Evaluation of an indirect ELISA for detection of antibodies in bulk milk against bluetongue virus infections in the Netherlands.

After the introduction of bluetongue virus serotype 8 (BTV-8) in western Europe in 2006, an indirect ELISA for detection of serogroup-specific antibodies against BTV in serum samples was validated for individual milk samples by the Central Veterinary Institute and the Animal Health Service in the Netherlands (Kramps et al., 2008). In order to develop a cost-effective monitoring tool, we now have evaluated this ELISA also for use in bulk milk. Therefore, bulk milk samples and individual milk samples were collected from 92 herds in the affected southern region in the Netherlands in 2007, before the start of the vaccination campaign. In addition, bulk milk samples collected from 88 herds before the bluetongue introduction in 2006 ("historically negative" samples) have been tested. With these results ROC analyses were performed and herd specificity and herd sensitivity of the bulk milk ELISA were estimated. All "historically negative" bulk milk samples were negative in the ELISA, with a mean S/P ratio of 10 ± 0.8%. The herd sensitivity and herd specificity of the ELISA in bulk milk samples depend on the cut-off that is chosen. In order to detect a within-herd-prevalence of 1%, the optimal cut-off S/P ratio 13% was found. A few herds with one or two milk-positive animals would then be missed. The specificity will be 100%. A within-herd-prevalence of 10% can be detected with 100% sensitivity at a cut-off S/P ratio of 96%. In conclusion, the indirect ELISA in bulk milk samples is a very specific and sensitive test which can be implemented in monitoring and surveillance systems in unvaccinated populations.