Scrapie, disease and tests
Scrapie (rida in Icelandic) has been endemic in Iceland for approximately 140 years and has heavily affected sheep farming through the years, since whole flocks are culled after diagnosis of classical scrapie on a farm. Scrapie is an infectious fatal neurodegenerative disease, with no available vaccine or treatment. It is characterised by the deposition of PrPSc, a pathologic isoform of a normal prion protein PrPC, which is found in all mammals.
The diagnosis of the disease is based on detection of the pathologic prion protein, PrPSc, in brain samples. At Keldur, Elisa tests have been used for scrapie testing since 2005, replacing for the most part histopathological staining. The scrapie testing is performed in a specialized laboratory designated for that type of research. The Elisa test (TeSeE, Bio-Rad) is based on isolation of PrPSc from brain tissue; obex and cerebellum, and measuring the amount with the help of specific antibodies. If samples test positive by Elisa, they are tested with western blot for confirmation.
Besides BSE and classical scrapie, the Elisa test detects atypical scrapie cases called Nor98. They have been detected all over Europe, but the first Nor98 case in Iceland was detected in 2004 and since then on six additional farms. These cases are often found in older sheep and usually there is only one positive sheep found in each flock. These cases show different pattern of tissue damage and depoition of PrPSc compared to classical scrapie. The diagnosis is based on a specific band pattern in western blotting.
Since 1995 the relationship between PrP genotypes and scrapie in sheep has been studied at Keldur, but it is known that polymorphism within the prion gene is important for how sensitive sheep are to scrapie. Different haplotypes at codons 136, 154 and 171 are related to risk (VRQ) and lower sensitivity (AHQ) to classical scrapie in Icelandic sheep, however the haplotype recognised for the most protective properties (ARR) has not been detected in Icelandic sheep.
All scrapie cases detected are genotyped and a selected group from the scrapie affected flocks. Through the years two cases of classical scrapie have been detected carrying AHQ, but that genotype had been thought of as the most promising genotype towards decreased sensitivity in Icelandic sheep. Those cases were not showing any clinical signs at the time of diagnosis. This genotype is detected in many of the Nor98 cases, but that scrapie type shows a different pattern in regard to sensitivity of PrP genotypes than classical scrapie.