Thought this posting might interest you prion-watchers.
peace
misha
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From: ProMED-AHEAD
Date: Tue, 13 Oct 1998 21:59:17 -0400 (EDT)
Subject: PRO/AH> BSE - Switzerland (02)
BSE - SWITZERLAND (02)
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A ProMED-mail post
In June Swiss scientists using immunological and immuno-histochemical
tests for the BSE prion found 8 cases of BSE infection among 1761
apparently healthy herd mates of Swiss cattle which had developed BSE
(a prevalence of 4.5/1000). It was then decided to test for the prion
in 3000 healthy cattle over the age of 30 months being slaughtered at
abattoirs for human consumption. In late September one infected cow
was found, a four-year-old sent for slaughter because her milk output
had fallen due apparently to mastitis. This is the first time BSE
infection has been detected in a cow that would otherwise have been
eaten, in time to take it out of the food chain.
All 3,000 have now been tested with the fast Western blot developed by
the Zurich-based firm Prionics. Of those 2,200 have also been tested
using slower immuno-histochemical methods. All the results so far
agree, including the one positive result. One possible reason for the
good agreement between tests (in the previous study different tests
agreed on only 4 of the 8 positives) is that this time, brains were
divided into hemispheres and each was sent for one test or the other.
Prion distribution seems to be laterally symmetric, so this reduced
sampling error.
The apparent prevalence of 1/3,000 is less than the 1/1,000 infected
cattle in apparently healthy herds in Switzerland calculated by Doherr
and colleagues, based on observed clinical incidence and estimated
incubation time of the disease. The discovery of only one case does
not allow the empirical calculation of a clinically significant rate.
But if the prevalence is 1/3,000, some 50 infected cattle over the age
of 30 months are being eaten per year in Switzerland.
The results of the Prionics test were available within 24 hours, which
allowed the infected carcass to be destroyed before it was sold for
meat. Swiss authorities are now considering whether to mandate testing
of all cattle at slaughter. There are fears that too many false
positives would make this prohibitively expensive, as for each case
detected the entire herd would have to be destroyed. Prionics points
out that of the 3000 tests in the current series, 2999 were negative,
indicating that while there might be false negatives, the rate of
false positives is not substantial.
Professor Picoux pointed out [ProMed 6 October] that the cow found in
the current series may not have been strictly subclinical, as she had
displayed behavioural changes which were put down, possibly
erroneously, to pain from mastitis. The early symptoms of BSE are
notoriously difficult to distinguish from other syndromes with neural
involvement. This cow possibly exemplifies the reason for much
under-reporting of BSE on the Continent: older cattle with falling
milk output or odd symptoms are simply sent for slaughter. Some of
those could have been developing BSE, but are killed before they get a
chance to develop clear symptoms.
The European Commission wants all EU countries to test cattle in
abattoirs for such hidden infection. It is to be remembered that the
levels of BSE infection expected on the Continent simply on the basis
of British cattle exports, to say nothing of the continued feeding of
meat and bone meal of questionable hygiene to livestock, are well in
excess of what has been reported. That, incidents such as the recent
surge of cases in Portugal, and the continuing, and to many people
implausible, apparent absence of BSE in Germany, suggest substantial
under-reporting.
The implications of the Swiss result for Britain, which has had the
most BSE, are complex. Only cattle aged 30 months or younger are eaten
in Britain, on the assumption, based on feeding trials, that cattle of
that age, even if they were infected as calves, have not yet
accumulated enough prions to be infectious. But the youngest cow to
develop BSE on record in Britain was 20 months old, showing some are
fast incubators. Models predict that 200-300 cattle under 30 months
per year are infected with BSE and enter the food chain currently in
Britain. Of these 3-5 could be fast incubators and carrying detectable
quantities of prion.
If one were to test cattle routinely at abattoirs in Britain, it is
possible that only those 3-5 would be detectable, and thus could be
kept out of the food chain. So routine testing may not be
cost-effective. On the other hand, these predictions are based
entirely on modelling. Some think that at least a study similar to the
Swiss one should be carried out in Britain to actually measure the
extent of infection, especially if there is a subclinical strain that
is not reflected in models based on clinical incidence.
The Swiss data do not shed light on infection before 30 months. They
did not test younger cattle as relatively few of them would be
expected to have accumulated enough prion to be detectable, so a much
larger sample size than the government was prepared to pay for
initially would have been required to detect at least one case. The
30-month cut-off was also practical (perhaps one reason it was chosen
as the limiting age in Britain) as because of dental development, the
head of a cow 30 months or older can be readily distinguished from a
younger one at the abattoir.
Whether or not screening cattle in abattoirs can make meat safe is
debated. It is claimed by some, disputed by others, that infected
cattle which have not yet accumulated detectable quantities of prion
are not infectious. So animals that test negative are safe to eat
whether infected or not. The few private Swiss butchers now using the
Prionics test to screen beef before it is marketed advertise their
wares as BSE tested, not BSE free. The efficacy of screening at
preventing the transmission of infection to people also ultimately
depends on whether detectable levels of prion in brain occur at the
same time as potentially infectious levels in muscle. That is not
known.
- --
Debora MacKenzie,
New Scientist.
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Michele Gale-Sinex, communications manager
Center for Integrated Ag Systems
UW-Madison College of Ag and Life Sciences
Voice: (608) 262-8018 FAX: (608) 265-3020
http://www.wisc.edu/cias/
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