Year: 2008
Type: dissertation
Abstract: 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) the choice of the specific disease based on economic costs and potential savings from selection, ii) genetic parameters for the disease, such as the heritability (h2) and the genetic and phenotypic relationships with production traits, iii) prediction of the response to selection on a trait that is measured in only two classes (healthy or diseased), accounting for variable exposure and prevalence, and iv) modelling of the combined effect of increased genetic resistance and reduced pathogen burden as a result of selection.It is shown that the major sheep diseases in Britain, in terms of annual costs to the British sheep industry are gastro-intestinal parasites, footrot, Chlamydial abortions (enzootic abortions of ewes) and Toxoplasmosis, with estimated annual total (variable) costs of £84M (f 84M), £24M (E12M), £20M (E1M) and £12M (9M).A commercial service for selection against gastro -intestinal parasites already exists in Britain and elsewhere, and footrot is identified as an opportunity to make substantial savings to the industry.Footrot is a highly contagious disease, caused by the bacteria Dichelobacter nodosus, which is influenced by environmental factors.It affects the skin between the digits Of a hoof resulting in lameness.Vaccines exist, but are expensive and have short protection periods.Resistance to footrot has been reported to show additive genetic variability, and this is confirmed in this thesis.A total of 4340 Scottish Blackface on 5 farms and 726 mule ewes in 3 flocks were scored repeatedly for lesions that are indicative of footrot by a team at SAC. Results showed that in these adult sheep lesion scores are heritable, with the h2 typically around 0.2 (for footrot 0.19 in SBF, 0.12 in mules; for severe footrot 0.26 and 0.19 respectively).This simple binary score turned out to be as effective as more complex criteria that considered the severity of lesions.Phenotypic correlations between successive scores in the same animals were generally close to 0, but genetic correlations were higher.The repeatability of footrot -Publications related to this thesis
Author: G.J. Nieuwhof
Language: English
Subfield: Genetics
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2339156290