Growth in chick chorioallantoic membranes of strains of Newcastle disease virus of differing virulence.

The growth of eight strains of Newcastle disease virus in chick embryo chorioallantoic membranes was studied by comparing, at different times after infection, the amounts of haemagglutinin released into the allantoic fluid (extracellular haemagglutinin) with that associated with the membrane (cell-associated haemagglutinin). The virulence of the strains examined differed in that some killed chick embryos more rapidly than others. All strains released similar amounts of extracellular haemagglutinin and maximum titres were achieved about 12 hr. after infection. With virulent strains cell-associated haemagglutinin titres increased exponentially until the death of the host and maximum titres were much higher than those of extracellular haemagglutinin. With avirulent strains cell-associated haemagglutinin titres increased exponentially for only a limited time and titres were always lower than the titres of extracellular haemagglutinin.Similar results were obtained when the titres of neuraminidase and viral ribo-nucleoprotein were measured during the growth of two virulent and two avirulent strains. Virulence appears to be associated with the continued intracellular accumulation of viral antigens.