In financial terms, mastitis is the most costly disease in the dairy industry, totaling approximately $1.8 billion dollars lost annually. Cost of a single case of mastitis ranges from $104 (Hoblet et al. 1991) to $200 (NMC, 1996). These losses are a result of reduced milk production, discarded milk, replacement costs, extra labor, treatment and veterinary service costs. Increased prevalence of mastitis also results in greater risk of antibiotic residues in human food as well as milk quality issues.
Mastitis is an inflammation of the mammary gland. The term, inflammation describes the response of a tissue or organ to injury. The purpose of inflammation is to destroy or neutralize infectious agents and associated toxins, thus allowing the gland to return to normal function. Bacterial invasion of the mammary gland occurs by bacteria entering the teat sphincter and moving into the teat cistern and beyond. Bacterial presence within the udder results in the movement of white blood cells into the gland to help fight the disease. An uninfected mammary gland will maintain a low total cell count (< 25,000 to 200,000 cells/ml), with most cells being macrophages. Macrophages can be viewed as special surveillance cells, constantly monitoring for the presence of foreign particles or microorganisms. Once gland tissue becomes infected, numerous neutrophils will be drawn to the mammary gland, resulting in increased somatic cell counts.
Outcome from bacterial invasion of the udder depends upon pathogenicity of the bacterial species involved and competency of the cow’s immune system. In the best case scenario, the bacteria are cleared without subsequent colonization of mammary tissues. Successful bacterial colonization of mammary tissue can result in a wide spectrum of disease outcomes, ranging from subclinical (e.g., no obvious change to udder or milk) to peracute clinical (e.g., severe systemic disease symptoms with dramatic changes to udder and milk secretion) mastitis (NMC, 1996).
Contagious mastitis pathogens such as Streptococcus agalactia, Mycoplasma bovis and Staphylococcus aureus are most often associated with subclinical mastitis infections, recognized as elevated somatic cell counts (>200,000 cells/ml). Coliform bacteria such as E. coli and Klebsiella sp. are most often associated with acute clinical mastitis cases. The interaction between bacterial pathogenicity and immune response dictates the ultimate disease severity and duration. For example, differing strains of Staphylococcus aureus, based on virulence factors associated with capsular polysaccharides, can result in subclinical as well as acute clinical mastitis and everything in between.
Nutrition, Immunity and Mastitis