| Summary: | Background: Classical scrapie is a contagious, fatal neurodegenerative disease that affects sheep and goats. It has been endemic in Icelandic sheep for about 140 years. The infectious agent is a prion, a pathological form (PrPSc) of the cellular prion protein (PrPC), which is expressed by the Prnp gene. The infectivity of PrPSc is influenced by Prnp genotypes of the host, as they convey different resistance/susceptibility to classical scrapie. This knowledge led many countries to breed sheep with the resistant genotype ARR/ARR, but this genotype has never been found in Iceland. In 2005 it became mandatory for EU member states to select for resistance in their sheep breeding, under an EU regulation. Three years later Iceland, a non-member state, had removed all rams carrying a VRQ allele, which is most susceptible to classical scrapie, from the country’s breeding stations. Aims: This study assessed the effect of removing VRQ-allele-carrying rams from Icelandic sheep breeding stations on the distribution of Prnp genotypes in classical scrapie flocks and their clinical suspect index samples, as well as the age of the index samples. Materials and methods: The above-mentioned assessment was performed by comparing classical scrapie flocks, as well as their clinical suspect index samples, from the years 2010-2019 (experimental group, n = 1450 and 10, respectively) to 1998-2007 (control group, n = 1081 and 32, respectively). The process for genotyping samples was DNA isolation, PCR, RFLP and electrophoresis. The age of the index cases was obtained from their ear tags. When comparing the distribution of Prnp genotypes between the groups, for both the flocks and the index samples, a Chi square test was used, and a Mann-Whitney test was used for age comparison. Results: A significant difference was detected in the frequency of the corresponding amino acids at codons 136 and 154 (p < 0.0001 for both) in the classical scrapie flocks. However, there was no difference when comparing codon 136, in the clinical suspect index ...
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