Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Veterinary Medical Sciences - Pathobiological Sciences

First Advisor

Johannes Storz


The early events in the infection of human rectal tumor cells by bovine coronavirus were investigated by immunoelectron microscopy and by studies with lysosomotropic weak bases. Virus particles labeled with colloidal gold were endocytosed by synchronously infected cells and accumulated in vacuoles that resembled secondary lysosomes. Sites of fusion between the virus envelope and the plasmalemma were observed but fusion events along intracellular membranes were not found. Exposure of cells to ammonium chloride or to mythylamine during the first hour of infection had little inhibitory effect on the production of infectious virus. Chloroquine treatments were inhibitory but this effect was found to result from relatively late events in the infectious process. These studies indicate that an acidic intracellular compartment is not required for infectious entry by bovine coronavirus. Bovine coronavirus appears to penetrate the host cell barrier by direct fusion with the plasmalemma. Consistent with this interpretation, analyses of the culture environment conducive to virus-induced cell fusion revealed that the coronaviral fusion factor was active at alkaline pH levels. Viral components were responsible for the induction of cell-to-cell fusion. Immunoelectron microscopy was used to demonstrate that viral antigens were expressed in the plasma membrane of infected cells despite the intracellular mode for assembly of this virus. Cytopathic expression of the virus in cell monolayers produced turbid plaques. Scanning electron microscopy was used to characterize cells that remained in these plaques after the lysis of cells susceptible to the virus.