Isolation and biological properties of some Moroccan strains of Newcastle disease virus.

Newcastle disease virus was isolated from six field cases in Morocco. On the basis of the mean death time of chicken embryos, the intracerebral pathogenicity index, and plaque formation on chicken embryo fibroblast monolayers, five isolates were determined to be of the velogenic pathotype. One of these differed from the others in that it agglutinated equine erythrocytes. The sixth isolate was found to be of low virulence but differed from the vaccinal strain tested.