Overcoming the susceptibility gap between maternal antibody disappearance and auto-antibody production.

In the first 10-14 days of a chick's life, protection is conferred by maternal antibodies. Further broiler protection is achieved by active vaccination. However, the high level of maternal antibodies interferes with the induction of an effective immune response by vaccination at a young age. As a result, there is a gap between the reduction in protective maternal antibodies and elevation of self-produced antibodies following active vaccination. The major aim of this study was to test an approach consisting of passive and active vaccination to overcome this gap and to provide continuous resistance to infectious viral diseases during the broiler's growth period. Newcastle disease virus (NDV), which is one of the world's most prevalent infectious diseases of poultry, was tested as a model. Following subcutaneous injection of 18 hemagglutination-inhibiting (HI) units of anti-NDV immunoglobulin Y per 1-day-old chick, protective log2 antibody titers above 4 could be detected to at least 17 days of age. The combination of passive immunization on day 1 of age with attenuated live vaccination on day 10 led to high protective titers throughout the entire growth period, up to 41 days of age. Moreover, the HI titers in the group of birds immunized with the combined vaccination were significantly more homogeneous than those in the group vaccinated only with live virus. Thus, full protection against NDV of all broilers in flock during their entire growth period was achieved by a vaccination regime that combines passive immunization and live vaccination.