Protection conferred by recombinant turkey herpesvirus avian influenza (rHVT-H5) vaccine in the rearing period in two commercial layer chicken breeds in Egypt

The effectiveness of recombinant turkey herpesvirus avian influenza (A/swan/Hungary/4999/2006(H5N1)) clade 2.2 virus (rHVT-H5) vaccine was evaluated in two layer chicken breeds (White Bovans [WB] and Brown Shaver [BS]). One dose of rHVT-H5 vaccine was administered at day 1 and birds were monitored serologically (haemagglutination inhibition test) and virologically for 19 weeks. Maternally-derived antibody and post-vaccination H5 antibody titres were measured using the Chinese (A/Goose/Guangdong/1/96(H5N1)) HA and the Egyptian (A/chicken/Egypt/128s/2012(H5N1)) HA as antigens. The challenge was conducted at 19 weeks of age and on six experimental groups: Groups I (WB) and II (BS), both vaccinated and challenged; Groups III (WB) and IV (BS), both vaccinated but not challenged; Groups V and VI, unvaccinated specific pathogen free chickens, serving respectively as positive and negative controls. The challenge virus was the clade 2.2.1 highly pathogenic avian influenza H5N1 A/chicken/Egypt/128s/2012 at a dose of 106median embryo infective dose. For both breeds, complete maternally-derived antibody waning occurred at the age of 4 weeks. The immune response to rHVT-H5 vaccination was detected from the sixth week. The seroconversion rates for both breeds reached 85.7 to 100% in the eighth week of age. Protection levels of 73.3%, 60% and 0% were respectively recorded in Groups I, II and V. No mortalities occurred in the unchallenged groups. Group I showed superior results for all measured post-challenge parameters. In conclusion, a single rHVT-H5 hatchery vaccination conferred a high level of protection for a relatively extended period. This vaccine could be an important tool for future A/H5N1 prevention/control in endemic countries. Further studies on persistence of immunity beyond 19 weeks, need for booster with inactivated vaccines, breed susceptibility and vaccinal response, and transmissibility are recommended.