Development of the AG68L strain of Newcastle disease vaccine. (1) Modification of the existing AG68L vaccine by clone purification and its subsequent testing.

After preliminary work had shown that the Newcastle disease vaccinal strain AG68L had considerable potential as a lentogenic vaccine for administration in the drinking water, it was studied for viral homogeneity and purified by plaque picking and cell culture passage. The fifth passage of a small plaque was used to immunise birds. Its performance was compared with LaSota virus and with the parent AG68L. The attenuated vaccine was found to be significantly more immunogenic than LaSota virus when administered in the drinking water to either fully susceptible or maternally immune chicks; it was shown to be significantly less pathogenic than the parent strain and slightly more pathogenic than the LaSota strain when given as an aerosol but without detectable pathogenicity when given in the drinking water.