Studies on genetic resistance of chickens to Marek's disease—A review

Abstract

Marek's disease (MD) in chickens is caused by a herpesvirus and is characterized by development of lymphoid tumors. Although vaccines have dramatically reduced losses from MD, genetic resistance, as well as vaccine-induced immunity are essential to minimize the incidence of this disease. Genetic variation in resistance of chickens to MD has been amply demonstrated. Parallel comparisons of vaccinated and non-vaccinated strains showed that MD incidence was decreased by vaccination but this had not altered the ranking of strains on MD incidence. Significant genetic variation has also been found in resistance of chickens to the MD tumor transplant JMV but data on the correlation of this resistance with resistance to MD were inconsistent and the use of JMV for testing genetic resistance to MD is of questionable value. Estimates of heritability of MD resistance were high (61%) suggesting good potential for genetic improvement. Resistance to MD tends to be genetically associated with lower body weight, lower egg weight and higher egg production. Thus there is a danger of reducing MD resistance in populations under selection for rapid growth and/or high egg weight, while populations selected for high egg production would be expected to maintain or improve their MD resistance. Conclusive evidence is available that the major histocompatibility locus plays a role in MD resistance and this has potential value in commercial poultry breeding.