The Combined Use of rHVT-H5 and rHVT-F Vector Vaccines in the Hatchery Enhances Immunity against Highly Pathogenic Avian Influenza H5N1 and Velogenic Newcastle Disease Viral Infections in Commercial Chickens

Highly pathogenic avian influenza H5N1 and Newcastle disease viral infections cause severe illness in chickens and vaccination is a strategic tool of controlling these diseases. Hence, this study was conducted to evaluate the efficacy of using both recombinant herpesvirus of turkey (rHVT-H5 and rHVT-F) vector vaccines at day-old, in the hatchery, under field conditions. Vaccinated chickens were challenged at 33 days of age with 100 µL containing 106 embryonated infective dose50 of either highly pathogenic avian influenza H5N1 or very virulent (velogenic) Newcastle disease viral strains through the intranasal route and monitored for 7 days for clinico-pathological changes. Tracheal and cloacal swabs and blood samples were also collected for determination of viral shedding using RT-PCR and immune responses using hemagglutination inhibition test. Absolute (100%) protection was recorded in vaccinated group against challenge with H5N1. In all time points, the challenge virus shedding was either not detected or greatly reduced in the trachea and the cloaca of vaccinated chickens compared to non-vaccinated, challenged chickens. Excretion of H5N1 challenge virus was not detected in the trachea of vaccinated birds at 3 and 5 days post-challenge and results of hemagglutination inhibition test revealed an average of 3.2 log2 titres at 5 days post-challenge. Protection achieved against challenge with very virulent Newcastle disease virus was 79%. There was 80-100% reduction in tracheal shedding at 3, 5, and 7 days post-challenge, and an average of 6.2, 6.0 and 6.4 log2 hemagglutination inhibition titres were recorded, respectively. Cloacal shedding of challenge Newcastle virus was greatly reduced in vaccinated groups compared to non-vaccinated, challenged chickens. These data support the efficacy of the combined use of rHVT-H5 and rHVT-F vector vaccines against highly pathogenic avian influenza and Newcastle disease viral infections under field conditions.