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.

Genetic resistance to two Marek's disease viruses and its relationship to production traits in chickens

“…Both these relationships are desirable negatively correlated with MD incidence or for improving genetic resistance to MD since mortality or total mortality, whereas correla-the early maturing genetic groups with higher tions for the age at first egg were significantly egg production generally showed lower MD incidence than the less-productive and latermaturing ones. The correlations between the susceptibility traits and egg weight were consistently negative ( incidence and age at sexual maturity were also found by Grunder et al (1974). …”

“…A more-detailed description ofthe origin and selection histories of the genetic groups was given by Gavora et al ( 1990 . Inoculum was administered intraperitoneally at 3 weeks of age (Spencer et al 1974 Relationships between the MD incidence and mortality as well as the total mortality in the challenge tests and production traits in the egg production test were analyzed by non-parametric methods. Pearson's product-moment correlations and Spearman's rank correlations corrected for ties in rank (Kendall 1962\ were calculated based on the means of the 23 genetic groups.…”

“…Based on field data, von Krosigk et al (1912) estimated the average heritability for MD resistance at 0.14. Gavora et al (1914) Grunder et al (1974) found that rank correlations between egg production of sires' sibs and incidence of MD in sires' progeny were mostly negative and nonsignificant. But correlations between MD incidence and age at sexual maturity or egg weight were positive.…”

“…A combination of vaccination and genetic resistance is essential to maximize protection Gavora et al 1974; Gavora and Spencer 1979); hence there continues to be a need to increase levels of genetic resistance.…”

Evaluation of resistance to Marek's disease in three white Plymouth rock unes and in two reciprocal crosses

“…The introduction of MD resistance into susceptible commercial strains may offer a practical means for natural control of the disease. This is very important since it has been established that over the years resistance to MD decreases in populations under selection for rapid growth; also a negative correlation between body-weight and MD resistance is observed (Hartmann and Sanz, 1971 ;Han and Smyth, 1972;Gavora et al, 1974). The more MD resistant lines that are developed in a breeding programme for introducing MD resistance into flocks the more helpful this will be in arriving at the most resistant combination.…”