Disease & heat resistance

Overview of 20 years of selection for scrapie resistance in the Canadian sheep industry

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Abstract

Scrapie is a fatal disease affecting the central nervous system which has spread almost worldwide. In Canada, the disease was first reported in sheep in 1938 and, like many other countries, has been designated for national surveillance. Genetic selection to increase overall flocks' resistance was initiated around 2005 with the National Genotyping Survey. Since then, most breeders have selected for haplotypes known to increased resistance, signed by the PRNP gene's 136A, 154R, and 171R codons. It is estimated that this intensive selection, combined with the culling of many herds due to the presence of the disease, has contributed to a decrease in genetic diversity. The aim of this study was to document, twenty years after the first national study, the haplotype frequencies and to look into effects that this selection may have had on the genome of the Romanov breed, the most common purebred raised in Canada. After removal of first-degree relatives, 700 animals genotyped between 2023 and 2025 and 41,419 filtered SNPs were retained for analysis. PRNP haplotypes' frequencies were compared to those from previous Canadian studies (i.e., 2003, 2012, and 2017). Selection signatures were detected by comparing methods based on haplotypes (normalized nSL and integrated haplotype score (iHS)), reduced local variability (runs of homozygosity; ROH) and site frequency spectrum (Tajima's D statistics). The results showed a 61.2% increase of the resistant ARR and a 57.7% decrease of the wildtype ARQ haplotypes since 2003. Linear regression over today's and previous data showed an annual allelic change of 3% and -3%, respectively. VRQ haplotype, which stood at 4.2% in 2003, now stands at 0.2%, matching ARH frequency. Analysis of selection signatures revealed a strong sweep around the PRNP gene, evidenced by the presence of PRNP polymorphisms in the significant (p The quasi-fixed ARR haplotype demonstrates the extent of the effort made over the past 20 years to increase the overall scrapie resistance of Canadian sheep flocks but also raises concerns about the impact that this type of selection could have on local genetic diversity, as shown by the observed selection sweep. Recently, Canadian breeders have begun selecting for other disease resistance alleles, such as those related to lentivirus infection; the genomic data generated by this project will act as an initial basis for further studies.

Keywords: 2026

How to Cite:

Poisson, W., Michaud, C., Cameron, J., Gilbert, I., Perecin-Nociti, R. & Robert, C., (2026) “Overview of 20 years of selection for scrapie resistance in the Canadian sheep industry”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2287106. doi: https://doi.org/10.31274/wcgalp.24212

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