Scrapie has been endemic in Iceland for more than 140 years. The disease affects the central nervous system and is incurable. The infectious agent is an abnormal form of an endogenous protein, PrPC. The abnormal form, PrPSc, forms amyloid deposits and is resistant to proteolytic digestion.
Scrapie diagnosis is based on detection of PrPSc in brain samples. Prior to 2005, scrapie diagnosis at Keldur was mainly based on immunohistochemistry. In 2005 the Institute started to use ELISA for screening. A dedicated laboratory was built up for this task. The ELISA method (TeSeE, Bio-Rad) is based on isolation of PrPSc from brain tissue, i.e. the medulla oblongata and cerebellum. Samples that are positive by ELISA are confirmed by Western-blot.
The ELISA method can detect classical scrapie and BSE (bovine spongiform encephalopathy) as well as atypical scrapie, or Nor98. Nor98 has been detected in several European countries since it was first described in Norway. It has been diagnosed on seven Icelandic farms since 2004. This atypical scrapie is most often detected in older sheep, and is usually restricted to one sheep per herd. Nor98 differs from classical scrapie in terms of pathology, i.e. it has a different distribution of tissue damage and the aggregation pattern of PrPSc in brain tissue differs. Furthermore, sheep affected by Nor98 have different PrP genotypes compared to sheep affected by classical scrapie. Nor98 is diagnosed by Western blot after ELISA as it has a different band pattern than classical scrapie.