Characteristics of a local virulent strain of Newcastle disease virus

A local virulent strain, VLT, of Newcastle disease virus formed 3- to 4-mm plaques on monolayers of primary chicken embryo cultures on the 4th day after inoculation. It agglutinated chicken and human 0 erythrocytes. Its hemagglutinin was stable at 56 C when compared with those of Komarov (K) and F vaccinal strains of the same virus. The viral titer of infected allantoic fluid dropped from 10(8.1) plaque-forming units to 10(1.0) plaque-forming units/ml within 2 hours when incubated at 56 C. The strain was ether-sensitive; it adsorbed readily on monolayers of chicken embryo cells and did not diffuse through agar. Its intracerebral pathogenicity index, chicken dose LD50, and embryo mean death time (hours) were 1.8, 9.0, and 48, respectively. Two virulent strains isolated in 1974 and 1975 were found to be identical to the VLT strain in terms of certain physical and biological properties. On the basis of plaque morphologic characteristics, hemagglutination spectrum, and hemagglutinin inactivation at 56 C, it was possible to identify readily the field isolate when it was compared with vaccinal strains (K and F) commonly used in Lebanon.