Cut-off value of somatic cell count and validation of differential somatic cell count by the fluoro-optical method in subclinical mastitis milk in three sheep breeds

Somatic cell count (SCC) is a test used for mastitis control in ewes, but there is no consensus on the cut-off value. Other markers of subclinical mastitis (SCM) have been proposed as potentially more efficient, sensitive and specific, such as differential somatic cell count (DSCC). Several studies have been conducted on cow milk DSCC application, especially since the development of a method (Foss DSCC method) that allows the rapid and simultaneous determination of both SCC and DSCC in cow milk. The aim of this study was to investigate the Foss DSCC method in ewe milk. We first calculated a SCC cut-off for sheep half-udder milk samples from three dairy breeds to be used with bacteriological analysis to define SCM; second, we validated the Foss DSCC method following the validation conducted on bovine milk; and finally, the DSCC cut-off study was conducted for the sheep SCM milk. For this purpose, 4074 ewe half-udder milk samples from three breeds were analysed for bacteriological, SCC and DSCC investigation. The validation of the Foss DSCC method followed that previously conducted on cow milk, while the optimal cut-off values were chosen based on the Youden method after generating receiver operating characteristic curves and calculating the relative area under curve values. The specificity, repeatability and robustness of the Foss DSCC method for sheep milk were comparable to those of the method for bovine milk. The optimal cut-off resulted in 500 × 10³ cells/mL and 71.5% for SCC and DSCC, respectively. For the first time, the Foss DSCC method was validated in sheep milk, and SCC and DSCC cut-off values were determined for three important dairy milk breeds of ewes in Italy. These results will allow developing further studies to improve mastitis screening and will help farmers, veterinarians and technicians to identify SCM in flocks.