Rapid in vitro assessment of the virulence of Newcastle disease virus isolates using the ligase chain reaction

The ligase chain reaction was used to assess the virulence of isolates of Newcastle disease virus. In the main study, 18/18 virulent isolates whose nucleotide sequences that code for the cleavage site and fusor peptide regions were known, successfully ligated oligonucleotides in a primer mix for virulent viruses termed VPM. Five of these isolates yielded a more intense ligated product with a second primer mix for virulent viruses called VPM1. No ligation was evident with eight avirulent isolates in tests with VPM or VPM1, however, each of these viruses did yield a strong ligated product with the primer mix for avirulent viruses (AVPM) as did one virulent isolate considered to be a mixture. Two virulent Australian isolates, 1238/1998 and 1248/1998, showed low but seemingly specific ligation with AVPM. In a blind study, 8/9 virulent isolates whose sequences were unknown ligated primers in VPM. Three avirulent and one virulent isolate, the latter again probably a mixture, ligated primers in AVPM. Ligation of oligonucleotides in VPM and AVPM was detectable in mixtures where virulent and avirulent isolates represented 0.1% and 0.01% by volume respectively of the viral population. The results indicate that LCR offers a potential in vitro alternative to current in vivo tests for virulence determination of Newcastle disease virus isolates.