Effects of 'in ovo' vaccination with herpes virus of turkeys against Marek's disease on chicken performance in the absence of a challenge with Marek's disease virus

This study tested the effects of in ovo vaccination with or without herpes virus of turkeys (HVT) on the performance of broiler chickens in the absence of a challenge with Marek's Disease virus. It also investigated how the concentration of HVT, determined using real-time quantitative PCR, varied with vaccine dose and age of the bird. Female Cobb 500 broiler chickens (n = 320) were either not vaccinated, vaccinated in ovo on day 18 of embryonation with 4000 or 8000 pfu of HVT, or injected with diluent alone (sham-vaccination). Measurements were made of liveweight, feed intake, feed conversion ratio, and weights of breast muscle, small intestine, abdominal fat pad, liver, bursa of Fabricius, thymus and spleen. Administration of HVT in ovo at the highest dose reduced liveweight and feed intake relative to the sham-vaccinated treatment and had no major effects on feed conversion ratio or organ weight. The process of in ovo injection alone improved feed conversion at week 7. These results suggest that the process of in ovo injection is beneficial to performance in the absence of a challenge, but administration of HVT vaccine during in ovo vaccination neutralises this effect. The concentration of HVT recovered from spleen tissue of HVT-vaccinated birds increased over time, but the dose of vaccine had no effect.