[Variability and the virulence of the Newcastle disease virus].

Six isolates of the Newcastle Disease virus (NDV), taken at different periods of time after provocation of immune birds with velogenous viscerotrophic strain, are used. It is shown that the virulence of the initial NDV challenge strain remains unchanges up the 46th day of persistence. The isolate, taken after 70 days of carriage, manifests a considerably decreased virulence. It is established that the reduced virulence is not genetically stable and reverts towards the initial one following passage through appropriate host organism. The latent infection with NDV is examined from ecological and practical aspects.