Bayesian Estimation of qPCR and Bacterial Culture Accuracy for Detection of Bovine Coagulase-Negative Staphylococci from Milk and Teat Apex at Different Test Cut-off Points

Aim
primarily to estimate the sensitivity (Se) and specificity (Sp) of the commercially available Mastit4 qPCR assay and bacterial culture (BC) for diagnosis of intramammary infections (IMI) and teat apex colonization (TAC) with coagulase‐negative staphylococci (CNS) at different cut‐offs for qPCR cycle threshold (Ct) values using Bayesian latent class analysis (LCA). A secondary objective was to evaluate two cut‐offs of BC for diagnosis of IMI and TAC with CNS.

Methods and Results
we randomly selected 13 to 20 cows with subclinical mastitis from 8 dairy herds. Teat skin samples and aseptically collected foremilk samples were collected from the right hindquarters (n = 149) for BC and qPCR analysis. The Se of qPCR was always higher than BCSe in diagnosis of IMI, however; the Sp of BC was higher than qPCRSp. BCSe and BCSp showed no substantial difference between the tested BC cut‐offs. In contrast to IMI, estimates of BC and qPCR in diagnosing TAC were different. BCSe was higher than qPCRSe at all tested cut‐offs, however; qPCRSp was higher than BCSp.

Conclusion
the overall performance of qPCR is higher than BC in the diagnosis of IMI however; the performance of BC is better than qPCR in diagnosis of TAC. The qPCR and BC are valid diagnostics for bovine IMI with CNS. Whereas for TAC, both techniques require further investigation to reduce the uncertainty of the true status of the quarter and teat skin.

Significance and Impact of the Study
we reported, for the first time, the diagnostic performance of new mastitis technology (Mastit4 PCR) and culture for detection of CNS in milk and non‐milk samples in dairy herds with automatic milking systems. Our findings will improve the interpretation of the test results of culture and qPCR assay and subsequently, will strengthen the control of IMI with CNS in dairy cows.