Selection of thermostable Newcastle disease virus progeny from reference and vaccine strains

In a study of low-virulence Newcastle disease virus (NDV) isolates from poultry, 38% of the isolates had a more thermostable hemagglutinin than the lentogenic reference strains B1 and La Sota or live vaccines derived from those strains. Whether those strains with a more thermostable hemagglutinin are truly indigenous or whether they could have originated from vaccines used in the flocks was unknown. Seven monovalent NDV vaccines of B1 or La Sota type and reference B1 and La Sota strains were heat treated at 56 C to select variants more thermostable than the parent virus. Four thermal treatment cycles were completed, and virus propagated from the second and fourth heat treatments was assayed for changes in thermostability and antigenicity. The hemagglutinin thermostability of all vaccine and reference strain variants increased from the initial < or =10 min to > or =120 min after four treatments. Antigenic changes evaluated by hemagglutination inhibition against NDV monoclonal antibodies identified changes in only the heat-treated La Sota strains. The results demonstrate that the field isolates with a more thermostable hemagglutinin could have been derived by selection from the heterogenous NDV populations in vaccine strains and that minor antigenic changes may be a result of that selection.