| Summary: | The aim of this thesis is to quantify the benefits of selection for resistance to an important sheep disease in Britain. Specific aspects addressed are <i>i</i>) the choice of the specific disease based on economic costs and potential savings from selection, <i>ii)</i> genetic parameters for the disease, such as the heritability <i>(h</i><sup>2</sup>) and the relation with production traits, <i>iii)</i> prediction of the response to selection on a trait that is measured in only two classes (healthy or diseased) and depending on environmental factors affecting exposure and prevalence, and <i>iv)</i> modelling of the combined effect of increased genetic resistance and reduced pathogen burden as a result of selection. It is concluded that footrot is a disease of economic importance, with additive genetic variability. Selection for resistance can be effective if based on simple binary scores, especially if animals are scored repeatedly. The response to such selection can be predicted with a newly developed theory for binary traits, which also covers situations when exposure to infection is variable. Selection for resistance is expected to result in animals that can better cope with the disease, in terms of reduced weight loss. A new epidemiological model predicts likely responses to selection, showing a considerable additional decrease in the prevalence of footrot compared to purely genetic predictions. It is concluded that selection for increased resistance to footrot can be expected to be successful in reducing costs of the disease to the British sheep industry.
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