Modification in Virulence of Newcastle Disease Virus Isolates

FLAVIA ZANETTI; MARIANO JUNCO; EVANGELINA GÓMEZ; ANALÍA BERINSTEIN; ELISA CARRILLO

Buenos Aires

Congreso; XIX Congreso Panamericano de Ciencias Veterinarias; 2004

Newcastle disease virus (NDV), an important avian pathogen that belongs to the Paramyxoviridae family, comprises a variety of strains which differ widely in virulence for chicken, its natural host. The difference in virulence is determined by the fusion (F) protein cleavage site amino acid sequence. F protein cleavage site of avirulent and low virulent strains has fewer basic amino acids than the F protein of intermediate and high virulent strains. However, the NDV HN proteins has been suggested as pathogenic marker, HN0 616, the biologically inactive precursor, needs proteolytic cleavage and has been found only for avirulent strains, whereas HN577 and HN571 are already active translation products and they are shared by high, intermediate and low virulent strains. In 1997, Argentina has been declared NDV virulent-strains-free for commercial poultry. However, wild birds are considered a natural reservoir for the virus and they represent a real hazard for the farms. The goal of the present study was to evaluate the potentiality of avirulent NDV isolated from wild birds to become virulent for chickens. The genetic and phenotypic characteristics of viruses during sequential passages in chicken embryo were evaluated by biological and molecular means. A mean death time (MDT) of 83 hs. was obtained for viruses after 20 passages in chicken embryos whereas the original isolate could not kill the embryos inoculated in the same test (MDT= infinite). The wild isolate did not produce clinical signs after intracerebral inoculation in day-old chickens and no virus could be isolated from brain. The viruses isolated after 20 passages produced illness in chickens and were able to replicate in brain. The F protein cleavage site amino acid sequence in initial viruses was 112G-K-Q-G-R-L117. This motif changed to 112G-R-Q-K-R-F117 after 5 passages and did not change thereafter. The HN C-terminal nucleotide sequence allowed to deduce a 616 amino acids HN for the initial viruses and a 571 amino acids HN was found in viruses which were passed 20 times. This results showed a virulence increase of NDV isolates from wild birds after these viruses were passed in a new host. These data support the fact that wild birds represents a real hazard for commercial farms and suggest that vaccination is a needed tool to preserve the health status of commercial bird populations.