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Title: Genetic resistance to helminth infections of sheep
Author: Altaif, Khalil Ibrahim
ISNI:       0000 0001 3418 5558
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1975
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That some breeds of sheep and individuals within these breeds thrive better than others in parasite-infested localities has probably been recognised by stockmen for centuries, but it is only within the past 50 years, and largely as a result of field surveys that the existence of genetically-determined differences in host resistance to parasitic infections has become widely recognised. As yet remarkably little is known about the genotypes responsible for resistance or susceptibility and even less of the mechanisms involved. Within the past decade several reports have suggested that resistance to some parasites is associated with the animal's haemoglobin type, sheep with HbA being more resistant as judged by faecal egg counts and venous haematocrits than those with HbB. Since the frequency of these haemoglobin types varies in different breeds it has been tacitly assumed that inter-breed variations in resistance are related to differences in the relative haemoglobin type gene frequencies, but none of these studies has indicated whether such resistance is expressed primarily as a resistance to parasite establishment or resistance to the parasites' specific pathogenic effects. In view of its potential practical significance it was considered worthwhile to examine the concept of breed and haemoglobin type resistances to parasitic infections and their possible relationship to the immunological status of the host under carefully controlled experimental conditions. The work represented in this thesis is essentially a study of the role of genetic factors in resistance to two important gastrointestinal helminth parasites of sheep, i.e. Haemonchus contortus and Ostertagia circumcincta. The first section deals with the inter-relationship between haemoglobin type and breed and the response of sheep to primary infections with H. contortus. For this purpose worm-free Scottish Blackface and Finn Dorset sheep with different haemoglobin types were each infected with 350 H. contortus larvae/kg bodyweight and their responses monitored and compared by a combination of clinical, radioisotopic and parasitological techniques. The results obtained showed that sheep with HbA developed less severe clinical and pathophysiological disturbances, passed fewer eggs and harboured fewer worms at necropsy than animals with HbB and that Scottish Blackface sheep exhibited similar advantages over Finn Dorsets with the same haemoglobin type. Since variations in the severity of the disease as judged by pathophysiological effects correlated closely with worm numbers it was concluded (a) that genetic resistance operates at the level of parasite establishment which in turn is controlled by the immune response elicited, (b) that although HbA is a useful genetic marker for resistance, the degree of protection with which it is associated is very much influenced by other, and as yet undefined "breed" characteristics, and (c) on the basis of a second experiment demonstrating that sheep of each haemoglobin type were equally susceptible to the establishment and pathogenic effects of H. contortus when heavily infected (1400 larvae/kg) , it would appear that the magnitude of the larval intake is an additional factor involved. The second and third sections are devoted to an examination of the influence of breed and haemoglobin type on acquired resistance to H. contortus, the former dealing with the acquisition of resistance from primary infections terminated by anthelmintic treatment, the latter with the well-known "self-cure" phenomenon. The results of the third section demonstrated that individuals and breeds with high resistance to primary infections, i.e. animals with HbA and belonging to the Scottish Blackface breed are also more resistant to reinfection than for example HbB and Finn Dorset sheep. The final section examines the genetic control of antibody production to non-parasitic antigens. No difference was observed between HbA and HbB type sheep, or between Scottish Blackfaces and Finn Dorsets in respect of immune elimination of horse gamma globulin, but on tlie basis of a significantly better response to human serum albumin, and suggestive evidence of a better response to rabbit red cells in HbA than in HbB Scottish Blackface sheep, it seemed reasonable to conclude that the advantages exhibited by the former in relation to parasitic infections were associated with a superior immunological competence.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available