Danish Ministry of Food, Agriculture and Fisheries
Danish Institute for Food and Veterinary Research
Estimation of the relative BSE infectivity
(risk) of meat and bone meal (MBM), bone
meal (BM), greaves and tallow
BSE: relative infectivity
2
Summary
Animal fat including bovine fat can be used in ruminant feeds. A risk assessment
published in 2001 showed that bovine tallow could contain BSE infectivity if
produced from an animal with clinical BSE. The amount of infectivity was
dependent on the method used to produce the tallow, the raw material and the
allowable concentration of protein impurities. Three different processes produce
bovine fat or tallow: whole carcass rendering, bone processing or fat melting.
Using the results of the earlier published model, we have estimated the relative
infectivity in bovine tallow vs. the other end products, meat and bone meal, bone
meal and greaves.
The results demonstrate that there has been a great decrease in the residual
BSE infectivity in fat and other meat by-products that have been used for cattle
feed. Generally, the by products (MBM, BM and greaves) contained higher
concentrations of BSE infectivity than the fat produced by the same method.
Introduction
Several EU member states, including Denmark, have reported BSE cases that
were born after the MBM ban in 1997. The source of exposure to the BSE agent
is thought to be primarily cross contamination with feeds for other species (pigs,
chickens) that could contain MBM until 2000. However, other possible sources of
BSE exposure including calf milk replacer, vaccines, environmental
contamination and others have been proposed but never demonstrated. An
assessment of the risk of BSE transmission by tallow based calf milk replacer
(CMR) suggests that some BSE cases may have resulted from the feeding of
CMR. However, the results are associated with much uncertainty (Paisley and
Hostrup-Pedersen, in press).
BSE: relative infectivity
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Cattle feeds and fat samples in Denmark have been found to contain detectable,
but unquantifiable, traces of bone and muscle fibers (PDIR pers com). In
addition, traces of MBM have been found in cattle feed processing equipment in
2004, seven years after the ban on MBM in cattle feed. The importance of these
findings is unknown.
Concern over sporadic findings of traces of MBM (bone fragments and muscle
fibers) in feed and fat samples prompted a request from the Danish Plant
Directorate (PDIR) and the Danish Veterinary and Food Administration (FDIR) for
an estimate of the relative BSE infectivity of MBM, BM and fat. This knowledge
might be useful in identifying risk management options.
The deterministic risk assessment model of Kamphues et al., (2001) was used to
estimate the relative BSE of bovine tallow, MBM, bone meal and greaves. The
model is based on the rendering, bone processing and fat melting industries in
Germany and the results may not be applicable in all countries. Never the less,
the results provide an indication of the relative risks associated with bovine meat
by-products over time. The underlying assumption in the model was that the CNS
of one BSE case was present in 1000 kg of raw material. The amount of
infectivity in CNS tissue was assumed to be 10 ID50/g. The total BSE infectivity
in a clinical BSE case before processing was 7000 ID50. An ID50 is the dose
that, on average, will infect 50% of animals receiving it.
Carcass rendering
Before October 1, 2000, whole carcasses including the skull (brain) and spinal
column (spinal cord) were rendered producing fat and MBM. The maximum
allowable impurities in fat were 0.5% but this was decreased to 0.15% on
January 1, 2001. It was assumed that the impurities in the fat were primarily
protein. After 1 October 2000 both the MBM and fat underwent inactivation by
133 ºC/ 3 bars/ 20 minute processing. Fat from this process could have been
used in animal feeds. The effect of these changes on the amount of BSE
BSE: relative infectivity
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infectivity in MBM and fat are shown in Table 1. For comparison, the amounts of
BSE infectivity associated with MBM and fat in the UK prior to 1990 (before the
ban on specified bovine offal (SBO)) are shown. The amounts listed are based
on processing 1000 kg. of raw material.
Bone processing
Bones from cattle are processed to produce bone meal (BM) and bone fat. As
with carcass rendering, the skull and spinal column were included in the raw
material prior to 1 July 2000. After 1 October 2000, the skull, brain and spinal
cord were excluded but the spinal column was still used in bone processing. The
fat was extracted at low temperature such that no reduction of infectivity was
expected. The BM was treated with 133 ºC/ 3 bars/ 20 minutes. The allowable
impurities in fat were decreased from 0,5% to 0.15%. Fat from this process could
have been used in animal feeds. The effect of these changes on the amount of
BSE infectivity in BM and bone fat are shown in Table 2. The amounts listed are
based on processing 1000 kg. of raw material.
