title: Bayesian estimation of test characteristics of real-time PCR, bacteriological culture and California mastitis test for diagnosis of intramammary infections with Staphylococcus aureus in dairy cattle at routine milk recordings.
abstract: Danish farmers can order a real-time PCR mastitis diagnostic test on routinely taken cow-level samples from milk recordings. Validation of its performance in comparison to conventional mastitis diagnostics under field conditions is essential for efficient control of intramammary infections (IMI) with Staphylococcus aureus (S. aureus). Therefore, the objective of this study was to estimate the sensitivity (Se) and specificity (Sp) of real-time PCR, bacterial culture (BC) and California mastitis test (CMT) for the diagnosis of the naturally occurring IMI with S. aureus in routinely collected milk samples using latent class analysis (LCA) to avoid the assumption of a perfect reference test. Using systematic random sampling, a total of 609 lactating dairy cows were selected from 6 dairy herds with bulk tank milk PCR cycle threshold (Ct) value ≤39 for S. aureus. At routine milk recordings, automatically obtained cow-level (composite) milk samples were analyzed by PCR and at the same milking, 2436 quarter milk samples were collected aseptically for BC and CMT. Results showed that 140 cows (23%) were positive for S. aureus IMI by BC while 170 cows (28%) were positive by PCR. Estimates of Se and Sp for PCR were higher than test estimates of BC and CMT. SeCMT was higher than SeBC however, SpBC was higher than SpCMT. SePCR was 91%, while SeBC was 53%, and SeCMT was 61%. SpPCR was 99%, while SpBC was 89%, and SpCMT was 65%. In conclusion, PCR has a higher performance than the conventional diagnostic tests (BC and CMT) suggesting its usefulness as a routine test for accurate diagnosis of S. aureus IMI from dairy cows at routine milk recordings. The use of LCA provided estimates of the test characteristics for two currently diagnostic tests (BC, CMT) and a novel technique (real-time PCR) for diagnosing S. aureus IMI under field conditions at routine milk recordings in Denmark.
authors.affiliation: HERD: Centre for Herd-oriented Education, Research and Development, Department of Large Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, DK-1870 Frederiksberg C, Copenhagen, Denmark; Infectious Diseases, Department of Animal Medicine, Faculty of Veterinary Medicine, Zagazig University, Zagazig City 44511, Sharkia Province, Egypt. Electronic address: yasser@sund.ku.dk.
authors.forename: Yasser S
authors.initials: YS
authors.lastname: Mahmmod
authors.forename: Nils
authors.initials: N
authors.lastname: Toft
authors.forename: Jørgen
authors.initials: J
authors.lastname: Katholm
authors.forename: Carsten
authors.initials: C
authors.lastname: Grønbæk
authors.forename: Ilka C
authors.initials: IC
authors.lastname: Klaas
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D002417:Cattle; D002452:Cell Count / Q000379:methods* / Q000662:veterinary; D015169:Colony Count, Microbial / Q000379:methods* / Q000662:veterinary; D003612:Dairying; D003718:Denmark / Q000453:epidemiology; D005260:Female; D007774:Lactation; D008414:Mastitis, Bovine / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology; D008892:Milk / Q000382:microbiology; D015995:Prevalence; D060888:Real-Time Polymerase Chain Reaction / Q000379:methods* / Q000662:veterinary; D012680:Sensitivity and Specificity; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000453:epidemiology / Q000382:microbiology / Q000662:veterinary*; D013211:Staphylococcus aureus / Q000302:isolation & purification*
keywords: Conventional diagnostics; Dairy cows; PathoProof™ Mastitis PCR assay; Sensitivity and Specificity; Staphylococcus aureus; Test misclassification