Somatic Cells in Milk-Physiological Aspects and Relationship to Amount and Composition of Milk

Somatic cells in milk include epithelial cells from the gland and leukocytes from the blood. Epithelial cells are elevated in very early and late lactation. Leukocytes increase during mastitis infection or injury. They have phagocytic properties and combat invading organisms. Mean somatic cell counts of each milking over a 1-month period for cows with no udder infection, non-pathogens, or pathogens, were 169,500, 225,800, and 997,800 cells per ml, with coeffecients of variation of 94, 66, and 82%. Advanced age, late lactation, and a previous history of mastitis are related to elevated cells. Milk loss in subclinical mastitis is related to somatic cell counts. On a quarter basis, loss started at 500,000 cells per ml, progressed to 7.5% at 1 million, and 30% at 5 million. In cell counting programs associated with monthly testing of individual cows, those cows with two cell counts over 1 million cells per ml produced over 1,000 pounds of milk per lactation less than other cows in the same lactation whose cell count never exceeded 500,000 per ml. Use of cell counting on an individual cow basis improves its usefulness as a management tool for the dairyman compared to bulk tank counts. Literature data suggest the following changes in the milk composition from quarters definitely positive to mastitis screening tests based on somatic cell counts compared to normal quarters (values represent percent of normal): total solids (92), lactose (85), fat (88), total protein (100), caseins (82), whey protein (162), chloride (161), sodium (136), potassium (91), pH (105), lipase activity (116), and acid degree value (183).