The pathogenic strain Italien and the apathogenic strain Ulster of Newcastle disease virus have been compared with respect to organ tropism and spread of infection in 11-day-old chick embryos. After infection of the endodermal layer of the chorioallantoic membrane by intra-allantoic inoculation with strain Italien, high virus titres are found in all extra-embryonic membranes and fluids and in the embryo itself. Infection results in early death of the embryo. In contrast, after infection with strain Ulster by the same route of inoculation, high virus titres are found only in the allantoic sac and embryos are not killed. Inoculation with strain Italien on to the ectodermal layer through an artificial air sac results in rapid spread of infection in the chorioallantoic membrane and the embryo dies before the virus invades other tissues including the embryo. Under the same conditions of infection, strain Ulster neither spreads within chorioallantoic membrane nor does it kill the embryo. Virus spread in each germinal layer of the chorioallantoic membrane was analysed by immune fluorescence. These studies showed that endoderm as well as mesoderm and ectoderm allowed the spread of strain Italien, whereas only the endoderm is permissive for strain Ulster. These differences in host range are based upon differential activation of the virus glycoproteins by proteolytic cleavage. The glycoproteins of strain Italien are cleaved in each germinal layer, whereas those of strain Ulster are cleaved only in endoderm. These studies demonstrate that, in the system analysed here, spread of infection and organ tropism are important factors for pathogenicity and both of these factors are determined by the susceptibility of the virus glycoproteins to proteolytic cleavage.


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