The Isolation and Characterization of Heat-Resistant Mutants of the Najarian Strain of NDV

The heat resistance of the Najarian strain of Newcastle disease virus at 56°C ± 0.2 as measured by hemagglutination and egg infectivity tests is discussed. Subcultures of greater heat resistance were obtained by inoculating embryos with virus that survived exposure to various periods of heating at 56°C. The isolate was considered to be a mutant when it differed significantly from the original strain and was obtained from inocula which contained less than one infective unit per inoculated embryo. The infectivity of the mutant NAJ/H1 for embryos survived exposure to 56°C for 240 minutes as compared to the parent that survived for 60 minutes. The hemagglutinin of the mutant persisted for at least 300 minutes as compared to 45 minutes for the parent. It was shown that heat-resistant mutants were more readily isolated when short rather than long periods of heat treatment were employed. This was done in a series of cycles in which the virus contained in embryonic fluids was heated and then injected into embryonating eggs and allowed to multiply.