A comparison of indirect methods for diagnosis of subclinical intramammary infection in lactating dairy cows

Abstract

Eight parameters - somatic cell count, electrical conductivity, pH, N-acetyl-β-Dglucosaminidase (NAGase) activity, and the concentrations of sodium, potassium, lactose and al-antitrypsin - were measured in milk samples from individual quarters, in two experiments. Experiment 1 : The eight parameters were measured in strict foremilk, foremilk, midmilk and strippings from 20 individual quarters, nine of which were free of bacterial infection. The somatic cell count, pH, NAGase activity and sodium concentration increased during the milking process ; the opposite was true of the concentrations of lactose and potassium, for both infected and uninfected quarters. The differences in the concentrations of a given parameter between infected and uninfected udder quarters were generally greater in strippings than in foremilk fractions, although the interaction term was not significant for six of the parameters. Experiment 2 : The eight parameters were measured in quarter foremilk samples taken from 121 cows in three herds, at monthly intervals. Between 10% and 19% of quarters showed growth by a pathogen during the period. Bacterial infection of the udder caused statistically significant increases in the somatic cell count, NAGase activity, electrical conductivity and the concentrations of sodium and al-antitrypsin, and a statistically significant decrease in the concentration of lactose ; effects on the potassium concentration and on the pH of the milk were quantitatively smaller and differed between herds. The stage of lactation exerted significant effects on all of the parameters ; somatic cell count, sodium concentration, electrical conductivity, pH, NAGase activity and α1-antitrypsin concentration, increased during mid to late lactation, while the concentrations of potassium and lactose tended to decrease. Milk from older cows exhibited increased levels of NAGase activity, electrical conductivity and sodium ions, while the concentrations of lactose and of potassium were lower for older animals. The effects of the age of the cow on the somatic cell count and on the α1-antitrypsin concentration varied between herds.