Health
Scientific Committees
Scientific Steering Committee (former MDSC)
Outcome of discussions
Extract from
the minutes of the meeting of 14-15 September 2000 on some
TSE-issues that recently emerged in the scientific and
popular press
At its meeting of 14-15 September 2000,
the SSC also discussed the following issues as a follow-up
to the TSE/BSE
ad hoc Group meeting of 31 August 2000.
1. A recent brief Communication in Nature (10 August
2000), suggests that, on average, no more than two cases of
vCJD could arise from the consumption of one maximally
infected bovine, as opposed to previous estimates that this
number could exceed 100. This would result in a
significant reduction of the expected maximum number of
future vCJD cases.
The SSC noted that this short
Communication contains quite limited scientific details and
that the model leading to the results on which the
conclusions are based, contains many assumptions. It noted
that in an earlier publication of January 2000, the same
research team accepted a
much
wider range of up to more than 100 vCJD cases that
could arise from 1 maximally infected bovine.
The assumption is made that only those with the
presently observed susceptible genotype are at risk.
Also, the communication does not take into account the
most recent raise in numbers of vCJD cases. The SSC
considers that in the field of TSEs there are still so many
unknowns that it is very difficult to make predictions on
future numbers possibly to be expected, especially in this
stage of the epidemic.
2. Prof.S.Prusiner's recent hypothesis (reported on in
the Sunday Times of 23 July 2000) that BSE prions in
sheep may have been there all the time at very low levels
that pose no significant risk to humans but that unusual
circumstances might have allowed them to spread either
through the sheep or cattle population and accumulate to
levels hazardous to humans.
The SSC noted the outcome of the
e-consultation of Europe's main TSE experts organised by
its secretariat in July 2000, which was in line with the
final clarification provided on 2 August 2000 in a letter
from Prof.Prusiner to the SSC Secretariat. BSE was not
produce in mice by infecting them with material suffering
from scrapie but both sheep scrapie and BSE prions had been
transmitted to mice which are engineered to mimic cattle.
From his findings Prusiner
et al advance the
hypothetical scenario that "it is possible that low
levels of the BSE 'prion strain' are actually endemic in
the scrapie sheep population, but that the BSE prions never
surface as such because their presence is marked by the
more rapidly growing sheep scrapie strain. Any unusual
selective treatment, such as the change in rendering
process, could remove the less dangerous sheep scrapie
strain and allow the BSE strain to accumulate and spread to
the cattle population". In conclusion, "BSE prions in sheep
may thus have been there all the time at very low levels
that pose no significant risk to humans but unusual
circumstances might have allowed them to spread either
through the sheep or cattle population and accumulate to
levels hazardous to humans."
The SSC considered, as for the Collinge
paper (see hereafter), that the Prusiner hypothesis in its
present form (i.e., without published details) does
presently not trigger the need for an immediate revision of
the SSC's various "TSE in sheep" related opinions. It is,
however, obvious that Prof.Prusiner's research and similar
work by others, will be monitored carefully and, once
finalised and published, further assessed by the Working
Group presently carrying out a pro-active risk assessment
for the case that BSE in sheep would be found under natural
conditions.
3. The Hill
et al publication ("
the Collinge paper") in the Proceedings of the
National Academy of Sciences (August 2000) on the
possible presence of subclinical TSE infectivity in
certain animals and its implications for the definition
of cross-species barriers of TSE transmission.
The SSC considers that most of the
scientific evidence on which the paper is based, was either
known to it or anticipated by the SSC and has been
appropriately addressed in various scientific opinions. As
a matter of principle, the SSC has not based its risk
assessments with regard to the safety of animals on results
of end-point titrations with clinical disease in laboratory
animals as the sole parameters. Furthermore, the SSC
reviews and updates its existing opinions in the light of
emerging scientific data, on regular basis. The relevance
of 4 of the SSC opinions with regard to the Collinge-paper,
is summarised in annex. The following incomplete list of
SSC recommendations, can be presented as an example on how
the possible existence of subclinical infections with
important public health implications with relation to
dietary exposure, have already been taken into account in
the SSC opinions
:
-
Product safety (gelatine, tallow, meat-and-bone
meal, dicalcium phosphate, ...): the opinions start from
the hypothesis that an animal with TSE, but apparently
healthy, would be slaughtered and assume
a very low species barrier. The recommended
production standards aim, when a TSE risk cannot be
excluded, for safe sourcing of raw material, removal of
SRMs and severe processing conditions.
