RUMINANT PESTIVIRUSES

The ruminant pestiviruses, bovine virus diarrhoea virus (BVDV) and bor der disease virus (BDV) are highly successful and important pathogens which infect ruminant species worldwide. Although the serological rela tionships among ruminant pestiviruses require further clarification, t here is growing evidence for two antigenic groups, one of which predom inates in cattle and one in sheep. The success of pestiviruses stems f rom the ability of the non-cytopathic (NCP) biotype of the virus to cr oss the placenta and establish a persistent infection (PI) in the deve loping foetus. This biotype should be regarded as the 'normal' biotype with the cytopathic (CP) biotype being an abnormal virus that is usua lly isolated only from PI animals dying from mucosal disease. Recent m olecular evidence points to CP viruses arising from their NCP counterp arts by recombination events that include the insertion of host RNA an d/or the duplication of viral RNA sequences. However, the biological m echanism through which CP viruses kill cells remains unknown. Virtuall y all CP and NCP viruses cause only mild, transient clinical symptoms in healthy adult animals and stimulate a protective immune response. D espite the urgent requirement for a safe, effective vaccine, there is still no commercial vaccine that has been shown to immunize darns so t hat foetal infection is prevented. In the absence of an effective vacc ine, reliable diagnostic techniques are essential to implement effecti ve 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 These tests are specific, rapid, sensitive and reliable but may themselves become redundant as they are superceded by ever-increas ing molecular biology-based techniques.