Evidence for host genetic variation in aspects of disease resistance has been documented for more than 50 diseases, in all major domestic livestock species. Almost certainly there is genetic variation in resistance to many more diseases. However, the term disease resistance is used to mean many different things and definitions are important to avoid confusion. Infection may be defined as the colonization of a host by organisms such as viruses, bacteria, protozoa, helminths, and ectoparasites, whereas disease describes the pathogenic consequence of infection. Disease resistance is used generically to cover resistance to infection, i.e., a host's ability to moderate the pathogen or parasite life cycle, and also resistance to the disease consequence of infection. Sometimes tolerance is used to describe a host's ability to withstand pathogenic effects of infection.
Genetic variation in disease resistance may, sometimes, be due predominantly to allelic variation at a single gene. Examples include resistance to various forms of Escherichia coli diarrhoea in pigs and the PrP gene, which is associated with resistance of sheep to scrapie. In other cases, variation in resistance may be due to the combined effects of allelic variation at several or many genes, e.g., nematode resistance in grazing ruminants and mastitis resistance in dairy cattle and sheep. Additionally, although resistance and tolerance are sometimes qualitative phenomena, more often they are quantitative traits, i.e., they show continuous variation from one extreme to the other. Continuous variation is expected when resistance is due to the combined effects of several genes, along with environmental effects.
Many processes may control resistance or tolerance. For example:
• The host may have an appropriately targeted immune response, enabling it to successfully combat an infection or avoid pathogenic effects of disease.
• The host may have nonimmune response genes that preclude infection, or limit infection in target organs.
• The host may have physical attributes that make infection difficult, e.g., the role of skin thickness in helping to confer resistance to ticks.
• The host may have behavioral attributes that enable it to avoid infection. An example is the hygienic behavior of honeybees that helps in their defense against diseases such as American Foulbrood and Chalkbrood.
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