Brussels, 19 December 2000
asked questions about BSE-tests
What will a BSE-test look for?
The commonly accepted cause of BSE is a
misshaped prion protein (PrPres). This misshaped protein
causes other normal proteins to become misshaped. These
proteins clump together to form sheets. There are other
theories of the cause of BSE and some variations on the
above theory. The presence of PrPres is regarded as a
marker for the disease.
BSE tests will determine if a detectable
level of PrPres (either the cause of BSE or a marker for
the disease) is present in the tissue examined.
How many tests have been approved by the European
To date, the European Commission has
approved three rapid tests developed by:
- CEA (name of test: Biorad)
- Prionics AG (name of test: Prionic
- Enfer Technology Ltd.(Enfer test
Five further rapid tests are currently
How do the tests operate in practice?
All three tests operate by detecting the
infectious agent or marker PrPres in the central nervous
system. Following slaughter of the animal a sample of brain
or spinal cord is taken from the animal using a special
tool. This tissue is taken to the laboratory and tested for
the presence of PrPres. Rapid tests are quick and reliable,
and allow large numbers of samples to be tested.
What other methods are used to diagnose BSE?
Other laboratory techniques used to
diagnose BSE include histopathological examination
(detection of spongiform encephalopathy), examination of
BSE fibrils (equivalent to scrapie associated fibrils),
examination by immunohistochemistry.
For what purpose can the tests be used?
Tests may be used for surveillance and
also to provide additional protection for the
Tests can be used to determine if BSE
exists in a population and to obtain an indication of its
prevalence. Used over time these can be used to monitor
changes in the level of the disease. This type of
surveillance can be carried out by testing risk groups of
animals, especially cows which have died on farms or cows
presented for emergency slaughter. If BSE occurs, it is
more likely to be found in this population so the sampling
is more effective. Actively searching for the disease in
this manner is more likely to detect it in a population (if
it exists there) than passive monitoring, i.e. waiting for
farmers to report suspicious signs.
The European Union will apply such a
testing programme amongst its "at-risk" population on
animals over 30 months from 1.1.2001 onwards.
2. Additional Health protection
BSE is a relatively rare disease.
However, routine testing of animals prior to slaughter may
detect animals presented for slaughter which may have
unnoticed signs of BSE and also animals with the disease
which are not yet showing signs. The identification and
removal of these animals will be an additional protection
for the consumer. However, the prime method of consumer
protection is the removal of specified risk material like
brain or spinal cord from every animal slaughtered. These
tissues harbour almost all infectivity if any present.
Removal of specific risk materials is obligatory in the EU
PrPres is always found in brain and
central nervous tissue in animals with clinical signs of
the disease and in animals in the months before they
develop the disease. Its presence in cattle appears to
parallel the development of infectivity.
The EU will apply such a testing
programme on all bovine animals over 30 months of age from
1.7.2001 onwards. Until then, all animals over 30 months
which cannot be tested will need to be destroyed.
How did the European Commission evaluate BSE
Test developers who had test in the
final stages of development were asked to submit them for
possible evaluation. In all ten applications were received.
Following review by an expert group, the four most
promising tests were evaluated as follows:
Under supervision laboratories were
asked to test 1,400 samples taken from healthy and diseased
animals. The samples were coded so that the laboratory
staff did not know which type of animals they were from
(i.e. they were tested blind). In fact the 1400 samples
were made up of 1064 samples from 1000 different healthy
animals and 336 samples from 300 animals with confirmed
The evaluation was carried out in
accordance with the guidelines of the International Office
of Epizootics (OIE).
The detection limits of the tests were
also assessed by testing BSE positive brains serially
diluted in negative brain.
How were you sure that the positive animals had BSE
and that the negative animals were healthy?
In evaluating a test it is extremely
important to be sure that the samples from the diseased
animals really had BSE and that the samples from the
healthy animals were really not infected. Otherwise the
evaluation is not accurate. In this case the samples from
the diseased animals were obtained from cows from the UK
which had shown signs of BSE and in which the disease had
been confirmed by microscopic examination of the brain. The
samples from the healthy animals were obtained from New
Zealand as this is the country most recognised as free from
BSE. They were healthy cows and their brains were also
What were the results of the evaluation?
