Persistent infection of Vero cells by paramyxoviruses. A morphological and immunoelectron microscopic investigation.

Two Vero cell lines persistently infected with canine distemper virus (CDV) or with both CDV and canine parainfluenza (CPI) viruses were investigated. Cells in the CPI-CDV cell line were 90-100% positive for CPI antigen and exhibited 10-80% CPI hemadsorption. Cytoplasmic CDV antigen expressed in both singly and dually infected monolayers varied weekly from 1 to 100%. Numerous cytolytic crises were observed in both cell lines. Cell replication was severely depressed in both cell lines when compared with uninfected Vero cells. Infrequent interfering activity against lytic CPI virus was present in the CPI-CDV cell line but not between lytic CDV and progeny virus from the CDV or the CPI-CDV cell line. Ultrastructurally, Vero cells persistently infected with both paramyxoviruses contained two types of viral nucleocapsids (NC). By immunoelectron microscopy, smooth NC were identified as CPI virus and rough NC were of CDV origin. The viral NC never intermingled but rather were restricted to discrete cytoplasmic areas containing either one type of NC or the other.