Respiratory burst activity of blood and milk neutrophils in dairy cows during different stages of lactation

The non-stimulated and phorbol 12-myristate 13-acetate (PMA)stimulated lumi nol-augmented cellular chemiluminescence (CL) response and viability of mil k and blood polymorphonuclear leukocytes (PMN) were determined in lactating dairy cows during different, stages of lactation. In the first study, ten healthy cows each in early, mid and late lactation were compared. In a seco nd study, the same measurements as in the first study Were evaluated longit udinally in 12 cows during 1 month following parturition. Th CL activity an d myeloperoxidase (MPO) content of milk PMN and macrophages (M) were also c ompared. Milk M did not possess MPO activity and Were devoid of any luminol -enhanced CL. The CL activity of milk and blood PMN was significantly lower in early lactation than in mid and lace lactation (P<0.001). Whereas littl e changes were observed in viability of blood PMN, the viability of milk PM N was lower in early lactation than in mid and late lactation (P<0.061). Th e percentage of PMN in isolated milk cells was also lower during early lact ation than during mid and late lactation (P<0.001). The CL activity in resp onse to PMA during early, mid and late lactation increased 13, 59 and 42-fo ld in blood PMN and 1.7, 2.6 and 2.4-fold in milk PMN, respectively, in com parison with non-stimulated PMN. The CL activity, both in milk and blood PM N, the milk PMN viability and the percentage of milk PMN were lowest betwee n 3 d and 11 d post partum. These observed changes immediately after calvin g could contribute to a higher susceptibility to mastitis in that period.