Ultrastructural study of long-term canine distemper virus infection in tissue culture cells

The morphogenesis of canine distemper virus was studied in Vero cell cultures for 43 days post-inoculation. Active replication of the virus was observed by electron microscopy and assay from 12 h after inoculation on, and peak production was observed on days 5, 14, and 22. From day 28 on, constant but smaller amounts of infectious virus were detected. Two ultrastructural types of intracytoplasmic nucleoprotein filaments were observed; although they first appeared at different times, their subsequent chronological patterns of development were similar. The cells apparently became free of virus by a mechanism of vacuolation. Intranuclear filaments were seen about day 11 and appeared to increase in number thereafter, whereas the infectious titer declined. Possible mechanisms of persistence are discussed in the light of these findings.