Gastrointestinal parasites infect all grazing livestock and are a major cause of economic loss. Each year in the UK, gastrointestinal parasites cost the sheep industry an estimated £85 million. Current control strategies are based on anthelmintic treatment; however parasite resistance to anthelmintic compounds is becoming an ever-increasing problem worldwide. Thus alternative control measures are now sought. This thesis comprises a number of studies which aim to investigate the genetic control of several aspects of the host response, and thus the application of such knowledge to develop alternative control strategies for gastrointestinal parasites in a commercial sheep population. 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. For example at a mean age of 22 weeks the heritabilities (± SE) for fructosamine concentration, IgA activity, eosinophil count and pepsinogen activity were 0.39 ± 0.16, 0.57 ± 0.15, 0.35 ± 0.15 and 0.56 ± 0.16 respectively. Strong negative genetic correlations «-0.6) 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 systematically and dramatically over time. For example, for all worm development traits, genetic correlations with eosinophil count were initially positive and moderate to strong, and then declined dramatically eventually becoming moderate to strong and negative at 5 months of age. These results provide an insight into the evolution of the genetic basis of the host parasite 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 aresistant breed, Gulf Coast Native, and a susceptible breed, Suffolk. Both studies identified OTL 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. In the Blackface study OTL associated with specific IgA activity were identified in chromosomes 3 and 20, in regions close to IFNG (chromosome 3) and the MHC (chromosome 20). OTL associated with Nematodirus FEC were identified on chromosomes 2, 3 and 14 and OTL associated with non-Nematodirus Strongyle FEC were identified on chromosomes 3 and 20. In the Suffolk x Gulf Coast Native study OTL associated with packed cell volume (PCV) were identified on chromosomes 1, 9 and 19 and with FEC on chromosomes 1, 6 and 19. OTL 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 species within the host animal were investigated. Significant interactions were observed between Cooperia and Te/adorsagia circumcincta, and T. circumcincta 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 a specific species. In conclusion this thesis provides evidence that many aspects of the host response are under some level of genetic control. Highly heritable indicator traits have been identified along with OTL associated with resistance traits, both of which could be utilised as selection criteria to increase the response to selection for resistance to gastrointestinal parasites within a commercial sheep population. 14
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:486930 |
| Date | January 2006 |
| Creators | Davies, Gail |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/3236/ |
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