Cellular Resistance in Tissue Culture, Induced by Noncytopathogenic Strains, to a Cytopathogenic Strain of Virus Diarrhea Virus of Cattle

Abstract

An accurate and reproducible assay method became available for measurements of noncytopathogenic strains of virus diarrhea (VD) virus when it was found that cell cultures became resistant to a plaque-forming cytopathogenic strain of VD virus if the noncytopathogenic strain had been inoculated at least 3 days earlier. Neutalization of the noncytopathogenic strains by VD antibodies made possible serological comparisons of VD virus strains whether or not they were cytopathogenic. Identification of noncytopathogenic strains, therefore, became possible without the use of calves. The nature of cellular resistance induced by noncytopathogenic strains to plaque -forming cytopathogenic strains of VD virus had not been shown by these studies. It may be similar to the interference produced in MCN cell cultures by Newcastle disease, mumps or 6-6 viruses to vesicular stomatitis virus. More recently, the resistance to Rous sarcoma virus, induced by also was attributed to interference because resistance was not established until after 3 days. In agreement with these studies, initial tests with VD virus showed complete resistance to 50 PFU after 3 days but not after 2 days.