Properties of non-neurovirulent plaque-forming mutants of Newcaslte disease virus.
The mesogenic Hertfordshire (H) strain, a vaccine strain of Newcastle disease virus, represents a heterogeneous virus population differing in physical, chemical, and biological properties. Small (S) and large (L) plaque-forming mutants were isolted from strain H in chick embryo fibroblast cell cultures. Part of the S mutants lacked neurovirulence, with an intracerebral pathogenicity index  for day-old chicks of 0 to 0.46 vs 1.0 to 1.4 for L mutants. S mutants grew only to a very low titer in the brain of day-old chicks and cleared from the brain by the sixth day, whereas L mutants killed the chicks after a 1000-fold titer rise. The S mutants caused a complete cell destruction in fibroblast culture by 12 hours, but they had 10--20 times lower virus yields than the L mutants. The infective titers of S and L mutants grown in allantoic cells were identical, but the infectivity to hemagglutinin ratio was 10 to 50 times lower for S than for L. The thermostability of infectivity and hemagglutinin varied with the different mutants. As for the immunogenicity of the  mutants, the minimum dose of S mutants inducing full protection by subcutaneous inoculation was 10(6) plaque-forming units per chicken, thus being about 100 times less immunogenic than either the L mutants or the parent strain H.
D000818:Animals; D000956:Antigens, Viral; D001921:Brain / Q000382:microbiology; D002642:Chick Embryo; D006389:Hemagglutinins, Viral; D006358:Hot Temperature; D009154:Mutation; D009521:Newcastle Disease / Q000382:microbiology; D009522:Newcastle disease virus / Q000254:growth & development / Q000276:immunology; D010948:Viral Plaque Assay; D014774:Virulence
D000956:Antigens, Viral; D006389:Hemagglutinins, Viral