Investigating genetic aspects of the variation in the host response to gastrointestinal parasites in sheep

Analysis of data from 6-month old Scottish Blackface lambs exposed to a mixed, natural nematode infection demonstrated that the indicator traits, faecal egg counts (FEC), immunoglobulin A activity, eosinophil count, plasma pepsinogen activity and fructosamine concentration, investigated at 6 months of age were highly heritable and strongly correlated with the worm development traits. Strong negative genetic correlations (<-0.06) were often observed between worm development traits and eosinophil count, IgA activity and pepsinogen activity. A substantial genetic correlation was also observed between fructosamine concentration and worm length (0.67). However when such correlations were investigated across the 6-month time-period, the genetic correlations changed systemically and dramatically over time. These results provide an insight into the evolution of the genetic basis of the host prarasite interaction at a time when the host immune response is developing, and help to define optimal measurement ages for selection purposes. Two quantitative trait loci (QTL) studies were carried out on populations comprising different breeds and population structure; firstly a purebred Scottish Blackface flock and secondly a wide-breed cross flock developed from a resistant breed, Gulf Coast Native, and a susceptible breed, Suffolk. Both studies identified QTL associated with parasitic resistance traits, and although there is no concordance between the results, this is possibly due to the animals being infected with different nematode species. QTL such as those identified in this thesis could be utilised in a marker assisted selection scheme to increase resistance to parasitic infection. In the final study interactions between different parasite specifies within the host animal were investigated. Significant interactions were observed between Cooperia and Teladorsagia circumcincta, and T. circumcinta and Trichostrongylus vitrinus. Additionally Cooperia had a greater effect on FEC than T. circumcincta. The results from this study indicate that complex multi-parasitic relationships exist and hence when developing new control strategies it is essential to consider this background multi-parasitic infection and not simply focus on specific species