Abstract

Background: Newcastle disease virus (NDV) and infectious bursal disease virus (IBDV) remain major causes of immunosuppression and mortality in poultry, leading to substantial economic losses in endemic regions. Conventional vaccination often fails due to maternal antibody interference and limited cellular immune stimulation, necessitating innovative vaccine strategies. Aims: The purpose of this study was to examine the immunogenic efficacy and mechanism of action of the recombinant HVTND+IBD vaccination against Newcastle Disease (ND) and Infectious Bursal Disease (IBD) in one-day-old layer chicks in comparison to conventional live and killed vaccines. Additionally, it compares the recombinant HVT vaccine with Marek's disease protection induced on by HVT alone. Methods: A total of 150 Lohmann Brown chicks were divided into five groups at random: unvaccinated control, live ND+IBD, recombinant HVTND+IBD, HVT alone, and killed ND. ELISA and the DIVA (Differentiation of Infected from Vaccinated Animals) strategy, which differentiation of infected from vaccinated birds based on specific antibody profiles produced by recombinant vaccines, were used to evaluate humoral immune responses. RT-qPCR was used for quantifying viral replication in spleen and bursa tissues. Results: RT-qPCR revealed that recombinant HVT replication was slower than HVT alone, and the results demonstrated that the recombinant vaccine induced a slightly delayed but sustained antibody response, peaking at day 21. Around day 10, the recombinant virus was detected. It first replicated in the bursa and spleen before expressing the ND-FP and IBD-VP2 genes. When compared to killed vaccinations, histopathological and immunohistochemical investigations showed significantly higher levels of IL-2 and IL-4 expression in the recombinant and live vaccine groups, indicating enhanced lymphoid activation and stronger cell-mediated immunity. Conclusion: These findings support the use of recombinant vaccines for early immunization despite maternal antibody interference, particularly under high pathogen pressure conditions in developing poultry industries.