Newcastle disease: effects of vaccines and vaccination techniques of hemagglutination-inhibition (HI) antibodies and clinical resistance.

Different vaccines (the lentogenic Hitchner-, the avirulent Ulster- and the mesogenic Beaudette strain) and different vaccination schemes (spray, dip, drinking water) were used in 1380 broiler-type chicks on the first day of life and at four weeks. In all these experiments an immunity status was induced sufficient to resist to a strong individual challenge infection, two weeks after the second vaccination. This uniform resistance showed no correlation with the very heterogeneous HI titers which were obtained two weeks after the second vaccination. The titers were, however, strongly dependent both of the vaccine strain used and on the vaccination method. The Beaudette spray-vaccinations provoked the highest titers. The highest post-challenge titers were seen after vaccinations with the lentogenic and avirulent strains and the lowest after-challenge titers were seen when the mesogenic Beaudette strain was used. Birds with high HI titers showed a marked decrease of these HI titers two weeks after challenge, while there was an increase of HI titers in birds with low post-vaccinal titers. It may be concluded that clinical resistance to NCD infection is not directly correlated with high-vaccinal HI titers. Experimental infection by natural route is essential to judge the immune status of fowls.