Current status of scrapie. CAB review for Perspectives in Agriculture

Despite being controlled in many developed countries, scrapie remains endemic in many parts of the world. Economic losses result from mortality and culling of small ruminants and from market restrictions. Moreover, it is difficult to develop all-inclusive guidelines that could establish a scrapie-free status for a country. Unfortunately, the global picture remains incomplete because in many countries confusion still remains regarding the clinical picture of scrapie and information is not available owing to the absence of adequate epidemiosurveillance networks. Currently, the predominant theory is that PrPSc is the infectious agent where host genetic factors play a central role. The precise transmission routes of scrapie and their relative contributions to the overall transmission intensity remain poorly documented and the physiopathology is not fully understood. However, it is evident that the purchase of female sheep from scrapie flocks, sharing pastures with scrapie flocks, sharing breeding rams and genetic host susceptibility are the main risk factors for the spread of the disease. A better understanding of the epidemiology of scrapie would greatly aid the development and evaluation of control and eradication strategies that were mainly based on selective depopulation of infected animals and genetically susceptible and/or related animals and also on the biosecurity and the use of selective genetic breeding programmes in healthy flocks. Some numbers of a new transmissible spongiform encephalopathy (TSE) form in small ruminants (atypical scrapie) have meanwhile been identified by TSE rapid testing using an assay, which also recognizes comparatively less proteinase K-resistant PrPSc. Uncertainties remain regarding the pathogenesis of this new TSE form, as well as regarding its potential transmissibility within the affected species and to other species. Thus far, no bovine spongiform encephalopathy (BSE) cases have been confirmed in sheep under natural conditions (a report of vertical transmission after experimental infection merits attention), but two historical cases of BSE in goats born in the 1990s have been identified. Currently BSE must also be considered in the differential diagnosis of scrapie. The development of prevention and control programmes should be assisted by new scientific findings.