-
Geographical risk: a risk level is attributed to the
ruminants of countries or regions where BSE might be
present, even if no clinical case have been
detected.
-
SRMs: removal of the ruminant tissues with the
expected highest infectivity levels, should they be
slaughtered without a TSE having been diagnosed;
- No further use of
fallen stock and certain animal categories, as a TSE
could be at the basis of the death or could be
[undiscovered] present in certain animals.
- Avoidance of
intra-species recycling (see annex for
details).
The SSC concluded that there is at
present no need to revise its TSE-related opinions. It,
however, asks its secretariat to request from the UK and
the EC research authorities the most up-to-date research
results on TSEs in pigs, poultry and in fish. From an
analysis of this info should appear whether there is a need
to revisit certain opinions.
It also asks the TSE/BSE
ad hoc Group to assess whether there is a need for
an adaptation of TSE surveillance strategies in farmed
animals and whether there is a need to update the opinion
on breeding for scrapie resistant sheep as a means to
control TSEs in small ruminants.
Annex: The relevance of 4 SSC opinions with regard
to the Collinge paper (August 2000).
The TSE/BSE
ad hoc Group considers that most of the scientific
evidence on which the paper is based, was either known to
or anticipated by it and has been addressed in various
scientific opinions. To be mentioned especially are:
a. The Opinion of 24-25 June 1999 on The
risks of non conventional transmissible agents,
conventional infectious agents or other hazards such as
toxic substances entering the human food or animal feed
chains via raw material from fallen stock and dead animals
(including also: ruminants, pigs, poultry, fish,
wild/exotic/zoo animals, fur animals, cats, laboratory
animals and fish) or via condemned materials.
b. The opinion of 22-23 July 1999on the
policy of breeding and genotyping of sheep, i.e. The issue
of whether sheep should be bred to be resistant to
scrapie.
c. The opinion of 17 September 1999 on
the risk born by recycling animal by-products as feed with
regard to propagating TSE in non-ruminant farmed
animals.
d. The opinion of 13-14 April 2000 on
Oral exposure of humans to the BSE agent: infective dose
and species barrier. (Adopted following a public
consultation via internet between 6 and 27 march
2000)
The
first opinion considers the risk to the public, to
animals and to the environment from transmissible
biological and chemical agents which may be present in
fallen stock and dead animals. The opinion makes
recommendations on how such risks can be minimised. In the
light of experience with BSE this includes consideration of
unconventional and as yet unknown agents. The risk to man
from dead animals and condemned materials is considered to
depend on:
- The nature and level of the agent(s)
present in the dead animal / fallen stock, which in turn
relies on accurate diagnosis and measurement;
- The prospect of intra and interspecies
transmission;
- The actual processing / disposal
method used;
- The prospects of human exposure as a
consequence of the processing / disposal.
Whilst also addressing zoo-,
laboratory-, exotic- and pet animals, the opinion states,
with respect to the susceptibility of pigs, poultry and
fish to become infected with TSEs is concerned, that there
is only evidence that pigs can become infected through
intra-cerebral inoculation with infectious BSE material. To
date no experiments have shown that pigs, poultry or fish
could be infected with TSE
via the oral route. Whilst recognising that
experimental transmission
via the i/c route has been shown for a number of
animal species, the to date not proven hypothesis that
orally TSE-inoculated non-ruminants without any signs of
disease could carry the TSE-infection in their tissues has
is considered unlikely.
