ProMED-mail |

Published Date: 2002-06-21 23:50:00
Subject: PRO/AH> BSE, source, calf milk replacers: suspected
Archive Number: 20020621.4563

BSE, SOURCE, CALF MILK REPLACERS: SUSPECTED
***********************************
A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases
<http://www.isid.org>
Date: 21 Jun 2002
From: Marcus G. Doherr <marcus.doherr@itn.unibe.ch>

[In several recent postings, ProMED-mail has been dealing with
investigations into the source of BSE with special reference to calf milk
replacers (see references). We are most grateful for the authoritative
response obtained from Dr Marcus G. Doherr from the NeuroCenter & Division
of Clinical Research in the Department of Clinical Veterinary Medicine,
University of Bern. - Mod.AS]
The major problem in assessing the risk of specific feed components in the
transmission of BSE is that at the beginning of the epidemic in the UK,
meat and bone meal (MBM) was considered to be the major (if not the only)
source of BSE infectivity. Only afterwards was it shown that the MBM bans
were not preventing all new infections. Other products were considered and
their BSE risks assessed. Unfortunately, the 3-6-year delay between
feed-borne exposure (during calfhood) and detection of (clinical) BSE has
prevented any valid retrospective analysis of the feed components available
at the time of likely exposure.
In addition, a large number of different products from a range of small and
large feed mills have been (and still are) used in the various countries,
and the sourcing of the raw materials (including tallow) often changed on a
daily basis based on market prices. This, together with the fact that
typically there is no feed from the specific time period left to be tested
for infectivity in mice bioassays, so far has prevented a thorough
evaluation of specific feed components in Switzerland and other countries.
Therefore, the evidence that we have for all feed components and their
associated BSE risks are:
(a) the fact (or evidence) that CNS tissue has (or could accidentally have)
been included in the feed during processing;
(b) experimental evidence that the production processes were (are) not
sufficient to remove BSE infectivity once it had been introduced;
(c) epidemiological evidence that a measure to prevent CNS tissue from
BSE-infected cattle from entering recycling venues (such as the MBM ban)
has indeed resulted in a significant reduction of the number of BSE cases
in cattle born AFTER the introduction of the measure;
d) risk analyses attempting to quantify (at a population level) the BSE
risk of various feeds based on their ingredients, production parameters,
and usage in the cattle population.
The Scientific Steering Committee, in a Risk Assessment made public in
2001, concluded that "There is no evidence that tallow derived from
ruminant animals would constitute a TSE risk. The SSC considers that
possible TSE risks associated with tallow will result from protein
impurities that may be present in the end product, because it is expected
that TSE agents, if present in the product, would be associated with these
impurities."
<http://europa.eu.int/comm/food/fs/sc/ssc/out228_en.pdf>
They indicate that, when tissue sourcing could have allowed CNS tissue to
enter the tallow melting process, contamination of such tallow (with
respect to the protein impurities) with BSE infectivity could not be fully
excluded.
Recently 2 consecutive papers published in a German Journal (see references
1 and 2 below) indicated that, under the tissue sourcing and production
processes present in Germany until the end of 2000, milk replacer with
tallow of cattle origin (and the potential of CNS tissue being included in
this tallow) considered a non-negligible relative BSE risk (in comparison
to other products). This needs to be considered for other countries, too.
Whether one will ever be able to actually demonstrate infectivity in feed
samples collected from the field remains questionable. The main challenge
in research within this area is that once a product such as tallow has been
suspected as a potential source of BSE infectivity, preventive measures are
applied in the field as a precautionary principle before research has been
able to be completed to scientifically assess whether indeed this is, or
has been a risk factor or not. This approach renders field studies almost
useless, since all of them will have to work on feed production and usage
records from several years in the past, and cannot rely on current data and
biological samples.
In conclusion, at present it should not be fully excluded that BSE
infectivity through protein impurities within the tallow used in milk
replacers could be responsible for some new BSE infections; research on
this issue is ongoing.
