Characterization of isolates of Newcastle disease virus from migratory birds and turkeys

The susceptibility of wild birds to infection with Newcastle disease virus (NDV) has resulted in speculation about the role of these free-flying birds in the origin and transmission of the virus infection. Since several NDV isolates from free-flying waterfowl and turkey flocks in the Midwest were avirulent for chickens and had thermostable hemagglutinins, isolates were collected from both waterfowl and turkeys over a seven-year period, 1974-1980, for comparison. Strains from both waterfowl and turkeys were avirulent, based on mean death times in chicken embryos, and could be differentiated from licensed vaccine strains with similar mean death times by in vitro markers. Waterfowl strains were generally thermostable, were slow to elute from chicken red blood cells, and produced plaques in chicken embryo fibroblasts. Some of the turkey isolates were similar to the waterfowl strains in thermostability, elution, and production of plaques in chick embryo fibroblasts. However, in the latter years of the study, there was a tendency toward isolation from turkey flocks of vaccine-like strains that did not produce plaques without additives and whose hemagglutinin was thermolabile. The appearance of these strains reflects either a selection for this virus type by the host or replacement of field strains by vaccine strains as the result of an increased usage of vaccines by producers.