Impact of intramammary infections on mammary gland development in pregnant dairy heifers during late gestation

Intramammary infections are common in nonlactating dairy cattle and have been shown to disrupt mammary tissue architecture in nonpregnant heifers. However, their effect on mammary development during pregnancy remains unclear. This study assessed the effects of IMI on mammary gland development in pregnant dairy heifers during late gestation. The study used 21 pregnant Holstein heifers, divided across 3 gestational stages (∼5.75, 6.75, and 7.75 mo of gestation; corresponding to 180 ± 2, 208 ± 2, and 238 ± 2 d pregnant, respectively). Using a contralateral quarter-pair design, a single culture-negative quarter of each heifer was infused with saline (SAL), and the contralateral quarter was challenged with 5,000 cfu of Staphylococcus aureus (CHALL). Mammary secretion samples were collected at various time points until tissue harvest at 21 d postchallenge, when animals were 6.5, 7.5, and 8.5 mo pregnant. Mammary tissue samples from the center and edge parenchymal regions were collected and evaluated for immune cell infiltration and tissue morphometry. Secretions from CHALL quarters had greater SCC and a greater proportion of neutrophils compared with SAL quarters. Mammary tissues from CHALL quarters exhibited increased immune cell infiltration in both the luminal and intralobular stromal regions and lower secretion score compared with SAL, regardless of gestational stage. Additionally, tissues from animals at later gestational stages showed reduced adipose tissue area and larger lobular areas, regardless of quarter treatment. At 8.5 mo of pregnancy, luminal areas in the edge regions of CHALL quarters were nearly 50% smaller than in SAL quarters, suggesting an increased risk to restricting milk accumulation and secretion capacity in the mammary gland. Additionally, in 7.5-mo pregnant heifers, CHALL quarters showed decreased epithelial areas and increased intralobular stromal areas in the central region. Lobular, adipose, and extralobular stromal areas did not differ markedly between CHALL and SAL quarters. Overall, the results of this study indicate that IMI induces tissue damage in mammary glands of pregnant heifers, with a greater effect during late gestation, and that the IMI-induced changes in tissue architecture were not consistent across all tissue mammary gland regions or gestational ages.