Investigations of Newcastle disease virus (NDV)
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Newcastle disease (ND) is one of the most important avian diseases. The disease severity varies among species and with different strains of Newcastle disease virus (NDV). Virulent ND has a potential for very serious and rapid spread, and may cause serious economic impact and international trade restraint on the poultry industry. Therefore, the accurate virulence characterization of NDV isolates is extremely important. However, relatively little is known concerning the contributions of the various proteins other than the fusion protein to the virulence of NDV. It is essential to understand which genes and specific differences in those genes of NDV induce differing virulence in birds. This study was performed in order to improve the understanding of the roles of the fusion (F), hemagglutinin-neuraminidase (HN), and phosphoprotein (P) genes in NDV virulence. Three wild-type isolates and their ten infectious clones containing artificially created mutations or substitution of the F, HN, or P gene were utilized, and their virulence was compared by standard pathogenicity tests and by clinicopathologic assessment in chickens. The results suggest that the virulence of NDV is multigenic, and at least the F, HN, and P genes influence viral pathogenicity as well as clinicopathologic disease in chickens. During the course of the studies, a severe outbreak of Newcastle disease occurred in California. It was determined expedient to apply our techniques on this outbreak virus. Thus, the characterization of the virus causing the outbreak has become a part of the studies. The virus was investigated by standard pathogenicity tests and by clinicopathologic assessment in specific-pathogen-free (SPF) chickens, SPF turkeys, commercial turkeys, and pigeons. The virus was classified as a highly virulent strain by pathogenicity tests; however, the clinicopathologic features of disease varied among chickens, turkeys, and pigeons. Throughout these experimental NDV studies, immunohistochemistry and in situ hybridization were use to detect viral distribution and replication, respectively, on sections of the formalin-fixed, paraffin-embedded (FFPE) tissue. The reverse transcription polymerase chain reaction (RT-PCR) of the NDV matrix gene was also developed, as another potential method for viral detection in FFPE tissues. The RT-PCR with FFPE tissues was found to be a sensitive method to detect NDV.