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.
