Pathogenesis of Newcastle disease in commercial and specific pathogen-free turkeys experimentally infected with isolates of different virulence.

The pathogenesis of five different Newcastle disease virus (NDV) isolates representing all pathotypes was examined in commercial and specific pathogen-free (SPF) turkeys. Experimentally-infected birds were monitored clinically and euthanatized, with subsequent tissue collection, for examination by histopathology, by immunohistochemistry for the presence of NDV nucleoprotein, and by in situ hybridization for the presence of replicating virus. Clinically, the lentogenic pathotype did not cause overt clinical signs in either commercial or SPF turkeys. Mesogenic viruses caused depression in some birds. Turkeys infected with velogenic neurotropic and velogenic viscerotropic isolates showed severe depression, and neurologic signs. Histologic appearances for all strains had many similarities to lesions observed in chickens inoculated with the various isolates; that is, lesions were present predominantly in lymphoid, intestinal, and central nervous tissues. However, in general, disease among turkeys was less severe than in chickens, and turkeys could be considered a subclinical carrier for some of the isolates.