Experimental transmission of atypical scrapie to sheep

This report presents the successful experimental transmission, in 378 days, of atypical scrapie to a recipient sheep of homologous genotype with preservation of the pathological and molecular characteristics of the donor. This isolate also transmitted to ovinised transgenic mice (Tg338) with a murine phenotype indistinguishable from that of Nor 98.This result strengthens the opinion that these cases result from a distinct strain of scrapie agent, which is potentially transmissible in the natural host under field conditions.TSE in sheep have been recorded for more than 250 years in the form of classical scrapie. In classical scrapie, there is a clear link between susceptibility and sheep genotype [1], and many countries are now implementing disease control programmes based on breeding for resistance. Host genotype also appears to affect to some extent the ultimate pathological presentation of the disease [2-4] as do specific scrapie 'strains' [5] and this complex interaction is not yet clearly understood, although it is widely accepted that several variants of classical scrapie exist in the field. However, in 2003, a newly identified form of scrapie (Nor98) was reported in Norway, which was distinct from the established range of classical scrapie phenotypes [6].In 2002, regulatory requirements for active TSE surveillance were implemented across the EU, and a number of countries have subsequently reported atypical isolates, full details of which can be found in the EFSA opinion of 2005 [7]. It has also recently been reported for the first time in the Southern hemisphere, in a sheep native to the Falkland Islands [8].The current minimum scrapie control measures within the EU require selective culling within a flock, based on genotype susceptibility to classical scrapie. However, the susceptible genotype range for atypical cases differs from that of classical disease [7,9-11] with most cases occurring in genotypes associated with relative resistance to classical scrapie