Somatic Cell Count as an Indicator of Udder Health Status under Modern Dairy Production: a Review

Mastitis, an important disease of dairy animals, influences the quality and quantity of milk. Penetration of pathogenic microorganisms in the teat canal irritates and invades the delicate mammary tissue, causing an inflammatory response and consequent changes occur in the milk. Degree of these changes depends on the infecting agent and the inflammatory response. Mastitis, the inflammation of the udder regardless of the cause, is the most costly disease of dairy cattle and results in severe economic losses from reduced milk production, treatment cost, increased labor, withheld milk following treatment and premature culling. Among infectious agents, bacterial pathogens are major threat to mammary gland. These microorganisms are often contagious, widely distributed in the environment of dairy animals and thus increase prevalence rate of intra-mammary infections (IMI). Field surveys of major livestock diseases in Pakistan have ranked mastitis as number one disease of dairy animals (Candy et al., 1983). The disease in Pakistan is mostly contagious in nature (Ali et al., 2008). It affects the milk quality and production of cow along with changes in the composition of milk, and the extent to which various compositional changes occur depends on the inflammatory response (Kitchen, 1981; Korhonen and Kaartinen, 1995). Main changes in the udder include; leakage of ions, proteins and enzymes from the blood into the milk due to an increased permeability, invasion of phagocytising cells into the milk compartment, and a decrease in the synthetic capacity of the gland, resulting in decreased concentration of certain milk constituents (Korhonen and Kaartinen, 1995). The affected quarter may also produce substances related to the inflammatory reaction such as acute phase proteins (Eckersall et al., 2001). Sub-clinical mastitis is important due to the fact that it is 15 to 40 times more prevalent than the clinical form, is of long duration, difficult to detect, adversely affects milk quality and production of dairy animals and constitutes a reservoir of microorganisms that can affect other animals within the herd due to its contagious nature (Schultz et al., 1978). Losses due to mastitis may even be higher in developing countries because standard mastitis control and prevention practices (e.g. pre and post milking antiseptic teat dipping and dry period antibiotic therapy) recommended by National Mastitis Council (NMC) of USA are not being carried out in these countries (Nickerson, 1994). Milk available to our human population is contaminated with pus due to high incidence of mastitis in our dairy animals (Allore, 1993; Khan and Muhammad, 2005), with high occurrence of pathogenic microbes in dairy buffaloes (Ahmad, 2001). The gold standard to measure inflammation is the cytological examination i.e. milk somatic cell count (SCC) and other methods are compared with SCC (Hamann, 2002). The diagnosis of mastitis according to the International Dairy Federation (IDF, 1971) recommendations is based on the SCC and microbiological status of the quarter. This paper reviews the importance of detection of sub-clinical phase of mastitis based on SCC and deleterious effect of high SCC on milk production and composition.