Fat melting
Discrete fatty tissues, trimming from cutting plants and some bones, including the
spinal column were minced and heated (< 100 ºC) to produce fat and greaves
meal. Until 1 Jan 2000, the legal limit on maximum allowable impurities in the fat
was 0.5%. This was reduced to 0.15% on 1 January 2001. After 1 October 2000
the SRM was excluded from the raw materials. The raw material for premier jus,
the highest quality fat used for human consumption and calf milk replacer,
contains no bones and has a maximum allowable impurity level of 0.02%. The
effect of these changes on the amount of BSE infectivity in greaves and fat are
shown in Table 3. The amounts listed are based on processing 1000 kg. of raw
material. Table 4 compares the relative BSE risk of MBM, bone meal and
greaves meal and fat during the same time periods. Table 5 shows the potential
relative BSE infectivity in 2001. Table 6 shows the percent reduction in potential
BSE: relative infectivity
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BSE infectivity due to the processing changes, SRM ban and changes in the
maximum allowable impurities.
Discussion
The estimation of the relative risk of MBM, BM or greaves vs. fat is not a
straightforward exercise. One must consider the type of raw material used, the
type of processing, the EU legislation, voluntary industry standards, the GBR-C
status of the source country as well as the likely uses of the end product
(EC,1998; EC, 2001). Because of the complexity, this estimate is based on a
single deterministic risk assessment model for one country (Kamphues et al.,
2001). Currently, there is a ban on MBM in animal feeds but bovine (and other
animal) fats are still allowed, provided that the fat was produced from animals fit
for human consumption, the SRM was excluded and contamination with
potentially BSE infected materials is prevented. In addition, it is recommended, in
countries with GBR-C status of II, III or IV, the edible tallow be pre-sterilized with
133 ºC/ 3 bars/ 20 minutes (EC, 2001). Sterilization of premier jus for human
consumption or calf milk replacer is not required.
The results demonstrate that there has been a great decrease in the amount of
residual BSE infectivity in fat and other meat by-products that have been used for
cattle feed. Generally, the by products (MBM, BM and greaves) contained higher
concentrations of BSE infectivity than the fat produced by the same method.
The results presented can be considered worst-case scenarios because the
percentage of impurities in the fat used in the calculations were the maximum
legal values. It is likely, that the real values were less because of voluntary
industry standards. In addition, current estimates of the ID50/g of BSE infected
CNS is closer to 2 ID50/g in contrast to the 10 ID50/g used in this model (Paisley
and Hostrup-Pedersen, in press). It should also be stressed that the values are
deterministic and do not address the uncertainties inherent in the assumptions or
the probabilities of actually observing these values.
BSE: relative infectivity
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In this assessment fat is equivalent to "tallow". However, fat for animal feed can
be of mixed animal or plant origin. Fat would only present a risk if it contained
ruminant (bovine or ovine) fat. Again, the assumption in this assessment was that
the fat was produced from raw material that contained CNS material from one
clinical, BSE infected bovine under a German production system. There is no
experimental evidence that BSE can be transmitted by tallow.
It is not possible, at this time, to make a definitive statement regarding the risk
presented by the traces of meat and bones in the fats and feeds. Suffice it to say,
that if the traces of muscle and bone were from a BSE infected animal in the late
incubation period, if the raw material contained CNS or other SRM from a BSE
infected animal, if the raw material were not pre- or post-sterilized (133 ºC/ 3
bars/ 20 minutes) the traces would present a BSE risk.
The current theory is that the BSE "risk" (probability of infection given exposure)
is proportional to the dose ingested. This means that there is no safe threshold!
Thus, prions present a "risk" at any dosage. This is the justification for the total
animal protein ban.
Since we can neither determine the species of origin nor the amount of "foreign"
material in the fat or feed we cannot quantitate the BSE risk therefore, cannot
state with any confidence whether the traces of muscle and bone have or have
had an influence on the BSE situation in Denmark.
It seems a paradox that there is “zero tolerance” for MBM in ruminant feeds yet
bovine tallow which can be used in animal feed can legally contain a maximum
0.15% impurities. It is possible that the source of traces of bone and muscle
fibers found in feed samples is the fat.
BSE: relative infectivity
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Selected references
E.C. (European Commission), 1998. Scientific Steering Committee, Opinion on
the safety of tallow derived from ruminant tissues.
http://europa.eu.int/comm/food/fs/sc/ssc/out11_en.pdf
E.C. (European Commission) 2001, Scientific Steering Committee, Revised
Opinion and Report: The safety of tallow obtained from ruminant slaughter byproducts. http://europa.eu.int/comm/food/fs/sc/ssc/out228en.pdf
Kamphues, J., Zentek, J., Oberthür, R.C., Flachowsky, G., Coenen, M., 2001.
Futtermittel tierscher als mögliche verbreitungsursache für die Bovine
Spongiforme Enzephalopathie (BSE) in Deutschland. Dtsch. tierärztl Wschr. 108,
283-290.
