Influence of Maternal Antibody on Efficacy of Embryo Vaccination with Cell-Associated and Cell-Free Marek's Disease Vaccine

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Abstract
Vaccination with turkey herpesvirus (HVT) of 18 day old chicken embryos from a commercial source or from a cross (15 .times. 7) of 2 inbred lines induced better protection against early post-hatch challenge with virulent Marek''s disease virus (MDV) than vaccination at hatch, despite the presence in embryos of maternally derived antibodies to HVT or to HVT and MDV. PD50 (50% protective dose) assays revealed that maternal antibodies in embryos reduced vaccine efficacy. PD50 assays were conducted by vaccinating 15 .times. 7 embryos with serial dilutions of HVT on day 18 of incubation. Embryonally vaccinated and unvaccinated chicks were challenged with MDV on hatch day. In the absence of maternal antibodies, PD50 values for cell-associated and cell-free HVT were 57 and 328 plaque forming units (PFU), respectively. In the presence of maternal antibodies, PD50 values for cell-associated and cell-free HVT were 105 and > 4000 PFU, respectively.