The growth cycle of visna virus in monolayer cultures of sheep cells

Abstract

The growth of visna virus was studied in monolayer cultures of sheep cells kept at 37°C and infected with virus at an input multiplicity of about 20 TCID 50. A latent period of 20–24 hours was followed by a period of rapid virus formation lasting for about 15 hours, after which the virus titer increased only slightly. At this time the cell layer showed very pronounced cytopathic changes, the cells beginning to detach from the glass. Increase in free virus occurred almost concurrently with the increase in cell-associated virus (CAV), and during the whole period of viral growth less virus was found in the cell layer than in the surrounding fluid. About 95–99% of the CAV could be neutralized by visna antiserum applied to the intact cell layer. The results indicate that visna virus is released into the surrounding fluid very shortly after its formation in the host cells and that most of the newly formed virus is to be found on the external cell surface, not within the cells. The present study thus agrees with a previous electron microscopic study which suggested that visna virus was formed by a budding process in the cell membrane.