Ruminant pestiviruses.

The ruminant pestiviruses, bovine virus diarrhoea virus (BVDV) and border disease virus (BDV) are highly successful and important pathogens which infect ruminant species worldwide. Although the serological relationships among ruminant pestiviruses require further clarification, there is growing evidence for two antigenic groups, one of which predominates in cattle and one in sheep. The success of pestiviruses stems from the ability of the non-cytopathic (NCP) biotype of the virus to cross the placenta and establish a persistent infection (PI) in the developing foetus. This biotype should be regarded as the 'normal' biotype with the cytopathic (CP) biotype being an abnormal virus that is usually isolated only from PI animals dying from mucosal disease. Recent molecular evidence points to CP viruses arising from their NCP counterparts by recombination events that include the insertion of host RNA and/or the duplication of viral RNA sequences. However, the biological mechanism through which CP viruses kill cells remains unknown. Virtually all CP and NCP viruses cause only mild, transient clinical symptoms in healthy adult animals and stimulate a protective immune response. Despite the urgent requirement for a safe, effective vaccine, there is still no commercial vaccine that has been shown to immunize dams so that foetal infection is prevented. In the absence of an effective vaccine, reliable diagnostic techniques are essential to implement effective control measures. There is now a range of monoclonal antibody-based enzyme-linked immunosorbent assays for identifying PI or convalescent animals. These tests are specific, rapid, sensitive and reliable but may themselves become redundant as they are superceded by ever-increasing molecular biology-based techniques.