Test selection for antibody detection according to the seroprevalence level of Schmallenberg virus in sheep

Schmallenberg virus (SBV), initially identified in Germany in 2011, spread rapidly throughout Europe causing significant economic losses in ruminant livestock. The ability to correctly detect emerging and re-emerging diseases such as SBV with reliable tests is of high importance. Firstly, we tested diagnostic performance, specificity, and sensitivity of three different assays used in SBV antibody detection using control sheep samples of determined status. After obtaining the results from the control samples, we assessed the potential of the assays to detect previously infected animals in field situations. The samples were investigated using IDEXX Schmallenberg virus Antibody Test Kit, ID Screen Schmallenberg virus Competition Multi-species ELISA and Serum Neutralisation Test (SNT). Analysis of control samples revealed that SNT was the most suitable test, which was therefore used to calculate concordance and test performance for the two other ELISA tests. To evaluate whether different assay performances had an impact under field conditions, sheep samples from two different contexts were tested: the emergence of SBV in Ireland and the re-emergence of SBV in Belgium. Comparing the results obtained from different assays to the non-reference standard assay SNT, we showed considerable differences in estimates of their sensitivity to detect SBV antibodies and to measure seroprevalence of the sheep flocks. Finally, a calculation of the number of randomly selected animals that needs to be screened from a finite flock, showed that SNT and ID.Vet are the most suitable to detect an introduction of the disease in low seroprevalence situations. The IDEXX ELISA test was only able to detect SBV antibodies in a higher seroprevalence context, which is not optimal for monitoring freedom of disease and surveillance studies.