Histopathologic response of the bovine mammary gland to experimentally induced Staphylococcus aureus infection

At 2 and 10 days after Staphylococcus aureus mastitis was induced in dairy cattle, mammary tissue from 2 zones in infected quarters was morphometrically compared with similar zones in contralateral control quarters. In comparison with control tissue, infected tissue exhibited more interalveolar stromal area, reduced alveolar luminal area, and more damaged and involuted alveolar epithelial area. These changes appeared more advanced at 10 days than at 2 days after infection was induced, indicating that by day 10, glandular parenchyma was losing secretory potential. Quarter zones differed greatly in the responses to infection. Parenchyma taken from a zone of an infected quarter close to the glandular part of the lactiferous sinus appeared to undergo involution more rapidly than a zone further from the sinus. Involuted parenchyma displayed reduced secretory activity, diminished luminal areas, and abundant connective tissue. A portion of nonsecretory epithelia taken from involuted areas of a zone close to the glandular part of the sinus stained for presence of immunoglobulin A.