Influence of maternal antibody on efficacy of embryo vaccination with cell-associated and cell-free Marek's disease vaccine

Vaccination with turkey herpesvirus (HVT) of 18-day-old chicken embryos from a commercial source or from a cross (15 X 7) of two 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. However, 50%-protective-dose (PD50) assays revealed that maternal antibodies in embryos reduced vaccine efficacy. The PD50 assays were conducted by vaccinating 15 X 7 embryos with serial dilutions of HVT at the 18th day of incubation. Embryonally vaccinated and unvaccinated chicks were challenged with MDV on the day of hatch. In the absence of maternal antibodies, the PD50 values in plaque-forming units for cell-associated and cell-free HVT were 57 and 328, respectively. In the presence of maternal antibodies, PD50 values for cell-associated and cell-free HVT were 105 and greater than 4,000, respectively.     

Observations on the possible invasiveness of Clostridium botulinum for waterfowl.

Twenty-four ducklings given multiple doses of Clostridium botulinum type C spores and toxin per os and 27 affected waterfowl from four natural outbreaks of the disease were examined bacteriologically. No evidence of invasion of the blood or liver was found in any bird and it is suggested that invasiveness and toxigenesis in internal organs are probably of little, if any, importance in the causation of botulism in waterfowl.