ABSTRACT
Avian diseases in the broad sense include health problems arising from adverse physical environment or inadequate nutrition, developmental disorders, and infectious and parasitic diseases. Genetic improvement is of particular importance in the reduction of losses from the latter two groups of diseases. There is considerable knowledge of genetic mechanisms underlying many developmental disorders, mainly where major genes are involved, but their molecular basis is largely unknown. Ample evidence exists that genetic factors play an important role in resistance to infectious diseases. Such resistance appears to be controlled by polygenes, and major genes underlying the resistance have been identified in only a few instances. Knowledge of molecular bases of resistance mechanisms, such as the major histocompatibility system, immune response, and pathogen receptors, is rapidly increasing. Also, the molecular structure of many pathogen genomes is known and can be used in devising approaches to the improvement of host resistance. Molecular genetic information may provide tools for genetic improvement of resistance by selection using markers that are either linked to or are themselves components of resistance genes. Such selection does not require exposure to pathogens and, therefore, is desirable from the animal-welfare point of view. As well, DNA fingerprinting can improve the efficiency of transfer of genetic resistance by crossbreeding and backcrossing. For industrial applications, molecular gene transfer is justified primarily to introduce new resistance mechanisms, exemplified by pathogen-mediated resistance or antisense RNA, that do not exist in the avian species. Improvements of existing resistance mechanisms by gene transfer should await perfection of methods such as the culture of avian embryonic stem cells and homologous recombination. Progress in molecular mapping of avian genomes and further elucidation of molecular bases of resistance will play an important role in future improvements of genetic resistance to diseases in poultry.
