Recent findings of scrapie protective genotypes in Iceland supported by case-control studies in Icelandic scrapie flocks.

Scrapie in sheep is endemic in Northern Iceland and has endured rigorous methods to fight the disease in the past. Breeding for scrapie resistance could not be adopted until recently after a widespread search for scrapie protective prion protein variants in Iceland resulted in the first findings of Icelandic sheep carrying ARR. Furthermore, AT₁₃₇RQ, shown to be protective against scrapie in Sarda sheep in Italy, was found on several farms.

A case-control study was performed on 18 Icelandic scrapie flocks detected between 1997 and 2024 and originating from six different movement protection zones. The purpose of the study was to access the influence of possibly protective genotypes in Icelandic sheep. Total number of sheep analysed was 4978 of which 406 tested positive for scrapie. Total number of 20 different PRNP genotypes were detected, when polymorphism at codons 137, 138 and 151 was considered in addition to 136, 154 and 171. Among the observed genotypes with a significant lower scrapie susceptibility compared to the wild type (ARQ/ARQ) were AC₁₅₁/ARQ, AHQ/ARQ and AHQ/VRQ. No scrapie-positive sheep carrying T137 was identified; however, the frequency of genotypes with T137 in scrapie-affected flocks was too low for statistical validation.

A breeding program using ARR is already in use and based on the results of this case-control study, additional selection on three more alleles, AT₁₃₇RQ, AC₁₅₁RQ and AHQ, is recommended, both to assist in keeping the diversity of the sheep population and possibly provide protection against unforeseen changes in the scrapie infectious agent in the future.