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Investigation of the potential for the control of sheep scab using immunological approaches and the development of diagnostic tools - OD0553

Sheep scab is caused by the mite, Psoroptes ovis, and is the most important ectoparasitic disease of sheep in the UK. The mites are highly contagious and the developing skin lesion causes intense itching and distress to the sheep. In some animals fitting and death can occur. Since the deregulation in 1992 of organophosphate dipping, with limited re-licensing in 2001, its prevalence has escalated to become endemic in both hill and lowland sheep in all areas of the British Isles. Control of the mites is normally through dipping or injectable endectocides, although there are concerns over the use of these chemicals with respect to increased parasite resistance as well as environmental and animal welfare issues. Indeed, this concern has resulted in the recent deregulation of the cypermethrin dips, leaving only the organophosphate dip Diazinon and the injectable endectocides with which to treat infections. Because of this, alternative methods of control for scab mites are urgently sought, and one such strategy is to harness the natural immune response. However, to achieve this, a much greater understanding of the complex interactions between host and parasite are required and this proposal seeks to define, through a detailed study of the host and parasite genes, the process of infection and immunity.
Indications that this is a feasible alternative has come from our work on the immune responses to P.ovis, where we have shown that previous infection substantially curtails and reduces subsequent infection. However, for an effective vaccine we need to understand the immune mechanisms involved. Studies to date have demonstrated that infection induces an intense reaction in the skin, involving the activities of inflammatory cells and antibody directed against allergens. Early mite-host interactions are important in the initiation of these responses, since the mites are not invasive but induce a rapid reddening and fluid release on the skin within a few hours of being applied. Preliminary studies have shown that there are profound changes in certain skin cells (keratinocytes) including up and down regulation in a number of genes, which invokes a cascade inflammatory reaction resulting in lesion development. Modern technique of microarray will evaluate the changes that occur in the the skin cells, and identify those mite factors which induce these changes. These mite derived proteins are likely to be secreted/excreted antigens; and since their role is likely to be pivotal for mite establishment and/or feeding their identity will provide important vaccine candidates. Recombinant forms of these antigens will be tested for the ability to vaccinate against experimental challenge of mites.
The development of a diagnostic assay is also important; such an assay is required to establish the mite-free status of flocks, and to identify chronically or sub-clinically infected animals, which will be particularly important in any eradication program. This assay will be based on specific antibodies which are generated during infection. We have previously shown that a number of allergens are recognised in infected animals relatively early during infection, and these will be tested together with other candidates to detect specific antibodies in the blood. Recombinant forms of these proteins will be amenable to a ‘dipstick’ technology to provide a rapid, simple and sensitive assay, which could be used by the farmer as a ‘pen-side’ test.
The main outcomes of this study will therefore be a) greater understanding of the underlying factors involved in lesion development in sheep scab, b) identification of potential control by intervention or vaccines , and c) the development of a pen-side diagnostic test for sheep scab.
1. Perform experiment to collect tissue and mites for microarray and antibody test studies (April 2007-Dec 2007)
2. Generation of a P. ovis EST microarray (April 07-March 08)
3. Investigate skin responses to live mites with an ovine skin EST microarray (Jan 08-Oct 09)
4. Investigate mite responses to initiation of parasitism on sheep through the mite EST microarray (Jan 08-Oct 09)
5. Histological and immunohistological assessments on skin tissues (Jan08-Dec 08)
6. Confirm microarray findings by qRT-PCR on selected genes (Sept 09-March 10)
7. Immunoscreen P. ovis cDNA library with serum from timed (1 week-6 week) primary and secondary mite infestations. (Dec 2007-June 2008)
8. Use serum from objective 1 (above) to screen existing recombinant P. ovis antigens to isolate most rapidly identifiable diagnostic recombinant proteins. (June 2008-Jan 2009)
9. From the findings above, select mite candidate antigens as potential vaccine, generate recombinant proteins (Jan 2009-Dec 2009)
10. Perform vaccine trial with recombinant protein(s) (Dec 2009-Mar 2010)
11. Develop Enzyme-linked immunosorbent Assay (ELISA) for specific P.ovis antibodies and determine persistence of antibody post treatment (April 2007- Jan 2009)
12. Adapt assay to ‘dipstick’ technology for evaluation as a pen-side test (July 2008-June 2009)
13. Validate assay from field samples (Jan 2009-Mar 2010)
Time-Scale and Cost
From: 2007

To: 2010

Cost: £734,294
Contractor / Funded Organisations
Moredun Research Institute
Animal Diseases              
Animal Health              
Plants and Animals              
Sheep Scab              
Vaccine Development              
Fields of Study
Animal Health