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

Abstract

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 coeffedents of variation of94, 66, and 82% Advanced age, late lactation, and a previous historv of mastitis are related to elevated cells. Milk loss in subclinical mastiti~ 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 useful-ness as a management tool for the dairyman compared to bulk tank counts. Literature data suggest the following changes in the milk com-position from quarters definitely positive to mastitis screening tests based on somatic cell counts compared to normal quarters (values re-present 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). Regular monitoring of the number of somatic cells in bulk milk has become a rather universal practice in the United States and western Europe. Improved methodology now permits good accuracy in this determination. This development has permitted adoption of regulatory programs and has given general information on the mastitis status of herds. However, it has been of limited value to most dairymen. In the small herd, one high cow can have a disproportionate effect on the bulk tank count. In large herds the milk from several bad cows can be diluted out to acceptable levels in the bulk tank. Problem cows are not identified. There seems to be a general lack of understanding of how to interpret somatic cell counts. Often they are confused with bacterial counts. But. somatic cell counts are here to stay and increasing use can be made of them. Use on individual cows in production testing programs is increasing rapidly and markedly improves their value as a management tool. About 400 herds are now on such programs in