The
second opinion addresses, amongst other issue, that
an animal not showing clinical signs can be infectious. It
mentions the experimental evidence for the existence of
hidden infectivity. It clarifies that "resistant" sheep may
include animals that remain clinically free of scrapie
signs for normal life-span but could still harbour
the infectious agent, posing a threat by maintaining the
infectious agent. It points at the possible existence, for
sheep, of carrier animals with latent scrapie infection,
that will not itself develop disease but with the ability
to pass infection on to other sheep. This opinion also
states that cross species persistence may also occur:
hamster scrapie injected into mice does not produce disease
but the hamster scrapie remains in brain and spleen of the
mice and can be recovered in a form still able to infect
hamsters. The SSC concludes, amongst others, that the
possibility that sheep may harbour a latent scrapie
infection exists and if so, that they could pass an
infection to other sheep.
The
third opinion, on intra-species recycling, stated in
mid 1999, that:
"a. So far no scientific evidence exists
to demonstrate the natural occurrence of TSE in farmed
pigs, poultry and fish, which may create a basis for an
intra-species progression of a TSE infection due to
intra-species recycling.
b. Given the limitations of the
surveillance in certain areas, and the length of the
incubation time in relation to the normal (=economic or
commercial) life span of the animals, it can not be
excluded that cases occur and that, perhaps more important,
an undetected pool of infectivity is build up.
c. Because of these two preceding
points, the SSC wants to underline that in scientific terms
absence of evidence is neitherevidence of absence nor of
presence of a risk. However, it is impossible to exclude,
on the basis of the available evidence, that TSEs are
already present (albeit undetected) in non-ruminant farmed
animals, in particular not if there is reason to assume
that these species have been (and might still be) exposed
to BSE-contaminated feed (produced from ruminants).
d. Recycling of animal material, in
general, will increase the risk that cases occur or
undetected infectivity pools develop,in particular if
potentially BSE (TSE) contaminated material is recycled to
ruminants or (possibly) susceptible non-ruminants.
e. Intra-species recycling will, due to
the absence of a species barrier, increase the risk
further.
f. If recycling, and in particular
intra-species recycling, of animal material to farmed
animals can not be avoided, all measures that reduce the
recycled infectivity would reduce the risk.
g. Measures that reduce the recycled
infectivity include:
- exposing the recycled animal material
to a treatment by 133°/20'/3b or equivalent
conditions,
- excluding those tissues known to carry
the highest infectious load (SRM),
- excluding fallen stock from the
production of feed,
- stop feeding pig, poultry or fish
potentially contaminated feed a sufficiently long period of
time before slaughter in order to reduce the risk of
recycling infectivity via the gut-content.
h. It has to be understood that
- the possible measures would not be
able to reach a zero risk should infectivity enter the
recycling loop, and
- that due to the long incubation time
of this type of disease a significant risk would have build
up before an incidence becomes visible (as has been seen in
the case of BSE in the UK).
i. The SSC considers R&D in the
field of surveillance and (pre-clinical) diagnostic of TSEs
and the experimental transmission of TSEs to farmed
(non-ruminant) animals to be of highest priority."
The
fourth opinion states, on species barrier: "The size
of the species barrier for BSE-in-ruminants to
BSE-in-humans is not known and may be large (for example a
barrier of the order of 1000, as assumed in some risk
assessments) or small. Given the conflicting scientific
data and thus the uncertainties about the bovine-to-human
species barrier as outlined in this document, the
assumption of a worst case scenario considering no (=1)
barrier should be included, although available evidence
indicates that values greater than 1 are likely to be more
realistic. The Working Group therefore recommends that,
until more scientific data are available, for risk
assessments of human exposure to potentially BSE
contaminated products, a species barrier of about 1 should
considered as a worst case scenario and that, in risk
assessments, the range from 10
4 to 10
1 is considered. The latter order of magnitude
would imply that the minimal infective dose value(s)
considered/accepted to be valid for animals, should also be
applied for humans."
[
©]
- [
HEALTH] - [
SCIENTIFIC COMMITTEES] - [
SCIENTIFIC STEERING COMMITTEE] -
[
OUTCOME OF DISCUSSIONS]
|