Serological response of broiler chicks to Newcastle disease vaccine as influenced by vaccination against infectious bursal disease

The effect of a modified live IBD [Infectious bursal disease] vaccine containing an intermediate strain of IBD virus on the antibody response of broiler chicks to live ND [Newcastle disease] vaccine was evaluated. Seventy chicks, all from the same breeder flock were used in this study. The chicks were randomly divided into four groups. Group A was simultaneously vaccinated against IBD and ND at day-old. Group B was given the IBD vaccine and ND vaccine at one and seven days old, respectively. Group C was immunized against ND at seven days old and against IBD at ten days of age. Group D received only the ND vaccine at seven days old, and served as control. All birds in the four treatment groups were revaccinated for ND at 21 days of age. The bursa of Fabricius were collected from three chicks in Groups A, B, and C, five days post-IBD vaccination. Bursa of Fabricius from 3, 7, 14, and 21 days old chicks in group D were likewise collected, examined for gross and histologic changes, and compared with those of the birds group A, B, and C. Grossly, the bursa of Fabricius from the IBD-vaccinated birds had creamy striated appearance due to the presence of serosal transudate and prominent longitudinal folds. Microscopic changes such as lymphocyte degeneration, lymphocyte depletion, cyst formation, interfollicular edema and hemorrhages were found to be more extensive in the bursa of birds vaccinated against IBD at day-old than in those vaccinated at ten days of age. In contrast, the bursa of Fabricius taken from day-old chicks prior to IBD vaccination and those from the non-IBD vaccinated chicks, showed the presence of dense bursal follicles populated with normal lymphocytes. Low to moderate levels of maternal antibodies to ND and IBD virus were detected in the sera collected prior to ND and IBD vaccination by the HI and ELISA test, respectively. Evidence of immunosuppression was found by measuring the serum HI antibody titres to ND vaccine in IBD-vaccinated birds. The primary and secondary antibody responses to ND vaccine were consistently and significantly lower in the IBD-vaccinated birds than in the control group. Suppression of humoral immunity was more dramatic in group A and B which were given the IBD vaccine at day-old than in group C which was vaccinated at 10 days of age. Results of the present study showed the potential of a modified live intermediate IBD vaccine to cause serious damage to the bursa of Fabricius with consequent depression of the antibody response to live ND vaccine when given particularly to one-day old broiler chicks with low to moderate levels of maternal antibody to IBDV