Reverse plaque formation by hog cholera virus of the GPE-strain inducing heterologous interference

A simple and rapid plaque procedure was developed for the assay of hog cholera virus (HCV) of a particular strain, GPE-, based on its intrinsic interference with vesicular stomatitis virus (VSV) on the primary swine testicle cells and on an established swine kidney cell line; the procedure is called the reverse plaque formation (RPF) method. The plaques were produced as colonies of HCV-infected cells which were VSV-sensitive, disintegrated cell sheet. These plaques became visible after 15 to 20 h of superinfection with VSV done 2 days after an initial inoculation of the GPE- strain. The plaque formation was inhibited by a specific antiserum against HCV. All cells within the plaque had HCV antigen detectable by fluorescent-antibody staining. The variations of reverse plaque count were low enough to permit virus titration. The relationship between virus concentration and the number of plaques was essentially linear. The titer measured by the RPF method was a little higher than that of the tube culture interference method.