prevalence of mammary infection

Résumé

A total of 9,592 milk samples of half udders (3,908 and 5,684 samples for lactations prior and subsequent to antibiotic dry therapy, respectively) were collected aseptically in monthly samplings throughout lactation from 1,322 Churra ewe lactations belonging to 7 separate flocks. Dry therapy allowed to establish two different prevalences of intramammary infection (IMI): high (pretreatment lactations) and low (posttreatment lactations), and somatic cell count (SCC) thresholds were studied for each IMI prevalence. Only half-udders infected by major pathogen (novobiocin-sensitive coagulase negative staphylococi included) were considered as positive in this study. For pretreatment lactations (IMI prevalence: 30.7%), the SCC threshold of 300 x 103 cells/ml showed values of 84.1%, 74.5%, 88.4%, 8.1%, 7.8%, 73.9% and 88.7% for half-udders correctly classified, sensitivity, specificity, false positive, false negative, and predictive values of positive and negative results, respectively. After dry therapy (IMI prevalence: 7.7%) the SCC threshold which showed a reliable discrimination of infection status was 600 x 103 cells/ml with values of 94.4%, 64.1%, 96.9%, 2.8%, 2.7%, 63.4% and 97.0% for half-udders correctly classified, sensitivity, specificity, false positive, false negative, and predictive values of positive and negative results, respectively. As a result, the variation in prevalence affects the accuracy of predicting infected half-udders, so it is essential to make SCC interpretation and mastitis control recommendations according to prevalence variations.