Poor cow health and high somatic cell counts (SCCs) often go hand in hand, and the result is often mastitis. This is most likely related to stress on the immune system which impairs its functionality, says Patrick Gorden, DVM, Dipl. ABVP-Dairy, Iowa State University. Additionally, antibacterial agents alone cannot control an intramammary infection (IMM) infection without the assistance of a functional immune system. Depending on the stage of lactation and presence of IMM infections, the immune system has varying levels of functionality.
The cow’s first line of defense against the introduction of mastitis-causing pathogens is the skin and muscular teat sphincter. The streak canal is lined with keratin that traps bacteria and then is removed with milking. Keratin is a waxy material that is derived from stratified squamous epithelium. Keratin’s structure enables trapping of invading bacteria, thus hindering their migration into the gland cistern. Within the keratin lining are natural antimicrobial agents which are bacteriostatic. Teat injuries, teat surgery and infusions all remove the protective keratin and increase the risk of mastitis. “An additional physical barrier is provided by the process of milking, which serves a protective function in that it flushes the contents out of the mammary gland on a regular basis,” Gorden says.
The most important defense system, however, is the innate immune system. “The innate system serves a role to detect and kill an array of pathogens,” Gorden notes. “A decrease in the function of even one component of the innate immune system can have detrimental effects on the cow’s immune response.”
Gorden explains that polymorphonuclear cells (PMN) including neutrophils, basophils, and eosinophils, play a key role in the innate immune system by detecting and containing infectious agents. While basophils and eosinophils have important functions, the neutrophil becomes the most important PMN for the mammary gland in dealing with IMM bac-terial infections.
Derailing the immune system
The lactating cow’s immune system is already somewhat derailed due to normal immune suppression associated with calving and lactogenesis. “Calving and lactogenesis seem to have independent effects on immune suppression,” Gorden says. “Negative energy balance during the pre- and post-partum period have a suppressing effect but negative energy balance during mid-lactation has been shown to only have a minimal effect. This suggests that there are factors associated with calving and lactogenesis that are associated here also.” Gorden adds that the development of ketosis has been shown to cause severe mastitis infection as compared to negative energy balance alone.Increased severity of mastitis.
Deficiency:Decreased neutrophil killing capability. Increased susceptibility to bactericidal infection.
Function: Linked to proper immune function. Essential for integrity of skin, physiologic barriers.
Deficiency: Decreased leukocyte function. Increased susceptibility to bacterial infection. High calcium diets can exacerbate zinc deficiency problems.
Source: Lorraine Sordillo, PhD, The Pennsylvania State University, “Mastitis and Immunology,” Bovine Veterinarian, July–August 1998.