Comparative Studies on Large‐ and Small‐Plaque‐Forming Clones of Newcastle Disease Virus (Miyadera Strain)

ABSTRACT The Miyadera strain of Newcastle disease virus (NDV) consisted predominantly of virus particles forming small plaques on monolayers of chick embryo fibroblasts (CEF), and contained small amounts of virus particles forming large plaques. These large‐ and small‐plaque‐forming clones of this virus (NDV‐L and NDV‐S) were isolated. The small size of the NDV‐S plaques did not appear to be due to an agar inhibitor. NDV‐L produced a much higher yield of infective virus particles in CEF and they were released more completely from the infected cells than were those produced by NDV‐S. The yield of infective virus of NDV‐L per cell from cultures of CEF was comparable to the yield from the allantoic cells. The infectivity/hemagglutinin ratio for NDV‐L from CEF was as high as the ratio for virus from the allantoic cells, but the ratio for NDV‐S from CEF was lower. NDV‐S demonstrated an autointerference phenomenon in CEF when infected at high multiplicities, but NDV‐L did not. Contrary to virus multiplication, NDV‐S exhibited a more rapid and marked cytopathic effect on monolayers of CEF than NDV‐L. In the allantoic cavity of eggs NDV‐S produced slightly higher virus yields than NDV‐L. No correlation existed between plaque size of the two viruses and the capacity to induce interferon synthesis or the susceptibility to the action of interferon. The properties of both distinctive plaque isolates were stable on egg passage.