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An assessment of effects of vaccination on genetic resistance to Marek's disease.

Abstract Relationship between resistance to Marek’s disease (MD) and production traits in vaccinated and non-vaccinated birds was studied over two generations in two Leghorn strains under selection for high egg production and related characters. In two experiments female progeny of a total of 116 sires and 758 dams were injected with MD virus at 3 weeks and survivors examined for lesions of MD at 12 weeks of age. Full sisters of the challenged birds were placed on 497 day egg production tests in which they were adventitiously exposed to MD. In the second generation all birds on the egg production test were vaccinated for MD. Average heritability of resistance to MD among birds injected with MD virus was .61. Heritability of laying house livability which included mortality from MD and from other causes was .11. Correlations between resistance to MD and production traits indicate that birds genetically more resistant to MD tend to have lower body weight, to mature earlier and to lay smaller eggs. Positive genetic correlations observed between resistance to MD and the egg production rate in vaccinated birds would tend to maintain or increase the genetic resistance to MD in populations under selection for high egg production, even though vaccination would reduce the effects of natural selection for resistance.