Control of bovine virus diarrhoea-mucosal disease in cattle: examples of the combined use of serological screening, viral antigen detection and vaccination.

As part of the preparatory phase of a disease control programme in three herds infected with bovine virus diarrhoea-mucosal disease (BVD-MD) virus (demonstrated by virus isolation), initial serological screening was performed on all livestock older than six months (351 animals) by blocking enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody recognising a common epitope to the different strains of BVD-MD virus. The presence of immunotolerant, persistently-infected animals was strongly suspected, as a high percentage (334 = 95.2%) of cattle showed positive serological reactions, while the other members of the herd (17 = 4.8%) continued to give negative results, even after vaccination with a live vaccine. Whole blood samples from all cattle were then tested individually for viral antigen by an ELISA technique which had previously been tested successfully. As a result, a total of nine viraemic animals were identified in the three herds. A confirmatory test was performed by the reference amplification method on cell culture with virus identification using specific fluorescein isothiocyanate-labelled monoclonal antibodies. The identification and elimination of the persistently-infected animals led to the recovery of a negative serological status for the herds. It was therefore recommended that protective measures should be taken to avoid the reappearance of viraemic animals, involving vaccination and systematic viral testing before introducing any new animal into the herd. It was advisable that these measures should be maintained until all the potential reservoirs and vectors of BVD-MD virus are better known.