Pestiviruses are a group of small enveloped, positive-strand RNA viruses well known to veterinary virologists. Based on their morphology and the polarity of their genome, they originally held generic status in the family Togaviridae. Pestiviruses have never been model viruses like other togoviruses, because they are difficult to work with. In cell cultures virus yield is usually low and virus purification is difficult if not impossible, because the virions are fragile and they seem to be intimately associated with the host cell membranes. However, the availability of modern virological techniques revived interest in pestiviruses in the early to mid 1980s and today there is a much broader understanding of this group of viruses. Although many unsolved questions remain important, details concerning the pathogenesis, genomic organization, and replication of pestiviruses have been established. However, notably among ruminant pestiviruses, cytopathogenic (cp) variants can arise during persistent infections with ncp viruses and they may play an important role in causing disease in their host animals. The name of the group is derived from the infectious agent of pestissuum (hog cholera virus, HoCVl). All pestiviruses are antigenically closely related, but no significant relationships with other known viruses have been detected, either at the antigenic or at the molecular level. Their spread is worldwide and, apart from the somewhat indirect evidence of pestivirus infections in humans, they are restricted to pigs and a variety of ruminant species, both domestic and wild living. Of the economic importance regarding domestic animals are: HoCV, bovine viral diarrhea virus (BVDV), and border disease virus (BDV) of sheep. These viruses are under extensive investigation and are the focus of this chapter. © 1992 Academic Press Inc.
