S Paul 1 , N Toft 1 , JF Agger 1 , Ø Angen 2 , AB Christoffersen 2 , JS Agerholm 1

1. Milk PCR Milk ELISA Blood ELISA Sensitivity and specificity of ELISA and PCR tests for identifying Coxiella burnetii seropositive and shedder cows using Bayesian analysis S Paul1, N Toft1, JF Agger1, Ø Angen2, AB Christoffersen2, JS Agerholm1 1Dept. of Large Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark; 2National Veterinary Institute, Technical University of Denmark BACKGROUND • Coxiella burnetii infection is prevalent worldwide including Denmark • High plevence was observed in Danish dairy cattle in recent years • Diagnosis is mainly based on the dectection of bacteria by PCR or dectection of anitibodies by immunological tests • However, very little is know about the characteristics of these commonly used tests OBJECTIVE To estimate sensitivity (Se) and specificity (Sp) of milk and blood antibody ELISA and milk PCR tests to determine Coxiella burnetii status in dairy cows MATERIALS AND METHODS 454 lactating cows from the 12 herds were included in the study 454 lactating cows • Se and Sp of the tests were estimated using latent class modeling in Bayesian analysis • Conditional dependency (COC) between milk and blood ELISA tests was assumed and estimated • Bulk tank milk (BTM) samples of 12 herds were tested for the level of C. burnetii antibodies by ELISA • BTM antibody level 64 was used as cut-off to stratify herds into high and low antibody level groups 230 lactating cows 224 lactating cows Milk PCR Milk ELISA Blood ELISA RESULTS Fig.1: Comparison between the sensitivite and 1- specificity plots of milk PCR, milk ELISA and blood ELISA at recommended vs best fit cut-offs Fig.2: A plot of the posterior mean prevalance estimates for two populations at different PCR cut-off values and at a cut-off of 30 for ELISAs • The best set of results was revealed at cut offs at 33 quantification cycles for PCR and at antibody level 30 for the ELISA tests (Fig.1) • Se and Sp of these tests at this cut-off combination: • PCR Se and Sp were significantly higher than those of both ELISAs • Se of milk ELISA was significantly higher than Se of blood ELISA • COC estimates confirmed the conditionally dependency of ELISAs • Prevalence increased with increasing quantification cycles for PCR (Fig.2) indicating an increased number of cows with low concentration of bacteria in milk CONCLUSIONS • PCR has higher Se and Sp than both ELISAs and therefore, is considered a better test for identifying shedder cows • However, high testing costs and risk of misclassifying intermittently shedding animals may be an argument for recommending PCR as screening test in a control program