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Short Communication: Evaluation of the Overall Accuracy of the DeLaval Cell Counter for Somatic Cell Count in Ovine Milk: Effect of Soak Time in Diluted and Undiluted Milk Samples

Departamento de Producción Animal, Facultad de Veterinaria, Campus de Vegazana, Universidad de León, 24071-León, Spain

¹ Corresponding author: c.gonzalo{at}unileon.es

This study evaluated the performance of the DeLaval cell counter (DCC) when analyzing ovine milk with different soak times (defined as the permanence time of samples within the DCC cassette before starting the DCC counting procedure) in diluted and undiluted milk samples in 2 dairy sheep breeds. A total of 101 composite ovine milk samples (50 from Assaf ewes and 51 from Churra ewes), ranging between 50 x 10³ and 2,200 x 10³ cells/mL, were divided into 10 aliquots/milk to be analyzed by DCC. Four undiluted aliquots and 4 aliquots diluted 1:1 in PBS were analyzed by using soak times of 0, 1, 2, and 3 min/aliquot, and the other 2 aliquots were diluted 1:1 in propidium iodide or ethidium bromide staining solutions and analyzed by DCC. Milk samples were also analyzed by the Fossomatic method, as a reference. All analyses were carried out in duplicate. Undiluted milk samples with soak times 1 min showed large coefficients of regression (b = 0.96 to 0.98) and correlation (r > 0.99) when compared with the Fossomatic method. In these samples, DCC gave repeatability standard deviations (s_r = 35 to 51 x 10³ cells/mL) lower than other DCC analytical conditions (s_r = 49 to 74 x 10³ cells/mL), and their log SCC means (5.51 to 5.52) were close to the reference value (5.56). The log SCC means corresponding to samples diluted 1:1 in staining solutions (5.55) did not differ from the reference value; however, these aliquots had lower regression coefficients (b: 0.92 to 0.93). Samples diluted 1:1 in PBS and undiluted samples with a 0-min soak time showed a global accuracy similar to or lower than undiluted samples with soak times 1 min. Breed did not seem to affect the results. We concluded that undiluted raw milk with a soak time 1 min and analyzed by DCC shows suitable overall accuracy in ovine milk compared with the reference method and can be considered as the best option for on-farm use from an operational point of view.

Key Words: sheep milk • somatic cell count • DeLaval cell counter • accuracy