Immunity to Teladorsagia circumcincta infection in Scottish blackface sheep : an investigation into the kinetics of the immune response, antigen recognition and the MHC
The kinetics of the host's immune responses to challenge infection were studied and identified clear patterns in plasma IgA activity, peripheral eosinophil counts, faecal egg counts and plasma pepsinogen concentrations but not in plasma IgG activity. It was determined that when used in parallel and when tested at multiple time points, these parameters have much greater potential as markers of resistance than when used individually or more importantly if only assessed on a single occasion. Further work investigated the recognition of stage specific parasite antigens by host plasma IgA by Western blotting. After adjusting for differences in the activity of IgA in each plasma sample the work in this thesis identified that preferential recognition of a different set of antigens was associated with resistance in the group of experimentally challenged animals compared to previous publications. Additionally, and for the first time this investigation was also carried out on naturally infected animals. There was little correlation in the patterns of antigen recognition between the experimentally challenged and naturally infected animals. Finally, the role of MHC was investigated and it was determined that MHC heterozygotes produced significantly more plasma IgA then MHC homozygotes but did not harbour significantly shorter worms. The analysis also confirmed in naturally infected sheep that there was no obvious relationship between MHC polymorphism and antigen recognition. The results suggested that resistance was due to the recognition of several molecules rather than a single molecule. The work detailed in this thesis has further increased our understanding of the complex host/parasite relationship and has confirmed that selective breeding using the various phenotypic and genetic markers studied is possible. However, this will only be viable if the tests involved in assessing these traits become cheaper and easier to perform, especially if they are to be carried out by the farmer, on the farm.