Mammary Tissue Damage during Bovine Mastitis: Causes and Control

¹ Department of Animal Science, McGill University, Ste. Anne de Bellevue, Québec, H9X 3V9, Canada
² Dairy and Swine Research and Development Centre, Agriculture and Agri-Food Canada, P.O. Box 90 STN Lennoxville, Sherbrooke, Quebec, J1M 1Z3, Canada

^* To whom correspondence should be addressed. E-mail: xin.zhao{at}mcgill.ca.

	Abstract

Mastitis is an inflammatory reaction of the mammary gland that is usually due to a microbial infection and is recognized as the most costly disease in dairy cattle. Decreased milk production accounts for approximately 70% of the total cost of mastitis. Mammary tissue damage reduces the number and activity of epithelial cells and consequently contributes to decreased milk production. The mammary tissue damage has been shown to be induced by either apoptosis or necrosis. These two distinct types of cell death can be distinguished by morphological, biochemical and molecular changes in dying cells. Both bacterial factors and host immune reactions contribute to epithelial tissue damage. During infection of mammary glands, the tissue damage can be initially caused by bacteria and their products. Certain bacteria produce toxins that destroy cell membranes and damage milk producing tissue, while other bacteria are able to invade and multiply within the bovine mammary epithelial cells before causing cell death. In addition, mastitis is characterized by an influx of somatic cells, primarily polymorphonuclear neutrophils, into the mammary gland. With more immune cells migrating into the mammary gland and breakdown of the blood-milk barrier, damage to the mammary epithelium worsens. It is well known that breakdown of the extracellular matrix can lead to death of epithelial cells. Meanwhile, polymorphonuclear neutrophils can harm the mammary tissue by releasing reactive oxygen intermediates and proteolytic enzymes. In vitro and in vivo studies suggest that use of antioxidants and other protective compounds in mastitis control programs is worth investigating because they may aid in alleviating damage to secretory cells and, thus, reduce subsequent milk loss.

Key Words: Bovine , mammary gland, mastitis, epithelial cells, apoptosis, tissue damage