PHYSIOLOGICAL-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 regula tory standard is generally considered as an indication of mastitis. Th erefore, milk with a SCC equal to or greater than the regulatory limit cannot be sold to the public because it is unsuitable for human consu mption, This study was performed to determine whether SCC levels above the regulatory limit observed in goats during late lactation are a ph ysiologic or a pathological response of the goat mammary gland. Differ ential counts of cells in nonmastitic goat milk samples during late la ctation revealed that approximately 80% of the cells were polymorphonu clear leukocytes (PMNs), In addition, microchemotaxis assay results in dicated that normal nonmastitic late-lactation-stage goat milk is sign ificantly higher (P < 0.001) in PMN chemotactic activity than early-la ctation-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 observ ed in normal late-lactation-stage goat milk is due to a PMN chemotacti c factor(s) that is different from the PMN chemotactic factor(s) prese nt 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 environm ent and may therefore be important in the augmentation of the immune s ystems of sucklings, These results indicate that the chemotactic facto r(s) present in the milk of normal late-lactation-stage goats is nonpa thological 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.