National Scrapie Plan
(NSP).
Updated: 17/02/2007
The NSP was set up to increase
the proportion of scrapie resistant sheep within the UK flock population and to
ultimately remove the susceptible sheep in order to remove any risk from Transmissible
Spongiform Encephalopathies (TSEs) in the food chain. The scheme is based on
genotype testing to identify resistant and susceptible sheep and the selective
breeding to increase the occurrence of the resistant ARR genotype. Recent
analysis of testing results has shown that several years down the line the
scheme is having the desired effect as this NSP report shows:
 NATIONAL
SCRAPIE PLAN
Year-upon-year improvements in the genotype distribution of ram lambs in
NSP flocks
Introduction
The National Scrapie Plan for Great Britain is now well into its third year of
sampling and to date has organised the collection of more than 950,000 blood
samples from over 11,000 flock visits. We are at a position where we can start
to review the progress (in terms of development of genotypes) of individual
breeds. This analysis will help us to judge if the goals of the NSP are being
met and should help us to ensure that an optimal balance is maintained between
genotype improvement and other breeding considerations.
Markers of Progress
We have performed two types of analysis. Firstly we have compared the NSP
genotypes of ram lambs born in 2002 and 2003. If the NSP is making an impact we
would anticipate that results will progressively improve from year to year - due
both to a general increase in the availability of NSP tested rams, and as a
result of genotype-informed breeding decisions taken within member flocks.
Secondly, we have compared the 2003 ram lamb genotypes of long-standing NSP
member flocks, with those of ram lambs tested in newly-joined flocks. Again we
would hope to observe a measurable (if temporary) advantage gained by those
flocks which have undergone previous rounds of testing and selection within the
NSP.
Although we have restricted the current analysis to just 10 numerically
important breeds, all of which have been heavily tested within the NSP, we hope
to review other breeds periodically. The 10 breeds discussed here were selected
on the basis that at least 15,000 samples were collected from animals of all
ages in 2003, of which 2000 or more were from ram lambs.
Results
The table in Figure 1 displays for 10 numerically important breeds the NSP
genotype patterns presented by 2002 and 2003 ram lambs. For simplicity, we have
grouped type 1 and type 2 results in a single category representing results with
at least 1 copy of ARR (but no VRQ). Similarly type 4 and 5 results are combined
in one category (VRQ- containing genotypes). Values are given to the nearest 0.1
percent.
Summary -
Across all 10 breeds, the 2003 ram lamb results contain a higher proportion of
combined type 1 & type 2 genotypes and proportionately fewer type 3 results
than in 2002. The breed average absolute movement in the proportion of types 1
& 2 (combined) and type 3 results is an increase of 6.5 and a reduction of
4.8 percentage points respectively. Furthermore with the exception of two breeds
with naturally low levels of VRQ (Suffolk and Bluefaced Leicester) all breeds
exhibit a reduction in the proportion of VRQ genotypes (breed average reduction
= 1.8 percentage points). Particularly striking changes include the 10 point
increase in combined type 1 and 2 percentage observed in the Blackface and the
double-figure elevation in the level of types 1 and 2 (combined), and reduction
in type 3 genotypes evident in the Texel.
The table in Figure 2 dissects the 2003 ram lamb results further by sorting
according to the level of NSP sampling previously undertaken in the flock.
Combined ram lamb results are presented for 1.) flocks that hosted their first
NSP visit earlier than August 2002 (so-called ‘’Established’’ flocks),
and 2.) Flocks receiving their first visit during 2003 (so called
‘’New’’ flocks). Values are given to the nearest 0.1 percent.
Summary -
In each of the 10 breeds considered here, the frequency of type 1 & 2
results (combined) in ram lambs sampled from Established flocks exceeds that of
ram lambs sampled in newly joined flocks (breed average advantage almost 10
percentage points). Similarly, the proportion of ram lambs testing in the type 3
category is on average nearly 8 points lower in Established flocks. Lastly, in
9/10 breeds, Established flocks present proportionately fewer ram lambs carrying
VRQ (breed average shortfall = 2.4 percentage points). The single exception is
the Swaledale in which Established flocks record a marginally higher proportion
of type 4/5 results in 2003, compared with ‘New’ flocks (7.2% vs. 7.0%). We
suspect that this difference is not statistically significant and that the lack
of a substantial differential may reflect the widespread availability of rams
typed in the Swaledale breed’s own scheme.
More
Details on the NSP.
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