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A comparison of indirect methods for diagnosis of subclinical intramammary infection in lactating dairy cows. Part 2: the discriminative ability of eight parameters in foremilk from individual quarters and cows

Authors: R.J. Holdaway, C.W. Holmes and I.J. Steffert

Abstract:

The ability of eight parameters to discriminate between infected and uninfected udder quarters, or cows, was assessed, using samples taken at monthly intervals from 121 cows within three herds. The somatic cell count correctly classified 80% of quarters and 75% of cows, while over 70% of quarters, and 67% of cows, were correctly classified by the sodium concentration and by the N-acetyl-β-D-glucosaminidase (NAGase) activity of the milk The remaining parameters each classified correctly fewer than 70% of quarters, and 60% of cows, as being infected or uninfected. The critical threshold value for each parameter varied between herds, with the greatest between-herd variation shown by somatic cell count, ranging from 85,000 to 375,000 cells/mL for quarters and between 73,000 and 344,000 for cows. Critical threshold values increased during lactation for somatic cell count and sodium concentration, but tended to decrease for NAGase activity. For identification of cows, the specificity of the somatic cell count was slightly lower than that of the NAGase activity or the sodium concentration, but the sensitivity of the somatic cell count was markedly higher than that of any other parameter. The discriminative ability of the somatic cell count showed little variation between herds or during lactation provided that the relevant critical threshold figure was used.

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