Three of the four examined tests managed
to identify every single one of the samples correctly. The
fourth test correctly identified 70% of the disease samples
and 90% of the healthy samples. The report of the
evaluation is on the Commissions web site at:
How do these results compare with tests for other
These results compare very well with
test for other diseases. For example tests for other animal
diseases such as tuberculosis or brucellosis are less
accurate that these even in the very late stages of these
What can the test do early in the infection?
No method will detect BSE early in the
infection. BSE has an average incubation period of 4-6
years. Therefore the EU testing programmes are targeted at
animals over 30 months. The PrPres has not been detected in
bovine brain or other nervous tissue very early in the
disease and infectivity has not been shown either. In
experimental infection where very high doses were
administered, infectivity has been found in the ileum, part
of the intestine. This has not been detected in natural
What does the dilution series tell us?
In the dilution series, brain tissue
from BSE infected cows of a known infectivity titre was
serially diluted in healthy brain tissue. This was done to
obtain a proxy to early infection when the levels of PrPres
in the brain are much less.
This is a useful proxy but must be
interpreted with care as certain test technologies may
affect a dilution series differently than material from
Why not use tissue from pre-clinical cows?
Such tissue is not available for routine
use. This tissue can only be produced by experimentally
infecting cows and then slaughtering them about three years
later before they develop symptoms.
Have the tests detected pre-clinical animals?
All three approved tests have identified
pre-clinical animals. These are animals not yet showing
clinical symptoms. The CEA/Biorad detected pre-clinical
animals in an experiment. The Enfer test has detected
several pre-clinical animals, particularly from herds which
have been slaughtered because one case of BSE has been
detected. The Prionics test has detected pre-clinical
animals from herds depopulated because of BSE and also in
animals at normal slaughter in Switzerland and now in
Germany. The two cases identified in Germany were in normal
animals presented for slaughter.
In the case of animals detected at
slaughter, we can be confident that no obvious signs of BSE
were present but we cannot know how advanced the disease
Several cases of BSE have also been
identified by the tests in animals presented for emergency
slaughter. These are not pre-clinical animals. They showed
some disease signs but these signs had not been treated as
suspicious for BSE.
How much have tests for pre-clinical animals been used
Tests have been used routinely in
Switzerland for the past three years. They have also been
used in Ireland in testing animals from depopulated herds
where BSE has been identified. In recent months they have
been used in other Member States, particularly France and
have resulted in an increase in detection of the disease.
The first cases identified in Germany were detected as a
result of a test.
How many cases do you expect to find?
As said earlier, BSE is a relatively
rare disease. The rate of detection in the general
population (i.e. normal slaughter cows) would be expected
to be many times less that in the at-risk groups of
animals that have died on farm or that re subjected to
slaughter because of health problems. In Switzerland the
ratio was calculated to be about one case in the normal
population for 186 cases in the at-risk
What parameters may be used to select a test?
The three tests use different
methodologies which will suit different laboratories.
Factors to take into account include the current expertise
in the laboratory, familiarity with the test technology,
degree of automation desired etc.
How much do the tests cost?
Member States purchase following
competitive tenders. The cost will depend on this and also
factors such as size of order, whether items such as
training are included etc. Tests cost about 15-20 at
present, not including laboratory work and items such as
the transport of samples etc.
How much BSE will affect a man?
We dont know. We cannot do the
experiment as this would involve infecting people with the
Surely it must be possible to make an estimate?
Because of the species barrier, it is
likely that a higher dose of BSE is needed to infect man
than will infect cattle.
Because of the route of administration,
it is likely that a higher dose of BSE is needed to infect
man by the oral route than will infect mice by injection
into the brain.
Other factors such as the genetic make
up of the host can be important and can make the host more
or less susceptible.
Is there a safe dose?
While we cannot experiment on man, in
other animals there is a minimum dose which is unlikely to
result in the development of disease.
Released on 20/12/2000
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