Vaccination of broilers against Newcastle disease in the presence of maternally derived antibodies.

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The aim of this study was to evaluate the efficacy of a live attenuated vaccine against Newcastle disease in broilers with different levels of maternally derived antibodies (MDA). While vaccination remains the single most important means for controlling Newcastle disease, presence of MDA may interfere with the vaccination of young birds and decrease the efficacy of the vaccine. Day-old chicks with variable levels of MDA (negative, low and high) were vaccinated with a live attenuated vaccine against Newcastle disease. Three most commonly used inoculation routes were compared; oculonasal, spray and oral (drinking water). Onset and duration of immunity were measured by serology and challenge with virulent virus. Immune response in vaccinated MDA-positive birds was delayed in comparison with SPF controls. Protection was well established already at 14days post vaccination in SPF birds while in MDA-positive birds it was 1-2weeks delayed and was lower throughout the study. Non-vaccinated MDA-positive birds lost passive protection completely at 3-4weeks of age and were significantly more susceptible to challenge than vaccinated hatch mates at all test points. The protection rate increased in vaccinated birds towards the end of the experiment and reached 70-100 % at the last test points (35-42days of age). Correlation of haemagglutination inhibition (HI) titre vs. protection rate revealed the importance of cellular and local immunity as most of the vaccinated birds with low HI titre were protected, contrary to their unvaccinated hatch mates with the same HI titre. Oculonasal route seems to provide slightly better protection than the other two routes. Although immune protection in vaccinated MDA-positive birds may be decreased or delayed, vaccination still provides high protection against ND challenge in comparison with the unvaccinated hatch mates. The degree of interference seems to be proportional to the level of MDA. Vaccination schedules therefore need to be designed according to the immune status of the flock.