Vaccination Against Marek’s Disease: Efficacy of Cell-Associated and Lyophilized Herepesvirus of Turkeys in Nine Strains of Leghorns

Abstract

Females from six strains selected for high egg production and from three unselected control strains (a total of 11,000 chicks) were pedigree hatched from dams that had been vaccinated with herpesvirus of turkeys (HVT). On the day of hatch, each family of full sisters was divided at random into two groups, one of which was vaccinated with cell-associated HVT (C-HVT) and the other with lyophilized HVT (L-HVT). The chicks were adventitiously exposed to Marek’s disease (MD) virus and were observed to 497 days of age. A third group, consisting of 413 females of one of the unselected strains, was kept with the vaccinated chicks to 139 days of age as non-vaccinated controls.Total rearing mortality of the non-vaccinated pullets was 10.4% and rearing mortality due to MD was 5.6%. A comparison with rearing mortality in the vaccinated chickens of the same strain indicated that both C-HVT and L-HVT protected the pullets against MD. However, on an overall basis, the C-HVT vaccinated groups had a lower total laying house mortality (6.8 vs. 8.5%), laying house mortality due to MD (1.4 vs. 3.0%) and higher hen-housed egg production (246 vs. 242 eggs) than the L-HVT vaccinated groups. In one of the strains tested the L-HVT vaccinated pullets had 7% higher laying house mortality and laid a dozen eggs less than the C-HVT vaccinated pullets. This observation pointed out the dangers associated with drawing general conclusions about effectiveness of vaccines from testing only a small number of genetic stocks of chickens.