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The EC
evaluation contained a small second part in which dilutions
of liquefied tissues were analyzed. The author of the VegSource
article erroneously concluded from the results of this study
that the Bio-Rad test must be more sensitive than the Prionics-Check.
In fact,
the EC never made such a claim and furthermore clarified in
an FAQ sheet published on the internet (http://europa.eu.int/comm/food/fs/bse/bse21_en.html)
that dilution experiments represent an artificial system falling
short in certain ways to represent the natural situation.
The reason
for this is twofold: The first reason has to do with the fact
that BSE-tests detect prion protein aggregates. The aggregates
as formed in visibly ill cattle do not adequately mimic the
rare aggregates formed during earlier stages of the disease.
Therefore, dilutions of aggregates from visibly ill cattle
do not in many ways represent the situation in BSE-infected
animals at earlier disease stages. The second reason why the
dilution study does not reflect field performance lies with
the technical specifications of Prionics-Check: the innovative
process of Prionics-Check already starts at the tissue preparation
using special proprietary solutions. The dilution-tests were
performed with standard liquefaction and not according to
the Prionics-Check procedure used in the field.
An adequate
method of determining a test's performance is the parallel
testing in the field with a gold standard (e.g. immunohistochemistry)
and the BSE-test which has to be evaluated. Such an evaluation,
however, is extremely time- and cost intensive, but it tells
you exactly how the test performs under field conditions and
with naturally infected BSE animals.
Such
an evaluation was done with Prionics-Check under the supervision
of the Swiss Veterinary Office (it took roughly a year to
get the whole Swiss evaluation done as compared to an afternoon
to test the few dilutions of the EC). The extensive comparative
field evaluation demonstrated that the Prionics-Check test
is at least as sensitive as optimally performed immunohistochemistry
in spotting naturally infected BSE animals under field conditions.
Such a study has not been done for the Bio-Rad test, however,
even in an experimental system performed under optimal conditions
on experimentally infected animals, this test was not more
sensitive than immunohistochemistry.
Reliability
was a key-feature of Prionics-Check to convince European countries
to follow the Swiss BSE-surveillance model and to introduce
Prionics-Check for mass screening. The screening was pioneered
by France and Denmark, the other countries followed.
BSE-cases
started to shoot up as a result of the increased surveillance
(the interested reader finds a short document on the milestones
leading to the general acceptance of BSE-surveillance by mass
screening at http://www.prionics.ch/PDF/surveillance_milestones.pdf).
Over 2.5 million Prionics-Check tests have been sold so far
throughout Europe of which over one million tests have already
been carried out. Germany and Italy, two countries which previously
had been allegedly BSE-free, had to face the first BSE-cases
shortly after the introduction of mass screening using Prionics-Check.
Should
we be afraid of introducing Prionics-Check mass testing in
countries without a previous record of BSE? After all, the
general picture Prionics-Check created in Europe is that whoever
introduces the test will have to face a harsh reality!
So why
introduce the test?
Well,
not every country that introduces testing will necessarily
find BSE: some may instead produce excellent evidence that
they are in fact BSE-free. Let's have a look at Europe again:
Both Italy and Germany were classified by the European Commission
as countries with a relatively high BSE-risk, even before
Prionics-Check detected the first BSE cases. This classification
was based on an analysis of the two countries import and feeding
practises. It was therefore not a big surprise for scientists
when the first BSE-cases were found.
In contrast,
both the U.S.A and Austria are classified as having a low
BSE-risk (The interested reader finds the detailed risk analysis
for the U.S.A. at http://europa.eu.int/comm/food/fs/sc/ssc/out137_en.pdf).
Austria has introduced Prionics-Check in January 2001 and
has tested so far over 30,000 animals without a single BSE-case
showing up. (In Germany a cow exported from Austria recently
tested falsely positive with a competitive rapid test, which
caused quite a crisis for a few days until it was clear that
the result was false positive. This episode of course confirmed
Austria in their decision to exclusively use Prionics-Check).
Although
in a Europe with open internal borders it cannot be excluded
that isolated rare BSE-cases might also show up in Austria,
the results of the mass screening program already demonstrates
that the country is essentially BSE-free compared to its neighboring
countries.
Prionics-Check
therefore does not only bring the bad news of BSE, it may
as well serve to demonstrate a country's BSE-free status.
The risk of detecting BSE versus the advantage of demonstrating
the absence of BSE - you can't have one without the other!
Dr. Markus
Moser
Prionics AG
University of Zurich
Switzerland
http://www.prionics.ch
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