Despite the possibly minor involvement of a single gene (prnp), what makes sheep susceptible to scrapie remains completely unknown. Government insistence that susceptibility only dependents on the prnp gene is false but is also a dangerous assumption. This present assumption may very well change in the near future. Limiting the national flock to one of limited genetic diversity will, in the next two years, cause the loss of both essential diversity and unique genetic material, possibly forever.
Several breeds have sufficient homozygous sheep, but often in only a few blood lines. Inbreeding will go up, often in association with deteriorating quality and undesirable characteristics. Breed traits associated with the ARQ/ARQ genotype will be lost. Despite the expert generalisations to the contrary, some breeds of sheep with this scrapie-susceptible genotype have never shown any sign of scrapie. These breeds should be used for breeding without any objection.
ARR/ARR sheep do not develop clinical signs but that does not mean that they do not carry the ‘Scrapie agent’. Breeding for scrapie resistance could well turn out to be a disaster, for eliminating genetic diversity exposes a population to pathogens that are unknown or that mutate from known pathogens.
* Scrapie control under new strain
Sheep believed to be resistant to scrapie are succumbing to atypical infections and a newly identified strain of the disease. Eradication programmes based on selective breeding should be reappraised.
Baylis & McIntyre
* TSE roadmap:
The Commission (EU) has discussed on several occasions with the Member States and the European Parliament about the next steps in the BSE policy on different points such as the definition and removal of Specified Risk Material (SRM), the feed ban and the age of testing. Based on the improved situation, the Commission has taken this initiative to present a roadmap on the BSE strategy in the short, medium and long-term.
* Identification of putative atypical scrapie in sheep in Portugal
Since its start over 30 000 animals have been tested, and the first seven cases of sheep with detectable PrPres deposition in the central nervous system have been identified in Portugal. Notably, four animals had genotypes rarely associated with scrapie, including one animal homozygous for A136R154R171.
Leonor Orge et al
* Neuronal accumulation of abnormal prion protein in sheep carrying a scrapie-resistant genotype(PrPARR/ARR)
After the introduction of large-scale rapid testing for scrapie, a number of so-called ‘atypical’ scrapie cases have been found in Germany and elsewhere. Among those cases were two supposedly scrapie-resistant sheep. Brain samples from these animals tested positive for abnormal PrP (PrPSc) in one of four rapid tests available.
Anne Buschmann et al
* Amyloidogenic Unfolding Intermediates Differentiate Sheep Prion Protein Variants
Sheep is a unique example among mammalian species to present a strong correlation between genotype and prion disease susceptibility phenotype. Indeed a well-defined set of PrP polymorphisms at positions 136, 154 and 171 (sheep numbering) govern scrapie susceptibility, ranging from very high susceptibility for V136-R154-Q171 variant (VRQ) to resistance for A136-R154-R171 variant (ARR).
* SUMMARY OF THE RESULTS OF SCRAPIE SURVEILLANCE IN SHEEP IN GREAT BRITAIN JANUARY 2002 - MARCH 2003