Original Research

Somatic cell count thresholds in composite and quarter milk samples as indicator of bovine intramammary infection status

Inge-Marié Petzer, Joanne Karzis, Edward F. Donkin, Edward C. Webb, Eric M.C. Etter
Onderstepoort Journal of Veterinary Research | Vol 84, No 1 | a1269 | DOI: https://doi.org/10.4102/ojvr.v84i1.1269 | © 2017 Inge-Marié Petzer, Joanne Karzis, Edward F. Donkin, Edward C. Webb, Eric M.C. Etter | This work is licensed under CC Attribution-NoDerivatives 4.0
Submitted: 22 May 2016 | Published: 24 March 2017

About the author(s)

Inge-Marié Petzer, Department of Production Animal Studies, University of Pretoria, South Africa
Joanne Karzis, Department of Production Animal Studies, University of Pretoria, South Africa
Edward F. Donkin, Department of Animal and Wildlife Sciences, University of Pretoria, South Africa
Edward C. Webb, Department of Animal and Wildlife Sciences, University of Pretoria, South Africa
Eric M.C. Etter, Department of Production Animal Studies, University of Pretoria , South Africa; Department Environment and Societies, French Agricultural Research Centre for International Development (CIRAD), France


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Abstract

The objective of the study was to establish an operational somatic cell count (SCC) threshold to predict the presence of intramammary infection (IMI) in composite milk samples and compare findings with those in quarter milk samples. South African dairy producers now preferred composite milk samples for herd udder health analysis because of increasing cow numbers, convenience of sampling and lower cost. A retrospective study was conducted on 345 461 composite and 89 638 quarter milk samples from South African herds. Variance estimates for the proportion of quarter samples testing positive were adjusted to account for the lack of their independence within individual cows. The IMI at SCC thresholds of 150 000 cells/mL and 200 000 cells/mL differed only by 3.26% in composite milk samples. Youden’s index indicated the optimum SCC thresholds for composite and quarter milk samples as 150 000 cells/mL and 200 000 cells/mL, respectively. At 150 000 cells/mL, sensitivity (95% confidence intervals [CI]) in composite milk samples was 65.3% (64.0%, 66.6%) and specificity was 66.8% (65.7%, 67.9%); and in quarter milk samples, sensitivity at 200 000 cells/ mL was 70.8% (69.5%, 72.0%) and specificity was 63.6% (62.4%, 64.8%). The likelihood of infection for udders and quarters, respectively, was 1.034 and 1.327 at an SCC threshold of 150 000 cells/mL and 0.864 cells/mL and 1.177 cells/mL at 200 000 cells/mL. The area under the curve of the receiver operating characteristics graph was 0.7084 and 0.7277 for composite and quarter samples, respectively, indicating that the SCC test could be considered as a good indicator of IMI in both sample types.

Keywords

Composite milk samples; somatic cell count threshold; intramammary infections; dairy cows

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