[Comparison of the thermostability of Newcastle disease virus strains isolated in Slovakia].

Reference strains of Newcastle disease virus of different virulence (two lentogenic, two mesogenic and three velogenic), isolated in Czechoslovakia and other countries, were compared with 11 field strains isolated in Slovakia in 1973, 1977, 1979 and 1980 as to the stability of their infectivity for cell cultures and as to their hemagglutination activity at a temperature of 56 degrees C for 120 minutes. The inactivation curves indicate that ten strains belong to the group of velogenic viruses and one of them is lentogenic. Although the data on hemagglutinin thermostability and infectivity do not suffice to characterize the virus strain, it is possible, by comparing the inactivation curves determined by the described method, to differentiate the field strains of Newcastle disease virus as lentogenic, mesogenic and velogenic. Some instability of the relationship between the thermostability and virulence of the virus is ascribed to the heterogeneity of the virus population.