Scrapie
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. TSE
road map
- 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).
Human
Rezaei et al
- SUMMARY OF THE RESULTS
OF SCRAPIE SURVEILLANCE IN SHEEP IN GREAT BRITAIN JANUARY 2002 - MARCH 2003
John
Wilesmith, Danny Matthews, Judi Ryan
disclamer
