The distribution of Staphylococcus aureus within herds seems to be related
to interactions among the shedding characteristics of the bacteria, their pathogenicity
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 quarter milk samples were
assessed using bacteriological and cytological tests. We evaluated differential cell
count, lysozyme concentration, N-acetyl-β-glucosaminidase (NAGase) activity, cell
viability and respiratory burst activity in randomly chosen quarter milk samples
from each cow. Staph. aureus intramammary infection elicited different responses in
the mammary gland immune defences investigated. Polymorphonuclear leucocytes
(PMN) as a proportion of total somatic cells in milk, 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. Mean values
differed among herds, but the differences were not significant. These changes were
associated with Staph. aureus infection. The reduced respiratory burst activity
together with the increase in the proportion of PMN suggests that both the number
and activity of PMN could influence the susceptibility of the mammary gland to
pathogens. Indeed, the logistic model adopted suggests that impairment of milk
immune factors could be concurrent with the development of an infection.