Elevated virulence of Newcastle disease virus strains following serial passages in kidney cells in vitro.

Serial passage of two lentogenic Newcastle disease virus (NDV) strains in kidney cell lines increased virulence and changed viral biological properties. In two cell lines (BHK and MDBK), elevation in virulence was demonstrated by plaque formation under an overlay with no additives, decrease in mean death times, elevated intracerebral pathogenicity index, and cytopathic effect in chicken embryo fibroblasts. Some other markers not directly correlated to virulence, such as heat inactivation of hemagglutinin and neuraminidase, were not influenced by passage in kidney cells. In addition, all strains were slow eluters. This observation emphasizes the importance of preventing the virus from reaching the viscera.