Studies on the pathology of Marek's disease following challenge in chicks vaccinated with three different vaccines

The pathology of Marek's disease (MD) was studied in chickens vaccinated separately with three different vaccines, namely herpesvirus of turkeys (HVT), inactivated Marek's disease virus (IMDV) and a bivalent vaccine. The MD-specific mortality following challenge was significantly lower (2.0 per cent) in bivalent vaccinated birds than in those vaccinated with either HVT (5.9 per cent) or IMDV (13.7 per cent) alone. The occurrence, extent and severity of gross lymphomas and lymphoproliferative lesions in visceral organs was lower in the bivalent-vaccinated birds than in other groups. Atrophy and degenerative changes were seen in unvaccinated and challenged birds but not in challenged and bivalent-vaccinated birds. Mild degenerative lesions and slight RE cell hyperplasia occurred in HVT- and IMDV-inoculated and challenged birds. The low intensity and frequency of gross and histological lesions in bivalent-vaccinated birds following challenge is highly suggestive of better vaccine-induced resistance.