AbstractsBiology & Animal Science

Summary Chapter 1- IntroductionIn the introductory chapter of this thesis, firstly, we present aspects of the distribution, immunology and pathogenesis of Mycobacterium avium subsp. paratuberculosis (MAP) infection of ruminants. The reader should realize that the prolonged latent stage and incubation period hamper the early serological diagnosis of MAP infection. Then, some of the strengths and limitations of validation of tests with non gold standard methodology and Receiver Operating Characteristic curve analysis with Bayesian Mixture Models are presented. The reader should realize that the methodology is more powerful and well suited for evaluation of the serological diagnosis of MAP infection in dairy sheep and goats. Chapter 2- Bayesian validation of a serum and milk ELISA for antibodies against Mycobacterium avium subspecies paratuberculosis in Greek dairy goats across lactationThe aim of the research presented in this chapter was to validate a commercial (IDEXX Pourquier, Montpellier, France) serum and milk indirect ELISA that detects antibodies against the MAP across lactation, in Greek dairy goats. Each lactating goat was sampled at four consecutive times starting from kidding and covering the early, mid and late lactation stage. A total of 1268 paired milk/colostrum and blood samples were collected during the seven-month-long lactation period. Bayesian mixture models, which allow for the continuous interpretation of test results, were used to derive the distribution of the serum- and milk-ELISA response for the healthy and the MAP-infected individuals at each lactation stage. Both serum- and milk-ELISA, in all lactation stages, were of average and similar overall discriminatory ability as measured by the area under the curve. For each test, the lowest overlap between the distribution of the healthy and the MAP infected does was at late lactation. At this stage the area under the curve (AUC) was 0.89, 95% credible interval (0.70; 0.98) and 0.92 (0.74; 0.99) for the milk- and the serum-ELISA, respectively. Both tests had comparable sensitivities and specificities at the recommended cut-offs, across lactation. Lowering the cutoffs led to an increase in the sensitivities without serious loss in the specificities. In conclusion, the milk-ELISA can be as accurate as the serum-ELISA especially at the late lactation stage. Thus, it could serve as the diagnostic tool of choice, especially during the implementation of MAP control programs that require frequent testing, because milk sampling is a non-invasive, rapid and easy process. Finally, there is no need for lactation-stage specific selection to detect the disease as the prevalence is constant.Chapter 3- Bayesian validation of a commercial milk and serum ELISA across lactation in dairy sheepThe aim of the research presented in this chapter was to evaluate a commercially available ELISA in sera and milk of a Greek dairy sheep flock. A total of 854 paired milk and blood samples were collected from ewes of a Greek flock and used to validate a commercial…