title: Determinants of resistance to nematode infection in Scottish blackface sheep
Creators: name: Benothman, Mohamed Ahmed Ezzeddin
Publisher: name: University of Glasgow
Year: 2005
Subjects: subject: Animal diseases, parasitology, veterinary science.

description: Gastrointestinal parasitism is one of the greatest causes of disease and lost productivity in domestic animals. It remains a major constraint on production and welfare in domestic animals in all geographical locations all over the world. Anthelmintic treatment is the mainstay of current control methods. However, with the widespread use of anthelmintics, the problem of parasite resistance has emerged. Other modalities of prevention and treatment are urgently needed. One of the most promising developments in this respect is the use of genetically resistant sheep. Selective breeding of animals resistant to gastrointestinal parasitism is particularly attractive, but identifying the phenotypic and genetic markers of resistance on which selection will be based is a major problem. Hence, this thesis has investigated some of the phenotypic and genetic determinants of resistance to nematode infection in Scottish Blackface sheep when naturally infected with the gastrointestinal nematodes, particularly Teladorsagia circumcincta, and how these determinants may facilitate the successful selection of resistant animals. Chapter three has investigated the seasonal pattern of T. circumcincta infection, as well as the changes in the mean and the distribution of faecal egg counts in Scottish Blackface sheep. There was no discernible pattern to egg counts with each season, and this could be attributed to factors like changes in grazing management, anthelmintic treatment, weather and humidity. Work described in chapter four has shown that there was considerable variation among populations. Faecal egg counts vary in naturally infected sheep, and mean egg counts vary among different populations and among the same population sampled at different times. In addition high mean faecal egg counts are not necessarily due to high intensities of infection, but probably reflect the contribution of species other than T. circumcincta. The present study has not found any correlations between plasma IgA activity against third-stage larvae of T. circumcincta and faecal egg (chapter five). Moreover, there was no significant correlation between growth rates in 24 week old lambs. The results suggest that IgA activity against fourth-stage larvae is probably a better marker of nematode resistance than IgA activity against third-stage larvae. This research has identified three new sequences at MHC class II DRB1 gene of Scottish Blackface sheep (chapter six). In addition, the study has provided evidence for QTL linked to parasitic infection and immune response on chromosome 3 and chromosome 20. The results reached in this study suggest that some aspects of parasite resistance are under strong genetic control, and with further research, this information could be used to select sheep for increased resistance to parasitic infection in marker assisted selection scheme. 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.