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Somatic cell count thresholds in composite and quarter milk samples as indicator of bovine intramammary infection
Inge-Marié Petzer1, Joanne Karzis, Edward F Donkin
1Department of Production Animal Studies, University of Pretoria. melklab@up.ac.za.
Insights
Establishing an operational somatic cell count (SCC) threshold is crucial for predicting intramammary infection (IMI) in dairy herds. This study determined optimal SCC thresholds for composite and quarter milk samples, aiding udder health management.
Area of Science:
- Veterinary Medicine
- Dairy Science
- Animal Health
Background:
- Composite milk samples are increasingly preferred by South African dairy producers for herd udder health analysis due to herd size, convenience, and cost-effectiveness.
- Accurate prediction of intramammary infection (IMI) is vital for maintaining dairy herd health and productivity.
Purpose of the Study:
- To establish an operational somatic cell count (SCC) threshold for predicting IMI in composite milk samples.
- To compare the predictive value of SCC thresholds in composite milk samples with those in quarter milk samples.
Main Methods:
- A retrospective study analyzed 345,461 composite and 89,638 quarter milk samples from South African dairy herds.
- Variance estimates were adjusted for the non-independence of quarter samples within individual cows.
- Youden's index and receiver operating characteristic (ROC) curves were used to determine optimal SCC thresholds and assess test performance.
Main Results:
- The optimal SCC threshold for composite milk samples was 150,000 cells/mL, while for quarter milk samples it was 200,000 cells/mL.
- At 150,000 cells/mL, composite samples showed 65.3% sensitivity and 66.8% specificity for IMI.
- At 200,000 cells/mL, quarter samples demonstrated 70.8% sensitivity and 63.6% specificity for IMI.
- The area under the ROC curve was 0.7084 for composite and 0.7277 for quarter samples, indicating SCC is a good IMI indicator for both.
Conclusions:
- The study successfully established operational SCC thresholds for predicting IMI in both composite and quarter milk samples.
- The findings support the use of composite milk samples for herd udder health monitoring, with an optimal SCC threshold of 150,000 cells/mL.
- Somatic cell count testing is confirmed as a valuable tool for identifying intramammary infection in dairy herds, regardless of sample type.
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.
