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From: TSS ()
Subject: Consumption of beef tongue: Human BSE risk associated with exposure to lymphoid tissue in bovine tongue in consideration of new research findings
Date: May 1, 2008 at 6:54 pm PST

Consumption of beef tongue: Human BSE risk associated with exposure to
lymphoid tissue in bovine tongue in consideration of new research findings -
Scientific Opinion of the Panel on Biological Hazards

Question number: EFSA-Q-2007-110

Adopted date: 17/04/2008




Following a request from the European Commission, the Panel on Biological
Hazards (BIOHAZ) was asked to deliver a scientific opinion on the human BSE
risk associated with exposure to lymphoid tissue in bovine tongue in
consideration of the findings included in a scientific article recently
published on the consumption of beef tongue and the risk for public health.

This scientific article describes the distribution of lymphoid tissue in
bovine tongue and the location of bovine lingual tonsil. In addition, it
concludes that the method currently prescribed for harvesting bovine tongues
in slaughterhouses is not appropriate for removing all specified risk
material (SRM) and proposes an alternative harvesting method.

EFSA was requested (i) to evaluate the design of the study and its
scientific validity in relation to the distribution of lymphoid tissue in
bovine tongue and (ii) to evaluate the conclusions and recommendations of
the study in relation to BSE risk from bovine tonsil following the
harvesting method currently prescribed by EU legislation compared to the
alternative one proposed in the study.

The BIOHAZ Panel reviewed the scientific article and concluded that the
study further confirms and extends observations that the lingual tonsil at
the base of the tongue may not be entirely eliminated when harvesting
tongues by means of the method currently prescribed.

In reply to the second request, the BIOHAZ Panel assessed different
parameters in order to quantify the human exposure risk to BSE from bovine
tonsil associated with the consumption of bovine tongue. It was concluded
that, overall, the level of infectivity in bovine tonsil is low. This,
together with the declining and overall low BSE prevalence and the current
policy on SRM removal, suggests a very low, if not negligible, human BSE
exposure risk associated with exposure to lymphoid tissue in bovine tongue
harvested as currently prescribed by EU legislation. The BIOHAZ Panel
further concluded that currently there are not sufficient quantitative data
available allowing a comparison of the human BSE exposure risk reduction
achieved by the alternative tongue harvesting method proposed by the study
in comparison to the harvesting method currently prescribed. However, it is
likely that the proposed method would only provide a marginal reduction in
the risk from bovine tonsil compared to the one currently prescribed.

Following to this, the BIOHAZ Panel made a series of recommendations on the
topics that might be addressed in future studies on the subject.

