Submitted to: International Dairy Federation Seminar
Publication Type: Proceedings
Publication Acceptance Date: 12/13/2001
Publication Date: N/A
Citation: Interpretive Summary: Lactating dairy cows are exposed to thousands of Gram-negative bacteria and harbor many potential pathogens on their skin and mucosal surfaces. Despite the high rate of exposure, intramammary infections by Gram- negative bacteria is low compared to infections by Gram-positive bacteria. This is due to the short duration of infection by Gram-negative bacteria, which are usually resolved within days or a few weeks. On average 1% of mammary quarters in the dairy cow population will have intramammary infections with Gram-negative bacteria compared to an infection rate of 35-50% for Gram-positive bacteria. It has been estimated that 2,000 to 4,000 milkings are required to observe one intramammary infection by a Gram-negative organism. The frequency for Gram-positive organisms is 1 in 600 milkings. However, infections by Gram-negative bacteria account for 30-50% of all clinical cases in a dairy herd. A great deal of research has sbeen directed toward unraveling and defining the natural defense mechanism of the mammary gland.
Technical Abstract: At calving, neutrophils are directed to the uterus and this phenomenon is accompanied by a decrease in the number of circulating neutrophils. Very immature neutrophils appear and many of their functions, such as, chemotaxis, diapedesis, phagocytosis and the production of free radicals, is altered. Also lymphocyte function has changed around parturition: blastogenesis is depressed, IgM production is impaired, and the T-cell helper function is lost. The higher incidence of mastitis at drying of could be attributed to many factors such as: the decreased ability of secretions to support phagocytosis, the inhibitory effect of secretions on phagocytosis, the diminished phagocyte function with advancing stage of the dry period and the flushing out effect of bacteria by the milking process, which is now absent. Here again, is not clear to what extent these phenomena are influenced by the animals mammary adaptations and systemic physiological adaptations (positive energy balance, lower cortisol, somatotrophin and prolactin, high steroid levels in the second halve of pregnancy,...).