Virulence and In-Vitro Characteristics of Four Mutants of Newcastle Disease Virus

The relation between virulence and plaque morphology was studied in four clones of Newcastle disease virus. Virulence was correlated with size but not with clarity of a plaque. Virulent Herts L, the large-plaque mutant, was characterized by a rapid rate of absorption to cells and a short lag phase, factors which might facilitate dissemination of the virus in nature. Differences in the rate of release of virus from cells were not correlated with distinctions in viral pathogenicity, nor was interferon related to the virulence of these clones. Increased activity of neuraminidase of Herts L was correlated with unstable attachment of virus to cells. However, a difference in the rate of release of virus from cells infected with mutants of Milano red and clear plaques was not correlated with enzymatic activity. In chick-embryo cells, the mutation rate from red to clear plaque and large to small plaque was 3.3 × 10−6 and 1 × 10−7 per particle per generation, respectively. The red to clear plaque mutation appeared to be reversible, while no revertants to large-plaque size could be demonstrated in the small-plaque population of Herts S. Loss of capacity to form large plaques and a low rate of mutation from small to large plaque may be associated with strain attenuation in nature.