[Characteristics of Newcastle virus isolated from live vaccine-immunized chickens--the unusual biological properties].

A strain of the Newcastle disease virus, denoted GF, was isolated from birds vaccinated twice with a live vaccine. In some flocks of the farm this strain has caused oligosymptomatic disease, and in others the infection has run a latent course. It was shown that the GF strain possessed unusual biologic properties--an extremely high virulence in vitro combined with a total lack of pathogenicity in vivo. With regard to the tests mean death time with 10-day-old chick embryos (44.6 h) and plaque morphology (a heterogeneous plaque population of clear plaques, sizing 0.5 to 3.5 mm) it was referred to the velogenic group. With regard to the intracerebral pathogenic index (1.21) it belonged to the mesogenic group. In terms of the intravenous pathogenic index (0) it had to be classified with the lentogenic group of of strains of the Newcastle disease virus. All these properties of the strain were shown to be stable. Besides the difficulties with its differentiation the behaviours of this strain pose a number of problems with regard to its origin and the variability of the Newcastle disease virus in nature, which are subject to discussion.