1. Kamphues, J.; Zentek, J.; Oberthur, R. C.; Flachowsky, G., and Coenen,
M. [Risk assessment for animal-derived feedstuffs as vectors for bovine
spongiform encephalopathy (BSE) in Germany. Part 1: Comparative risk
assessment for animal derived feedstuffs]. Dtsch Tierarztl Wochenschr.
2001; 108(7):283-290
2. Zentek, J.; Oberthur, R. C.; Kamphues, J.; Kreienbrock, L.; Flachowsky,
G., and Coenen, M. [Risk assessment for animal-derived feedstuffs as
vectors for bovine spongiform encephalopathy (BSE) in Germany. Part 2:
Compounded feed as vector for bovine spongiform encephalopathy (BSE) in
Germany]. Dtsch Tierarztl Wochenschr. 2002; 109(2):43-51
--
Dr. Marcus G. Doherr, Ph.D.
NeuroCenter & Division of Clinical Research
Department of Clinical Veterinary Medicine
University of Bern
Bremgartenstrasse 109a, CH - 3012 Bern
Email: <marcus.doherr@itn.unibe.ch>
<http://www.neurocenter-bern.ch>
[For our readers' convenience, here are the English summaries of the 2
German references, as they appear in the Deutsche Tierarztliche
Wochenschrift (German Veterinary Journal) of July 2001 (Part 1) and
February 2002 (Part 2):
Risk assessment for animal-derived feedstuffs as vectors for bovine
spongiform encephalopathy (BSE) in Germany:
Part 1: Comparative risk assessment for animal derived feedstuffs
Summary:
The occurrence of BSE cases in Germany after the ban of meat and bone meal
for ruminant feed in 1994 requires a detailed investigation of animal
derived feeding stuffs regarding their specific risks as vectors for the
diseases. Accepting the theory that BASE is a prion-transmitted disease,
the theoretical infectious potential was calculated for animal-derived
feedstuffs. This calculation was based on the assumption that risk
material (brain and spinal cord) of one clinically diseased cow was
rendered in the process as established in Germany (133 centigrade, 3 bar,
20 in) or, alternatively, that one diseased animal was slaughtered and its
by-products normally processed for human food production. From this risk
assessment it became obvious that meat and bone meal was one -- but
probably not the most important -- source for the spreading of BASE. Taking
into account the high sensitivity of calves, it can be speculated that
certain products, e.g., from bone processing (bonemeal) and fat melting
(mixed animal fats), commonly used for the formulation of milk replacers,
might have been more important as pathways.
As it can't be excluded retrospectively that infected meat and bonemeal was
imported from the UK, this non-calculable influence may have reactivated
the significance of the other products. The calculation model underlines
that efficient removal of specified risk material (brain and spinal cord)
and adequate processing (133 centigrade, 3 bar, 20 min) or alternatively
other equivalent treatments of fats are prerequisites for minimising the
risk of foodborne transmission of BSE by animal-derived feedstuffs. The
epidemiological consequences are part of a subsequent paper.
Part 2: Compounded feed as vector for bovine spongiform encephalopathy
(BSE) in Germany
Summary:
Specific conditions and practices of cattle feeding in Germany have to be
taken into account for assessing the risk of foodborne transmission of
bovine spongiform encephalopathy, especially regarding the situation before
the year 2000, when specific directives were introduced for feed
production. The present retrospective epidemiological study includes data
on feed production and the estimated amount of animal-derived feedstuffs
for the production of compounded feed for cattle. Risk assessment was
performed based on the "reproduction rate," which is defined as the
estimated number of infections resulting from the processing of brain and
spinal cord of BSE affected cattle that is recycled to bovines via feed.
Under the conditions as given in Germany until the year 2000 the
reproduction rate of BSE via the inclusion of animal-derived feedstuffs in
compounded feed production for cattle was estimated to be 1.1. Thus, it can
be expected that BSE could be reproduced in the system, but with
comparatively low efficiency. The expected incidence of BSE should be
considerably lower compared to the situation during the 90's in the UK, due
to the markedly lower recycling rate of animal protein in cattle feeding.
Animal fat could have been a significant factor for BSE transmission due to
contamination by proteinaceous brain and spinal cord material during the
production process. The relative significance of fat containing feedstuffs
for BSE transmission could have been higher in Germany compared to the
situation in the UK, where meat and bone meal was produced under different
conditions and frequently used in higher proportions as an ingredient for
compounded feed for ruminants. - Mod.AS]

See Also