Annex 1

3.1.2. Experimental BSE cases after oral challenge. • In a sequential kill
pathogenesis study of BSE in which calves were experimentally infected by
oral exposure to 100g of a pool of BSE affected brainstems (a dose
considered far in excess of most natural exposures (Arnold et al., 2007)),
palatine tonsil was assayed in conventional mouse strains from all time
points (2-40 months post-exposure). No infectivity was detected (Wells et
al., 1998; Wells et al., 2005). • Palatine tonsil from this sequential kill
pathogenesis study was further assayed by intracerebral inoculation of
cattle, which provides a 500 fold (log10 2.7) greater sensitivity of
detection of the BSE agent than the RIII mouse assay (Wells et al., 2005). A
pooled inoculum was prepared from palatine tonsil of cattle (3, 3, 3 and 1
respectively) at each of 6, 10, 18, and 26 month periods post-exposure
(corresponding to 10, 14, 22 and 30 months of age) from the oral challenge
study. One ml of a ten per cent tissue homogenate in saline was injected by
the intracerebral route into groups of 5 calves. Results indicated traces of
infectivity in the palatine tonsil of cattle killed ten months after
experimental oral exposure, with transmission occurring in 1 of the 5
challenged calves (Wells et al., 2005). An infection rate of 1 out of 5
suggests that the infectivity is close to the limit of detection of the
assay and that the titre of infectivity in tonsil is less than 1 cattle i.c.
ID50/g. The study was completed in 2006, without further transmission to any
of the 4 remaining calves (Veterinary Laboratory Agency, unpublished
data). • Palatine tonsil collected from a further sequential-kill
pathogenesis study of BSE in which 100 calves were exposed to 100g of a pool
of BSE affected brainstems (Arnold et al 2007) was also bioassayed. In this
study, Espinosa et al. (2007) inoculated tonsil tissue intracerebrally into
mice expressing bovine PrP (BoPrP-Tg110). The inocula originated from pooled
samples from three cattle at each of five time points (20, 24, 27, 30 and 33
months) after the oral exposure of calves. Infectivity was detected at all
of the time points tested, with infection rates in the mice of 1/6, 1/6,
1/5, 1/6 and 1/6 respectively. Interestingly, this would seem to indicate a
relatively low constant plateau level of infectivity in the tonsil
throughout this largely preclinical period. The low infection rate (1 out of
6) is consistent with a level of infectivity lower than 1 i.c.ID50 in Tgbov
mice. Buschmann and Groschup (2005) provide data on a Tgbov mice model
giving a 1.000-10.000 fold greater sensitivity than the cattle bioassay.
While the Tgbov mice (tg110) in the study of Espinosa et al. (2007) were
different from the TgbovXV mice used by Buschmann and Groschup (2005), their
sensitivity is likely to be similar based on comparable levels of over
expression of the bovine PrP gene in each model. From data provided by
titrations of BSE infectivity in brain by the i.c. and the oral routes in
cattle it has been calculated that one bovine oral ID50 = 105.5 bovine i.c.
ID50 (Wells et al., 2007). On this basis, the titre of infectivity in tonsil
is less than 10-5.5 bovine oral ID50/g; an estimate, using additional data,
that is closely similar to that provided previously (EFSA 2004). Given the
greater sensitivity of the Tgbov mouse assays than the cattle i.c. assay, it
would seem probable that even in cattle after exposure to a 100g oral dose,
the titre in terms of bovine oral ID50/g may be at least one order of
magnitude less (i.e. 10-6.5 bovine oral ID50/g), for at least part of the
incubation period. As the majority of exposures in the epidemic were
probably less than 1g (Arnold et al., 2007), the infectivity in tonsil might
well have occurred at an even lower titre and have peaked later in
incubation. The most optimistic estimate might be that with doses of the
order of less than 1g, tonsil never contains detectable levels at any time
in the disease course. A table summarising the key parameters of the
infectivity studies mentioned in section 3.1 can be found in Appendix A. In
summary, the available data on BSE infectivity in tonsil indicate that: •
The frequency with which detectable infectivity occurs in tonsillar tissue
of a BSE naturally infected animal is difficult to estimate. Although only
two cases have been investigated employing biological assay, infectivity has
not been detected in palatine tonsil from naturally occurring clinical
cases. • From sequential kill pathogenesis studies in experimentally
infected cattle, infectivity associated with tonsil could be detected at 10
months post-exposure and, in the preclinical period, 20-33 months
post-exposure (Wells et al., 2005; Espinosa et al., 2007), suggesting that,
in this model, infectivity in tonsil probably persists throughout the
disease course. • In the experimental exposure to a 100g dose, infectivity
titer can be estimated from the data available to be 10-6.5 bovine oral ID50
per gram of tonsilar lymphoid tissue. • It must also be noted that all
available data on infectivity relate to palatine tonsil. Occurrence and
comparable levels of infectivity in lingual tonsil is an assumption. 3.2.
Amount of lymphoid tissue associated with the tongue intended for human
consumption As mentioned above, the findings of the study from Casteleyn et
al. (2007) are consistent with those of other studies carried out in Great
Britain (Wells et al., 2005) and Germany (Rebmann et al., submitted for
publication), but provide no quantification of lymphoid tissue remaining in
tongues intended for human consumption. Examination of 251 tongues, derived
from 15 abattoirs in Great Britain after removal of SRM and intended for
human consumption, showed that visible identifiable lingual tonsillar
tissue, indicated by fossulae, remained in 76.5% of them (192 out of 251)
(Wells et al., 2005). Even in the tongues in which no visible tonsillar
tissue remained, histological examination revealed lymphoid tissue in more
than 90% of them. Variations in the distribution of the lingual tonsil
suggested that even after the most rigorous trimming of the tongue traces of
tonsillar tissue may remain. However, the histological examination did not
extend rostral to the most caudal of the filiform papillae, which occur
caudal to the most caudal vallate papillae. In Germany (Rebmann et al.,
submitted for publication), specimens of the lingual mucosa were taken from
thirty cattle immediately after slaughter. The main parameter to identify
the lymphoreticular tissue in this study was the immunohistochemical
identification of the follicular dendritic cells (FDC). Lymph follicles were
detected in areas up to 30 mm rostral to a given macroscopic landmark (i.e.
the most caudal of the vallate papilla). This is an area which would not be
removed from the tongue when implementing the measures foreseen by
Regulation (EC) No 999/2001. Alternative technical approaches for the
removal of the lingual tonsil’s tissue, similar to those by Casteleyn et al.
in 2007, are proposed by the authors. These scientific studies did not take
into account the rostral part of the tongue which can harbor solitary
primary lymph nodules or diffuse accumulations of T and B lymphocytes. Some
information on more rostral areas of the tongue was provided in a more
recent study (Kato and Sawada, 2008). In that study, examination of the
bovine tongue was carried out from the tip of the torus linguae to the root
of the tongue. The study confirmed previous results in relation to the
distribution of lymphoid tissue in bovine tongue. In addition, 1 out of the
20 specimens collected rostrally to the most rostral vallate papilla
contained lingual tonsillar tissue. Based on the above data, neither
qualitative nor quantitative estimation of the significance of the lymphoid
formations located rostrally to the most caudal vallate papilla is feasible
at present. In the DNV risk assessment cited in section 3.1 (EFSA, 2004;
SEAC, 2003), a total weight of bovine tonsil of 50g is assumed. However, in
the DNV risk assessment, it is stated that the weights of the various
tissues were taken from the LFRA ruminant products audit (LFRA, 1997). The
total weight of tonsillar tissue in a typical bovine, as given from
literature derived offal weights in the LFRA audit, is estimated at 200g.
This suggests that the value of 50g in the DNV report is in fact referring
to the weight of lingual tonsil. If the largest contribution to the total
weight of tonsillar material is the palatine tonsil, it would be reasonable
to estimate lingual tonsil as approximately 50g. It is further assumed in
the DNV risk assessment that, after removal of all visible tonsillar tissue,
the realistic upper limit of tissue that would remain in the tongue would be
10% i.e. 5g. Since the palatine tonsil is a circumscribed structure and
easily identified, complete removal, compared to the lingual tonsil, is
ensured, so this estimate remains valid as the quantity of tonsillar tissue
that might not be removed. In conclusion, based on the data available:

• it is not possible to know exactly the quantity of lymphoid tissue
remaining in bovine tongue intended for human consumption when harvested
according to the harvesting method currently applied, even if it can be
estimated to be 5g;

• it is not possible to estimate how much lymphoid tissue is removed by the
alternative harvesting method compared with the one currently applied.

snip...full text ;


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Wednesday, April 30, 2008

Consumption of beef tongue: Human BSE risk associated with exposure to
lymphoid tissue in bovine tongue in consideration of new research findings


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