Comparative pathology of ducks experimentally inoculated with three different recombinant Newcastle disease viruses

Although velogenic Newcastle disease viruses (NDVs) generally show low pathogenicity in waterfowl such as ducks, lethal infection by NDVs has been reported sporadically. A previous study revealed that a serially-passaged NDV isolate showed increased virulence in ducks, and that the viral proteins harbored a total of 11 amino acid substitutions. Among them, the fusion protein (F), matrix protein (M), and hemagglutinin-neuraminidase (HN) were suspected to be involved in enhanced virulence. The present study aimed to clarify the association between genetic mutations in NDVs and pathogenicity in ducks by comparing histopathological findings in ducks inoculated with three recombinant (r) NDVs: rM44 (Q44R substitution in M), rF142 (I142M substitution in F), and rM+F+HN (Q44R, N123D, and D342N substitutions in M; I142M and E304K substitutions in F; and G538R substitution in HN; three viral proteins compatible with amino acid sequences from passaged NDV). Seven-day-old ducks were intranasally inoculated with rM44, rF142, or rM+F+HN. Gross and histopathological lesions in the heart, thymus, brain, and pancreas were more severe in ducks inoculated with rM+F+HN than in ducks inoculated with rM44 or rF142. In particular, myocarditis associated with NDV antigens and mixed inflammatory cells was prominent in rM+F+HN-inoculated ducks. Therefore, the increased virulence of NDV in ducks is not due to a single amino acid substitution in one protein, but rather to the synergistic effects of multiple mutations in three viral proteins.