Persistent infections induced by viral agents in animals

: Several studies have been conducted on persistent infections induced by viruses which are usually lytic for the infected cells. At the onset of these infections at least four mechanisms seem to be involved, i.e., the defective interfering (DI) viral particles, the temperature-sensitive (ts) mutants, the interferon or the presence in the culture of integrated DNA proviral copies of RNA viruses. Actually these systems are very complex and their evolution could depend on several factors such as the type of virus, the growth temperature of the culture or the kind of culture. Beside the systems that have been discussed here there may be other still unknown factors which might be involved in persistent infections, both in vitro and in vivo. The discovery of these factors could eventually represent a realistic basis for a better understanding of the pathogenesis of slow virus diseases and also would offer the possibility to study the long-term reactions of the organism when subjected to vaccination with ts mutants.