Physiologic-chemoattractant-induced migration of polymorphonuclear leukocytes in milk

The somatic cell count (SCC; leukocytes and epithelial cells) in milk is used as an indicator of udder health status. A SCC above the regulatory standard is generally considered as an indication of mastitis. Therefore, milk with a SCC equal to or greater than the regulatory limit cannot be sold to the public because it is unsuitable for human consumption. This study was performed to determine whether SCC levels above the regulatory limit observed in goats during late lactation are a physiologic or a pathological response of the goat mammary gland. Differential counts of cells in nonmastitic goat milk samples during late lactation revealed that approximately 80% of the cells were polymorphonuclear leukocytes (PMNs). In addition, microchemotaxis assay results indicated that normal nonmastitic late-lactation-stage goat milk is significantly higher (P < 0.001) in PMN chemotactic activity than early-lactation-stage goat milk, with a mean chemotactic activity of 14.9 and 42.7/mg of protein for early and late lactation stages, respectively. Physicochemical analyses also suggest that the PMN infiltration observed in normal late-lactation-stage goat milk is due to a PMN chemotactic factor(s) that is different from the PMN chemotactic factor(s) present in mastitic milk. Interestingly, the PMN chemotactic factor in late-lactation-stage goat milk is highly acid resistant (pH 2), suggesting that the factor is able to survive the highly acidic gastric environment and may therefore be important in the augmentation of the immune systems of sucklings. These results indicate that the chemotactic factor(s) present in the milk of normal late-lactation-stage goats is nonpathological and may play a physiologic regulatory role in mammary gland involution. Hence, the regulatory standard for goat milk needs to be redefined in order to reflect this.