Genetic comparisons between lentogenic Newcastle disease virus isolated from waterfowl and velogenic variants.

Avirulent Newcastle disease viruses (NDV) harbored by waterfowl have the potential to become virulent after transmission to and circulation within chicken populations. In order to investigate how virulent viruses are selected from an avirulent background, we compared the complete sequences of the avirulent NDV isolate Goose/Alaska/415/91 and its virulent variant strain 9a5b, which was obtained by nine and five passages in the chick air sac and brain, respectively. Seven amino acid substitutions were detected in the M, F, and HN proteins. Two were detected between variants 9a3b and 9a5b (128P to H and 495E to K in HN protein) that were passed through the brain. Pathogenicity determined by the MDT and IVPI tests also differed between 9a3b and 9a5b. These results suggest that in addition to the F cleavage site sequence, these two amino acids in HN protein are also related to the pathogenicity of NDV in chickens.