Study on the relationship between milk immune factors and Staphylococcus aureus intramammary infections in dairy cows

The distribution of Staphylococcus aureus within herds seems to be related to interactions among the shedding characteristics of the bacteria, their p athogenicity and mammary gland immune status. The aim of the present study was to investigate the relationships between selected mammary gland immune factors and intramammary infections associated with Staph. aureus. Overall, 70 cows from five commercial dairy herds were included in the study and qu arter milk samples were assessed using bacteriological and cytological test s. We evaluated differential cell count, lysozyme concentration, N-acetyl-b eta-glucosaminidase (NAGase) activity, cell viability and respiratory burst activity in randomly chosen quarter milk samples from each cow. Staph, aur eus intramammary infection elicited different responses in the mammary glan d immune defences investigated. Polymorphonuclear leucocytes (PMN) as a pro portion of total somatic cells in milli, cell viability and NAGase activity were higher in infected quarters, while the proportions of macrophages and lymphocytes, respiratory burst activity and lysozyme levels were lower. Me an values differed among herds, but the differences were not significant. T hese changes were associated with Staph. aureus infection. The reduced resp iratory burst activity together with the increase in the proportion of PMN suggests that both the number and activity of PMN could influence the susce ptibility of the mammary gland to pathogens. Indeed, the logistic model ado pted suggests that impairment of milk immune factors could be concurrent wi th the development of an infection.