Control of Newcastle disease by vaccination.

Vaccination for Newcastle disease (ND) is routinely practised in countries where virulent strains of the Newcastle disease virus (NDV) are endemic and in countries where virulent strains do not exist but ill-timed infection by a low virulent field strain may have significant economic consequences for the producer. The types of vaccines and vaccination schedules used vary depending on the potential threat, virulence of the field challenge virus, type of production, and production schedules. A combination of live and inactivated ND vaccine, administered simultaneously, is shown to provide better protection against virulent NDV and has been successfully used in control programmes in areas of intense poultry production. A potential limiting factor in the use of live vaccines to control virulent ND is that live virus can interfere with surveillance and laboratory diagnosis. However, a new assay, the real-time reverse transcriptase-polymerase chain reaction (RRT-PCR), differentiates low virulent from virulent NDV, thus minimizing the disadvantage of live virus vaccines in the face of an outbreak and may facilitate the use of such vaccines to control outbreaks of virulent ND in the future.