Available empirical data on the natural occurrence of ruminant pestiviruses has shown that in cattle, bovine viral diarrhoea virus (BVDV) is nearly exclusively found, whereas both border disease virus (BDV) and BVDV can be isolated from sheep. During routine genetic typing of pestivirus RNA from UK cattle diagnosed as BVDV positive between 2006 and 2008, five samples that were classified as BDV positive yielded positive virus isolates in cell cultures. The samples originated from animals that had shown signs typical for BVD. Phylogenetic analysis of the bovine BDVs showed that two belonged to the BDV-1a group and three to the BDV-1b group, thereby matching the genetic diversity seen for previously described UK ovine BDVs. Antigenic typing with a set of monoclonal antibodies (MABs) showed that all bovine BDVs lacked one or more epitopes conserved among ovine BDV-1 isolates, and that they had gained reactivity with at least one BVDV-1 specific MAB. Serial passaging of two of the virus isolates in ovine cell cultures did not change the epitope expression pattern. These findings suggest that the presumed natural resistance of cattle against infection with BDV no longer holds. A consequence of this is that BVD diagnostic assays should be checked for their ability to also detect BDV, and also highlights the need for monitoring of the BDV status in sheep that may be in contact with cattle in areas with organised BVD control programmes.
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