Consumption of beef tongue: Human BSE risk associated with exposure to lymphoid tissue in bovine tongue in consideration of new
Posted Apr 09 2009 7:13pm
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
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.
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.