title: Sensitivity and specificity of PCR analysis and bacteriological culture of milk samples for identification of intramammary infections in dairy cows using latent class analysis.
abstract: Real-time PCR analysis of milk samples is a fast method to identify intramammary infections (IMI) in dairy cows, and has the potential to be used for routine analysis of test milking composite milk samples. However, the results of the PCR analysis can be difficult to interpret. The objective of this study was to compare the sensitivity (Se) and specificity (Sp) of PCR analysis of composite milk samples, and conventional bacteriological culturing (BC) of quarter milk samples, when used to identify cows with IMI. The comparisons were performed for IMI with four common udder pathogens; Staphylococcus aureus (S aureus), Streptococcus dysgalactiae (Str dysgalactiae), Str uberis and coagulase negative staphylococci (CoNS). The Se and Sp of real-time PCR (SePCR; SpPCR) and BC (SeBC;SpBC) was simultaneously estimated using latent class analysis (LCA), studying one pathogen at the time. Milk samples from 970 dairy cows from 25 herds were included. Aseptically collected quarter milk samples taken at the day before test milking (TM), at the day of TM, and at the day after TM, were analyzed using BC. Non-aseptically collected composite milk samples taken at the day of TM were analyzed using PCR. Moreover, the composite milk somatic cell count (SCC) was recorded and summarized by diagnostic test and bacterial finding. LCA was first performed using only test results from samples taken at the day of TM, but in a second analysis BC results from the three consecutive samplings, interpreted in parallel, were included. The SePCRwas significantly higher than the SeBCfor S aureus, Str dysgalactiae and CoNS in the first analysis, but only for CoNS in the second analysis. The SpPCRwas significantly lower than the SpBCfor Str dysgalactiae and CoNS. In conclusion, using PCR analysis of composite milk samples, as a diagnostic tool for identifying cows with IMI increased the Se for all the pathogens investigated (although not always significantly), while Sp in general remained on a similar level, compared to BC of quarter milk samples. The use of repeated quarter milk sampling improved the SeBC, making the results of PCR analysis and BC more similar. However, the SCC of cows with IMI according to BC was higher than for cows with IMI according to PCR, suggesting that some of the cows with IMI according to PCR did not have an active inflammation. Hence, extra caution is needed when decisions about treatment of IMI are based on PCR.
authors.affiliation: Department of Animal Health and Antimicrobial Strategies, National Veterinary Institute, 751 89, Uppsala, Sweden. Electronic address: ann.nyman@sva.se.
authors.forename: A-K
authors.initials: AK
authors.lastname: Nyman
authors.affiliation: Department of Animal Health and Antimicrobial Strategies, National Veterinary Institute, 751 89, Uppsala, Sweden; Department of Clinical Sciences, Swedish University of Agricultural Sciences, Box 7054, 750 07, Uppsala, Sweden.
authors.forename: K
authors.initials: K
authors.lastname: Persson Waller
authors.affiliation: Department of Clinical Sciences, Swedish University of Agricultural Sciences, Box 7054, 750 07, Uppsala, Sweden.
authors.forename: U
authors.initials: U
authors.lastname: Emanuelson
authors.affiliation: Department of Disease Control and Epidemiology, National Veterinary Institute, 751 89, Uppsala, Sweden; Department of Animal Environment and Health, Swedish University of Agricultural Sciences, Box 234, 532 23, Skara, Sweden.
authors.forename: J
authors.initials: J
authors.lastname: Frössling
mesh_terms: D000818:Animals; D001499:Bayes Theorem; D002417:Cattle; D005260:Female; D008414:Mastitis, Bovine / Q000175:diagnosis / Q000453:epidemiology* / Q000382:microbiology; D008892:Milk / Q000382:microbiology*; D060888:Real-Time Polymerase Chain Reaction / Q000662:veterinary*; D012680:Sensitivity and Specificity; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000453:epidemiology / Q000382:microbiology / Q000662:veterinary*; D013210:Staphylococcus / Q000145:classification / Q000235:genetics / Q000302:isolation & purification; D013290:Streptococcal Infections / Q000175:diagnosis / Q000453:epidemiology / Q000382:microbiology / Q000662:veterinary*; D013291:Streptococcus / Q000145:classification / Q000235:genetics / Q000302:isolation & purification; D013548:Sweden / Q000453:epidemiology
keywords: Bacteriological culturing; Bayesian latent class analysis; Dairy cow; Diagnostic test; Mastitis; PCR analysis; Quarter milk samples; Sensitivity; Specificity; Test milking (DHI) samples