Pestiviruses: how to outmaneuver your hosts.

Heterogeneity is a hallmark that encompasses all aspects of pestiviruses, including the genetic properties, spectrum of hosts, disease signs and virulence. As an additional feature, pestiviruses are found world-wide, in both domesticated and wild animals, which indicates that these viruses are highly successful. Starting with the strategy by which pestiviruses persist in their host population, we discuss the role virulence and attenuation play in viral transmission and focus on the interaction of these viruses with their hosts' immune systems. The most unusual feature of pestiviruses is their ability to infect their hosts either transiently or persistently. Persistent infection, best known in cattle infected with bovine viral diarrhea virus, is characterized by immunotolerance to the infecting viral strain. This strategy of evading from the adaptive immune response is unique among persistent viral infections. It obviates antigenic variation, as used by lentiviruses, or hiding by latency, as seen in herpesviruses. In addition to establishing and maintaining immunotolerance, bovine viral diarrhea virus also evades innate immunity during persistent infection. Investigations in cultured cells showed that this virus fails to induce interferon type-I. Moreover, while being resistant to the action of interferon during persistent infection in its own host cells, bovine viral diarrhea virus does not interfere with interferon action against unrelated viruses infecting these cells. This novel finding of discrimination between "self" and "non-self" may contribute to the good health status seen in many persistently infected cattle. In evolutionary terms, not globally interfering with a crucial first line of antiviral defence of its persistently infected hosts may improve the chances for bovine viral diarrhea viruses to transmit to new hosts. Copyright 2009 Elsevier B.V. All rights reserved.