A case–control study of scrapie Nor98 in Norwegian sheep flocks

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Abstract
Scrapie is a fatal, neurological disease of sheep and goats and belongs to the transmissible spongiform encephalopathies. In 1998, a new type of scrapie, designated scrapie Nor98, was detected in Norway. Scrapie Nor98 differs from classical scrapie in the distribution of pathological changes and of the scrapie prion protein, the Western blot profile of the prion protein, and with isolated cases usually being observed in the case flocks. In 2004, a case–control study was conducted on scrapie Nor98 with 28 cases and 102 randomly selected controls. The questionnaire included questions on demographic data, animal contact between sheep flocks, indirect contact with equipment, use of concentrate feed and supplemental feeds, and use of medicines and vaccines. The data were analysed by using logistic regression with the sheep flock as the statistical unit. In the final model, the detection of scrapie Nor98 was related to the practice of not removing all afterbirths, the use of vitamin and mineral feed supplements, the absence of concentrate feed of swine or poultry on the farm and the presence of dogs on the farm. The results show that the epidemiology of scrapie Nor98 differs from that of classical scrapie in that no risk factors that indicate transmission of scrapie Nor98 between flocks by movement or direct contact between animals were found. Furthermore, the association between scrapie Nor98 and mineral intake shown herein should be explored further. Although the possibility that scrapie Nor98 has a low transmissibility between animals under natural conditions cannot be ruled out, the results would also be in accordance with a spontaneous aetiology.