Genetic variability of resistance to Marek's Disease (MD), and relationships between MD incidence and egg production traits were studied in 23 genetic groups of experimental and commercial meat and egg chickens. In the challenge tests, chickens were injected with either moderately virulent BC-1 MD virus (BC-1) or highly virulent RB-1B MD virus (RB-1B). In the 497-d egg-production test, sisters of challenged birds were vaccinated for MD on the day of hatch and there were no outbreaks of MD. Heritabilities for MD incidence, MD mortality, and total mortality in the challenge tests, based on data from three unselected control strains of Leghorns were generally low (0.06, 0.13 and 0.20) with BC-1. With RB-1B, the heritability estimates were significantly (P <.05) higher (0.62, 1.00 and 0.96). Intraclass correlations, based on variation between and within the genetic groups were also higher with RB-1B. In the test with BC-1 there were deaths from an early mortality syndrome (EMS) prior to development of MD. Losses from EMS in Leghorns were highest in 2 genetic groups most resistant to MD. Based on genetic group means, correlations between MD incidence and egg production and egg weight were high and negative. In contrast the correlations between MD incidence, age at first egg, total blood spots, and egg specific gravity tended to be positive. No consistent relationships were found with adult body weight. Key words: Marek's disease, genetic resistance, heritability, genetic correlations, production traits, chickens
Influence of Selection for Egg Production and Marek's Disease Resistance on the Incidence of Endogenous Viral Genes in White Leghorns
