Effect of host selective pressure on Newcastle disease virus virulence.

Newcastle disease virus (NDV) causes an economically important disease that can vary from clinically inapparent to highly virulent forms. Generally, NDV strains isolated from wild birds are non-pathogenic for chicken. However, there are evidences supporting the fact that avirulent viruses maintained in feral birds could have caused outbreaks of virulent NDV in poultry. The strain-specific difference in virulence is determined by structural variations on the fusion glycoprotein (F). More basic amino acids are present in the F cleavage site of virulent strains. Nevertheless, other regions have been involved in virulence determination. When we subjected an avirulent NDV isolated from a wild bird to a host change we found that the virus arose was virulent for chicken. Nucleotide changes in the F protein cleavage site amino acid sequence and in the hemagglutinin-neuraminidase protein sequence are reported.