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 The
National Scrapie Plan (NSP) for Great Britain was launched in July
2001 following recommendations in 1999 from the sheep sub-group of
the Spongiform Encephalopathy Advisory Committee (SEAC), writes Dr
Richard Warner, scientific adviser with NSPAC.
The NSP is a long-term voluntary initiative to eradicate scrapie
(and hence address the theoretical risk of BSE) from the national
flock through a programme of selective breeding under which sheep
which are genetically susceptible to TSEs are removed over time.
Scrapie is a transmissible and invariably fatal brain disease of
sheep that is estimated to cause illness in somewhere between 5-10
000 animals in the GB every year.
Presently the NSP provides a voluntary blood sampling, genotyping
and advisory service to the pure-breeding sector. More recently however,
the EU has adopted legislation that requires the introduction of
compulsory genotype based breeding programmes in all Member States
from April 2005. The NSP wishes to encourage as many breeders to
join the voluntary schemes in advance of 2005.
The testing process
The central genotyping/ breeding programmes are the NSP Ram Genotyping Schemes. Once the membership contract is authorised, the National Scrapie Plan Administration Centre (NSPAC) agrees dates on which an NSP sampler will visit the holding (usually one visit per year for three years).
Over the period we hope to collect blood samples from all stock rams, and a proportion of that year's ram lamb crop. Precise numbers of animals tested depend on the size of the flock and the proportion of ram lambs/ shearlings sold on for breeding but will not be less than 40 animals per visit.
If the number of male animals available for testing falls below this minimum then we will make up the difference by testing females. Blood samples are sent to our laboratories directly after sampling and we undertake to return genotype results to the breeder within 15 working days.
Sampled animals receive an Electronic Identification (EID) Device in the form of a ruminal bolus. This is a very important aspect of the sampling visit, and allows NSPAC to maintain a traceable link between the animal, blood sample and genotype result.
NSP restrictions on breeding and sale
There is good evidence from field cases and from experimental studies that scrapie
infection and disease progression has a strong genetic basis.
The genetic element exists in 5 forms in sheep, defined by a three-letter code.
These are ARR, AHQ, ARH, ARQ and VRQ. Broadly, ARR is associated with a low
degree of scrapie susceptibility whereas VRQ (and to some extent ARQ) are linked
to
disease susceptibility. A sheep will inherit two copies of the gene from its
parents and the NSPAC two-part result (or "genotype") describes both
copies (e.g. ARR/VRQ or ARQ/ARQ). NSPAC have sorted the 15 known genotypes
into 5 categories or 'NSP types' defined by the restrictions placed on the
use of
male animals sampled within the Ram Genotyping Schemes.
No restrictions apply currently to use of females, whatever genotype. Of course
several breeds have long-promoted their own genotype based schemes for scrapie
resistance, including the Swaledale, and participating flocks should be well
positioned when joining the NSP.
The table below shows how the Swaledale classification relates to the genotypes
(2-part result) and types employed by the NSP. NSPAC issues genotype certificates
for males with results of type 1-3, and for females of type 1 & 2 and breeders
are obliged to arrange the slaughter or sterilisation of male animals with type
4 & 5 results. Males with type 3 results, ARQ/ARQ especially, are viewed
as having relatively disadvantageous genotypes.
These animals are themselves susceptible and pose the problem that if used for
breeding will pass on risk-associated genes to all progeny. However, as type
3s can account for a sizeable proportion of all animals tested (the proportion
differs from breed to breed) and can contain breeding animals with desirable
performance traits, breeders are permitted to use them for on farm breeding and
to sell for a restricted period.
Recently, the time-windows for use and sale have been further extended for hill
breeds in response to industry concerns. Results to date The chart above shows
the patterns of accumulated NSP genotype results for three breeds - the Texel,
Swaledale and Bluefaced Leicester (correct as of start September 2003, both males
and females represented).
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Bluefaced Leicester |
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Swaledale |
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| NSP Type |
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