Ultrastructural study of cell cultures infected with swinepox and orf viruses

Monolayer cells of a porcine kidney cell line were infected with the PP-1 strain of swinepox virus, while secondary or third subcultured monolayer cells of African green monkey kidney were inoculated with the Iwate BT-9 strain of orf virus. Those infected cells were fixed when CPE became remarkable in the monolayers and examined by electron microscope. In the cytoplasm of cells infected with both viruses, various immature forms of viral particles in different developmental stages were observed. Micells were detected very close to the opening of immature particles. Mature particles asssociated with a double membrane were frequently observed. From those observations it was suggested that the developmental sequence of both viruses is essentially the same as that of vaccinia (Dales, 1973). Besides various virus forms as well as factories, the following ultrastructural changes were noted in swinepox infected cells, i.e. 1) intranuclear inclusions which consist of very fine filaments, 2) fibrillar structures with cross striations located in the nuclear inclusions, and 3) similar striated fibrillar structures in or just adjacent to virus factories (B type inclusions) in the cytoplasm. Those observations made with in vitro cells are in good accordance with the descriptions by previous investigators on in vivo materials. Accordingly those ultrastructural changes characterize the swinepox infection.