Large‐scale reproductive loss in sheep due to Border disease virus infection, New South Wales, Australia

Border disease viruses (BDV) and bovine viral diarrhoea viruses (BVDV) are members of the Pestivirus genus in the family Flaviviridae. While BVDV is one of the most significant endemic viral infections of cattle in Australia, BDV infection is generally considered to be uncommon in Australian sheep. This study describes the widespread detection of BDV on two properties in southern New South Wales following an investigation into poor pregnancy rates, resorbing foetuses and stillborn lambs. Extensive cross-sectional serological studies identified a high seroprevalence in some groups of sheep and low prevalence in others, demonstrating both the extent of infection and the number of susceptible breeding sheep remaining at risk. BDV-specific qRT-PCR confirmed BDV infection of stillborn lambs, and a large number of 'hairy' lambs were confirmed as BDV infected by use of a pestivirus antigen ELISA at marking. In a group of BDV persistently infected lambs that were monitored for 12 months, postweaning survival was low, with 21 of 120 still alive at 5 months of age and 14 still alive at 12 months of age. This study highlights the potential impact BDV can have on production and how management strategies, including breeding young primiparous ewes under intensive management conditions can result in large-scale virus transmission and disease.