Paisley, L.G. and Hostrup-Pedersen, J. Quantitative assessment of the risk of
BSE transmission by tallow based calf milk replacer (In press)
BSE: relative infectivity
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Table 1. Whole carcass rendering (1000 kg raw material), 10 ID50/g CNS
material
After 1
After 1
Oct 20003 Jan 20014
Before 1
Oct 20002
UK 19901
Carcass Carcass
Carcass
Carcass
rendering + rendering + rendering rendering
(–) SRM (–) SRM
SRM
SRM
ID50/case
7000
7000
2300
2300
ID50 in fat
29
0.1
0.002
0.001
0.2636
0.0009
0.00002
0.00001
697
27.9
0.46
0.46
3.030
0.112
0.002
0.002
ID50/kg fat
ID50 in MBM after
rendering
ID50/kg MBM
Table 2. Bone processing (1000 kg raw material), 10 ID50/g CNS material
Before 1 Before 1
July 20001 Oct 20002
ID50/case
ID50 in fat
ID50/kg fat
ID50 in BM after
rendering
ID50/kg MBM
After 1
After 1
Oct 20003 Jan 20014
Bone
Bone
Bone
Bone
processing
processing processing - processing (–) spine
(–) SRM
(+) SRM
(+) SRM
(–) SRM
7000
7000
2300
2300
18.6
18.6
0.31
0.09
0.186
0.186
0.0031
0.0009
698
27.9
0.46
0.46
1.485
0.059
0.001
0.001
1
90% reduction in infectivity in MBM due to old rendering process
2
99.6% reduction in infectivity in MBM due to 133ºC / 3 bars/ 20 min
3
95% reduction in infectivity due to SRM removal, additional 99.6% reduction in infectivity in MBM due to
133ºC / 3 bars/ 20 min; 0.5% protein impurities
4
95% reduction in infectivity due to SRM removal, additional 99.6% reduction in infectivity in MBM due to
133ºC / 3 bars/ 20 min; 0.15% protein impurities
BSE: relative infectivity
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Table 3. Fat melting (1000 kg raw material), 10 ID50/g CNS material
ID50/case
ID50 in fat
ID50/kg fat
ID50 in greaves
after rendering
ID50/kg MBM
Before 1 After 1
After 1
After 1
After 1 Jan
July 20001 July 20002 Oct 20003 Jan 20014 20015
Fat Melting Fat Melting Fat Melting
Fat Melting Fat Melting (+) spine (+) spine (-) spine
(+) spine, (+) spine,
(-) SRM,
(-) SRM, (-) SRM
(+)SRM
(+)SRM
0.5% prot 0.15% prot 0.02% prot
2300
2300
2300
2300
2300
105
105
5.25
1.63
0.21
0.16
0.16
0.01
0.003
0.00033
220
2.75
8.78
0.110
0.44
0.006
0.45
0.006
0.45
0.006
1
90% reduction in infectivity in MBM due to old rendering process
2
99.6% reduction in infectivity in MBM due to 133ºC / 3 bars/ 20 min
3
95% reduction in infectivity due to SRM removal, additional 99.6% reduction in infectivity in greaves due to
133ºC / 3 bars/ 20 min; 0.5% protein impurities
4
95% reduction in infectivity due to SRM removal, additional 99.6% reduction in infectivity in greaves due to
133ºC / 3 bars/ 20 min; 0.15% protein impurities
5
95% reduction in infectivity due to SRM removal, additional 99.6% reduction in infectivity in greaves due to
133ºC / 3 bars/ 20 min; 0.0.2%% protein impurities
BSE: relative infectivity
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Table 4. Relative BSE infectivity of meat and bone meal / bonemeal vs. fat
(Infectivity of fat =1). Note that these are relative infectivity values and not
measures the actual amounts of infectivity.
Rendering – MBM + fat
1
UK before 1990
DE before 1 Oct 2000
DE after 1 Oct 2000 2
DE after 1 Jan 2001 3
11.49
122.76
101.20
202.40
DE before 1 July 2000
DE after 1 July 2000 4
DE after 1 Oct 2000
DE after 1 Jan 2001
Bone processing – Bone meal + fat
7.98
0.32
0.32
1.09
Fat melting – Greaves + fat
DE before 1 July 2000
DE after 1 July 2000 5
DE after 1 Oct 2000
DE after 1 Jan 2001 (0.15%)
DE after 1 Jan 2001 (0.02%)6
1
Specified bovine material ban
SRM ban
3
Ban on rendering the skull
4
Pressure sterilization of bone meal. 133º/3 bars / 30 minutes
5
Reduction of maximum protein in fat to 0.15%
6
Reduction of maximum protein in fat to 0.02%
2
17.02
0.68
0.68
2.24
17.4
BSE: relative infectivity
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Table 5. The relative BSE risks of MBM, bone meal or greaves vs. fat since
2001. Infectivity of fat =1. Note that these are relative infectivity values and
not measures the actual amounts of infectivity.
Rendering
Bone processing
Fat melting - 0.15%
Fat melting - 0.02%
BSE Risk
202
1
2
17
Table 6. The percent reduction in BSE infectivity in MBM and tallow since
July 1, 2000.
Percent reduction of BSE infectivity since 1 July 2000
MBM
FAT
98.4%
99.997%
Rendering
Bone processing
Fat melting - 0.15%
Fat melting - 0.02%
99.9%
99.8%
99.8%
99.5%
98.4%
99.8%