Acquisition of pathogenicity of a Newcastle disease virus isolated from a Japanese quail by intracerebral passage in chickens.

Newcastle disease virus (NDV) was isolated from a Japanese quail (Cotornix cotornix japonica). The effect of intracerebral and intranasal passages of the NDV in chickens on the pathogenicity was studied. Pathogenicity of the viruses of different passage levels was compared with that of the original isolate by the mean death time with the minimum lethal dose in chicken embryos, intracerebral pathogenicity index in day-old chicks, intravenous pathogenicity index with 6-week-old chickens and the mortality rates of chickens and quails inoculated intravenously or intranasally. The original isolate from the quail did not kill chickens but only embryos and some one-day-old chicks, exhibiting a mesogenic character. Pathogenicity of the virus of the 10th intranasal passage was not different from that of the original isolate. The viruses passaged intracerebrally, on the other hand, killed chickens of all ages by either route of inoculation, showing a velogenic property. Virus recovery from the blood and the brain was positive only in the chickens infected with brain-passaged viruses by any route of inoculation. Virus titers in the tissues of chickens infected with the brain-passaged viruses were higher than those with the original isolate and the virus of the 10th intranasal passage. These results indicate that the enhanced pathogenicity of the mesogenic NDV isolate from the quail for chickens was induced by acquiring the properties of neurotropism and pantropism through intracerebral passage in chickens.