Pathology of Staphylococcus aureus mastitis during lactogenesis: relationships with bovine mammary structure and function.

Pathological alterations of mammary parenchymal tissue from 5 dairy cows with Staphylococcus aureus mastitis were studied. Tissue from infected quarters exhibited less synthetic and secretory ability during lactogenesis, as indicated by lower percentages of luminal area, but higher percentages of stromal area compared with control tissue. Ultrastructural analysis of alveolar epithelium demonstrated decreased numbers of organelles associated with milk synthesis and secretion. Mammary secretion from 12 additional cows confirmed the loss of secretory potential in infected quarters as concentrations of alpha-lactalbumin were lower during the first 14 d of lactation compared with concentrations from controls. Higher concentrations of serum alpha-lactalbumin from cows with Staphylococcus aureus mastitis suggest leakage of mammary secretion through gaps left by damaged or sloughed alveolar epithelium. Macrophages and lymphocytes were observed frequently in large gaps between basal plasma membrane of secretory cells and the basal lamina which may contribute to epithelial damage. Ability of neutrophils and macrophages to phagocytize staphylococci may have been compromised in the prelactating gland due to the indiscriminate ingestion of accumulated fat and casein.