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Title: Molecular characterisation of the sheep scab mite, Psoroptes ovis
Author: Kenyon, Fiona
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2002
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Sheep scab is a highly contagious ectoparasitic disease that is caused by infestation with the mite, Psoroptes ovis (Acari: Psoroptidae). The disease is an important welfare issue due to the clinical symptoms induced, which include intense pruritis and severe exudative dermatitis. The host response is typical of an immediate hypersensitivity reaction however, there has been little definition of the antigens present in the mites to date. The main aims of this study were to identify the allergens associated with the sheep scab mites, and to determine the main antigens present. The proteases present in the water-soluble (SI) and membrane associated (S2) P. ovis extracts were characterised because of their allergenic potential by analogy with other organisms, particularly house dust mites. This was conducted using a combination of substrate gels and protein degradation assays. Substantial protease activity was present with cysteine proteases dominating, although aspartyl and metallo-proteases were also present. These proteases were found to degrade many substrates found in the mites' natural habitat, e.g. gelatin, collagen, fibronectin, and haemoglobin. The proteases were not inhibited by antibody from animals with a current infestation, however, this does not rule out the possibility that these proteases are targets for the host immune response. A complementary DNA (cDNA) library was prepared from mixed stage mites in a lambda TriplEx2 vector, for the dual purpose of immunoscreening and expressed sequence tag (EST) analysis. Immunoscreening was carried out using sera from animals which had been immunised and protected against mite challenge with mite extracts and 48 cDNA clones were selected. Of these, six were recognised in a differential immunoscreen using sera from strongly protected animals compared with sera from weakly protected animals. Subsequent sequence analysis established that these clones had homology to myosin or myosin-based proteins. The EST analysis of 500 randomly selected cDNA clones identified many novel proteins and this study focused attention on the allergens, proteases and antioxidant enzymes. Eight different allergens were identified including homologues of the major house dust mite (Dermatophagoides spp.) allergens. The group 2 allergens, e.g. Der f2, were most abundant in the EST dataset. Cathepsin B and L cysteine proteases were identified as well as several antioxidant enzymes, e.g. glutathione S-transferase, superoxide dismutase and thioredoxin peroxidase, which may help to protect the mite from toxic free radicals released in the host lesion. This study has shown that there are a range of proteases present in P. ovis mites. Eight putative allergens and three classes of antioxidant enzymes were also identified. These results provide an insight into the physiology of the mite, and the aetiology of the disease.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available