A poultry breeder's approach to avian neoplasia

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Abstract
Neoplastic disease in chickens is attributable to three groups of viruses: Marek's disease virus, reticuloen‐dothelial virus and avian leukosis virus. This paper reviews the different strategies employed in breeding companies to minimize their impact on performance in the field. The theme of the paper is variation, and the problems and opportunities that variation presents to the geneticist. The ideal of the geneticist is to breed chickens free of these pathogens and resistant to their effects if challenged. A successful programme would also require that the security systems in place exclude the re‐introduction of the viruses and that the vaccines present no risk of introducing infection. Improving genetic resistance of commercial strains is often proposed as the best way forward. However, the rate of evolution of pathogens and the dangers of narrowing the genetic base may undermine this approach. Nevertheless, a successful strategy with resistance to Marek's disease is described. Achieving and maintaining freedom from ALV is a major focus in all breeding companies. The direct costs of eradication programmes are substantial, but they also have an indirect cost by diverting time and energy from other traits under selection. Current performance in the field indicates that the combined use of vaccination, biosecurity and genetics to control these pathogens has been very effective.