ULTRASTRUCTURAL STUDIES OF ORF VIRUS-INFECTION AND REPLICATION IN FETAL LAMB FIBROCYTES

The cycle of replication of orf virus was identified in temporal in vi tro studies and a putative model was developed. During the first phase , which lasts about 5 h, the virus enters the cells by a phagocytic pr ocess and uncoats. The ''eclipse'' phase, with the virus apparently pr esent as strands of DNA, lasts for approximately the next 8-10 h. In t he final phase virions develop within well-defined zones of viroplasm from which mature virions migrate to the margins of the cell. There th ey apparently exit either by exocytosis or within microvillous project ions which are ''pinched off''; they can also be released by disintegr ation of the host cell.