[Plaque formation by strains of Newcastle disease virus in monolayers of chick embryo fibroblasts].

Comparative studies were carried out with 4 local velogenic strains (2 viscerotropic and 2 neutropic) and 6 standard strains of the Newcastle disease virus of the various pathogenic groups with regard to their plaque-forming capacity in a monolayer of tissue cultures (TC) of chick embryo fibroblasts (CEF) under agar cover, after a routinely employed method. It was found that the velogenic strains produce a heterogenic (in terms of plaque size) population of bright, mat-white plaques regardless of their belonging to the viscero-, resp, neurotropic pathogenic type. The mesogenic produced only small, bright plaques, while the lentogenic strains produced no plaques. The plaque-forming unit titers per cm3 in TC of CEF of the strains as well as their ratio to the respective infectious titers for chick embryos (EID50) and for TC of CEF overlaid with liquid nutrient medium by microscopic record of the cytopathic effect (TCID50) varied within a wide range. It was in favour of EID50, and in most of the strains - in favour of TCID50 no matter to which type of strains belonged. The results obtained with the use of the plaque method in TC of CEF confirmed the concept of a number of authors abroad that they could be used successfully as an index in typing also for the differentiation of the Newcastle disease virus strains.