Second European Symposium on: BVDV Control Session 1: Virus properties and diagnostic assays relevant for control of BVDV Selection of diagnostic assays for BVDV control programmes

A mandatory objective of any control programme aiming at eradicating BVD from an infected bovine population is the removal of animals persistently infected (PI) with BVDV. In practice this will require a selection of reliable diagnostic assays. Approaches on how to identify PI animals can be indirect, by first identifying herds with a high probability of containing PI animals through diagnostic surveillance. Subsequently individual testing all animals in suspicious herds may identify PI animals. Several factors influence what diagnostic tests should be chosen for a given BVD control programme. Currently two species of bovine viral diarrhoea virus (BVDV) have been recognised, and also within each of these species a certain degree of viral diversity can be seen. To some extent viral diversity may affect the ability to detect BVDV, or indirectly to diagnose infection with them. Furthermore, the choice of diagnostic tests to use will depend on the epidemiological status of the population. In high cattle density areas BVDV spreads more easily than elsewhere, and the initial value of serological surveillance is less. Nevertheless, once a control programme has been running successfully for some time, monitoring the rate of reinfection may be very important in high cattle density areas. To some extent serological surveillance may be difficult for populations vaccinated against BVD, since distinguishing between immunity derived from vaccination and natural infection cannot easily be done. Dependent on the production system, the nature of sample material easily available may differ considerably. Bulk tank milk samples have proven to be a very cost-effective and reliable way to monitor the BVD prevalence in non-vaccinated dairy herds. Similar surveillance of beef cattle herds is much less convenient since it requires sampling of individual animals. Finally, cost, investigation time and ease of use are parameters that influence whether an otherwise technically reliable test is suitable for mass testing during control programmes. Historically, most laboratory investigations for bovine viral diarrhoea (BVD) diagnosis have been performed in virological laboratories equipped with cell culture facilities. During the last 15 years, the development of enzyme-linked immuno-sorbent assays (ELISA) has revolutionised the capacity of large-scale diagnostic investigations, allowing the bulk of laboratory investigations for BVD to be performed away from the scrutiny of pestivirologists. Similarly, during the last decade many assays based on in-vitro amplification of nucleic acids (RT-PCRs) have been refined for diagnostic applications. Thus, the laboratory investigative capacity may today easily outperform the ability to analyse and describe the dynamics of infection on the individual farm, or in the cattle population under investigation. Both for testing of individual animals and serological surveillance, antibody ELISAs are the principal choice of assay today. One shortcoming over the reference test for serology, the virus neutralisation test, is that quantification of the antibody level can only be approximate with ELISAs, especially for individual sera. Antigen ELISAs are similarly convenient to identify PI animals, although RT-PCRs may soon become realistic alternatives. A principal advantage over reference assays based on isolation of BVDV in cell cultures is a much shorter turnaround time. Most importantly, in well organised BVD control programmes the performance properties of all diagnostic assays should be known, and used to design a diagnostic network that is able to operate at diagnostic sensitivity and specificity levels as close to 100% as possible.