Symposium review: The influences of heat stress on bovine mammary gland function.

Heat stress reduces cow milk yield and results in a significant economic loss for the dairy industry. During lactation, heat stress lowers milk production by 25 to 40% with half of the decrease in milk synthesis resulting from the reduced feed intake. In vitro studies indicate that primary bovine mammary epithelial cells display greater rates of programmed cell death when exposed to high ambient temperatures, which may lead to a decrease in the total number of mammary epithelial cells in the mammary gland, partially explaining the lower milk production of lactating cows under heat stress. The function of mammary cells is also altered by heat stress. In response to heat stress, mammary cells display higher gene expression of heat shock proteins, indicating a need for cytoprotection from protein aggregation and degradation. Further, heat stress results in increased gene expression without altering protein expression of mammary epithelial cell junction proteins, and does not substantially influence the integrity of mammary epithelium. These data suggest that the mammary gland strives to maintain cell-to-cell junction integrity by synthesizing more proteins to compensate for protein losses induced by heat stress. During the dry period, heat stress negatively affects mammary gland development by reducing mammary cell proliferation before parturition, resulting in a dramatic decrease in milk production in the subsequent lactation. In addition to mammary growth, the mammary gland of the heat-stressed dry cow has reduced protein expression of autophagic proteins in the early dry period, suggesting heat stress influences mammary involution. Emerging evidence also indicates that heifers born to cows that experience late-gestation heat stress have lower milk yield during their first lactation, implying that the maternal environment may alter mammary gland development of the offspring. It is not clear if this is due to a direct epigenetic modification of prenatal mammary gland development by maternal heat stress. More research is needed to elucidate the effect of heat stress on mammary gland development and function.

Effects of heat stress and dietary zinc source on performance and mammary epithelial integrity of lactating dairy cows.

Dietary Zn and heat stress alter gut integrity in monogastric animals. However, effects of Zn on mammary epithelial integrity in heat-stressed lactating dairy cows have not been studied. Multiparous lactating Holstein cows (n = 72) were randomly assigned to 1 of 4 treatments with a 2 × 2 factorial arrangement to study the effects of environment and Zn source on performance and mammary epithelial integrity. Treatments included 2 environments [cooled (CL) or not cooled (NC)] and 2 Zn sources [75 mg/kg of supplemental Zn as Zn hydroxychloride (IOZ) or 35 mg/kg of Zn hydroxychloride + 40 mg/kg of Zn-Met complex (ZMC)]. The experiment was divided into baseline and environmental challenge phases of 84 d each. All cows were cooled during the baseline phase (temperature-humidity index = 72.5), whereas NC cows were not cooled during environmental challenge (temperature-humidity index = 77.7). Mammary biopsies were collected on d 7 and 56 relative to the onset of environmental challenge to analyze gene expression of claudin 1, 4, and 8, zonula occludens 1, 2, and 3, occludin, and E-cadherin and protein expression of occludin and E-cadherin. Deprivation of cooling increased respiration rate (64.8 vs. 73.9 breaths/min) and vaginal temperature (39.03 vs. 39.94°C) and decreased dry matter intake (26.7 vs. 21.6 kg/d). Energy-corrected milk yield decreased for NC cows relative to CL cows (24.5 vs. 34.1 kg/d). An interaction between environment and Zn source occurred for milk fat content as CL cows fed ZMC had lower milk fat percentage than other groups. Relative to CL cows, NC cows had lower concentrations of lactose (4.69 vs. 4.56%) and solids-not-fat (8.46 vs. 8.32%) but a higher concentration of milk urea nitrogen (9.07 vs. 11.02 mg/mL). Compared with IOZ, cows fed ZMC had lower plasma lactose concentration during baseline and tended to have lower plasma lactose concentration during environmental challenge. Plasma lactose concentration tended to increase at 3, 5, and 41 d after the onset of environmental challenge in NC cows relative to CL cows. Treatment had no effect on milk BSA concentration. Cows fed ZMC tended to have higher gene expression of E-cadherin relative to IOZ. Compared with CL, NC cows had increased gene expression of occludin and E-cadherin and tended to have increased claudin 1 and zonula occludens 1 and 2 gene expression in the mammary gland. Protein expression of occludin and E-cadherin was unchanged. In conclusion, removing active cooling impairs lactation performance and affects gene expression of proteins involved in the mammary epithelial barrier, and feeding a portion of dietary zinc as ZMC improves the integrity of the mammary epithelium.

Nicaragua--dentistry against the odds.

In July 1988, Roxana Marisa Orellana, a young dentist, travelled from Nicaragua to study community dental health in the UK. Now, 16 months on, she writes about oral health and the dental service in Nicaragua, outlining some of the problems she and her fellow dentists encountered while at work in the strife-torn Central American country.