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Scientific Opinion on BSE/TSE infectivity in small ruminant tissues
The objectives addressed were i) to provide an update on TSE (Transmissible Spongiform Encephalopathy) infectivity distribution in small ruminant tissues; and ii) to indicate based on the current epidemiological situation as regards to BSE (Bovine Spongiform Encephalopathy) in the small ruminant population in the EU (European Union), whether a review of the existing SRM (Specified Risk Materials) list for small ruminants should be envisaged with regard to the potential exposure to the BSE agent. The appraisal was addressed by reviewing for Classical scrapie, BSE and Atypical scrapie in small ruminants aspects related to: i) tissue infectivity distribution according to the age and the genotype of sheep and goats; and ii) the infectious load in the different tissues. In order to perform the assessment all the currently available scientific results were reviewed, and data on TSE monitoring in small ruminants in the EU and on small ruminants slaughtered by species and age category in each EU Member State were considered. The reduction of the infectivity associated to the carcass of an infected individual achieved by the current SRM policy in small ruminants for Classical scrapie and BSE was estimated. The total number of Classical scrapie infected sheep and goats that could enter yearly into the food chain was provided. Moreover, considerations about Atypical scrapie were given. A set of simulations allowing estimating the impact of different policy options on the BSE infectious load potentially present in an infected sheep was provided.
© European Food Safety Authority,2010
Following a request from the European Commission (EC), the Panel on Biological Hazards (BIOHAZ Panel) was asked to deliver a scientific opinion on BSE/TSE infectivity in small ruminant tissues.
The most recent scientific opinion on TSE infectivity distribution in small ruminant tissues was published in January 2002 by the Scientific Steering Committees (SSC) and last amended in November 2002. In recent years new scientific data relating to the infectivity of some tissues in small ruminants became available. Some of those findings related to the tissues from sheep and goats might have an impact to the current measures in relation to the Specified Risk Material (SRM) list of the Regulation (EC) 999/2001.
Therefore, the EC asked EFSA: i) to update, as regards small ruminants and on the basis of the most recent scientific data, the SSC scientific opinion from 2002 on TSE infectivity distribution in ruminant tissues; ii) to indicate based on the current epidemiological situation as regards BSE in the small ruminant population in EU, whether a review of the existing SRM list for small ruminants should be envisaged with regard to the potential exposure to the BSE agent.
The BIOHAZ Panel addressed the mandate by reviewing individually for Classical scrapie, BSE and Atypical scrapie in small ruminants aspects related to: i) tissue infectivity distribution according to the age and the genotype of sheep and goats; and ii) the infectious load in the different tissues.
In order to perform the assessment all the currently available scientific results were reviewed. Data about the TSE monitoring in small ruminants in the EU were provided by the European Commission and information on small ruminants slaughtered by species and age category in each EU Member State were provided by the EFSA Focal Points Network.
It was emphasized that this assessment required several assumptions. Moreover, the estimates of the infectious load are based on a simple approach using computations based on a low and a high estimate of each of the parameters. This provides order of magnitude estimates of the infectious load of TSE agents entering into the food chain at EU 27 level. This approach could be replaced by a probabilistic model to provide more insight into the uncertainties. However, due to time and resources constraints it was not possible for the BIOHAZ Panel to develop and validate such a probabilistic model within the framework of this mandate.
Considering Classical scrapie in small ruminants it was concluded that the current SRM policy allows a reduction of the relative infectivity associated to the carcass of an infected animal of about 1 log10 (infectious load as expressed in IC ID50 in C57Bl6 mice). The infectivity load as expressed in the opinion (IC ID50 in C57Bl6 mice) cannot be related to any quantifiable dietary transmission risk in farmed animals or humans.
As regards to Classical scrapie in goats, it was further concluded that, according to the currently available knowledge, goat kids below 3 months of age, even coming from infected herds, represent a negligible source of infectivity for the food chain.
On the basis of data collected between 2007 and 2009, the total number of Classical scrapie infected animals that could enter yearly into the food chain in the EU27 as a whole was estimated to approximately range between 16,000 and 67,000 (most probable estimate 29, 000) for sheep and between 10,000 and 34,000 (most probable estimate 13,000) for goats.
The Panel pointed out that Classical scrapie is present in a majority of EU member states. However because differences in the prevalence of the disease, population size and production system (age at slaughter), there are significant differences between certain member states with regards to Classical scrapie infectivity load that may enter the food chain. This heterogeneity and the differences in consumption pattern between countries and regions mean that the dietary exposure to Classical scrapie cannot be considered to be homogeneous in the EU27.
It was furthermore concluded that at the EU27 level, the current SRM policy in force allows a global reduction of the potential exposure to Classical scrapie which can be estimated to be around 1 log10 (infectious load as expressed in IC ID50 in C57Bl6 mice).
When considering BSE in small ruminants, the Panel concluded that with 95% confidence the the number of BSE cases that could enter yearly into the food chain in the EU is ranging between 0 and 240 for sheep and between 0 and 381 for goats. This estimate argues against any current widespread BSE epidemic within the EU small ruminant population.
The BIOHAZ Panel indicated that the current SRM policy allows a reduction of the relative infectivity associated to the carcass of a BSE infected animal of about 1 log10 (infectious load as expressed in IC ID50 in C57Bl6 mice). The infectivity load as expressed in the opinion (IC ID50 in C57Bl6 mice) cannot be related to any quantifiable dietary transmission risk in farmed animals or humans.
It was further emphasized that preliminary biochemical and immunohistochemical data in goats suggest that there might be no major involvement of the lymphoid tissues in preclinical and clinical phase of the disease after oral experimental challenge. Before more complete information becomes available it is not possible to provide reliable specific estimates of the impact of SRM removal measures on the BSE exposure that would be associated with an infected goat entering into the food chain. The Panel highlighted that in this context the estimates of the impact of SRM removal measures on the BSE exposure provided for BSE in sheep could be considered as a worst case scenario for BSE in goats.
As regards to Atypical scrapie both in sheep and goats it was concluded that low levels of infectivity can be present in peripheral tissues (lymphoid tissues, nerves, skeletal muscle) in preclinical and clinical cases of Atypical scrapie harbouring various genotypes. Consequently SRM measures cannot be assumed to prevent the entry of the Atypical scrapie agent into the food chain.
It was highlighted that there is currently no data on the kinetics of distribution of the Atypical scrapie agent into peripheral tissues of incubating small ruminants and that there are uncertainties on the Atypical scrapie pathogenesis and its true prevalence in the EU small ruminant population. Therefore, the Panel was not in position to provide an assessment of the current Atypical scrapie infectious load entering into the food chain.
In answering to the first Term of Reference, the BIOHAZ Panel revised the TSE tissue infectivity distribution in small ruminants and provided updated information within the body of the opinion (section 2, tables 1 to 12).
Considering the second Term of Reference, the BIOHAZ Panel provided a set of simulations illustrating the impact of different policy options on the BSE infectious load potentially present in an infected sheep. According to these simulations, the use of the dressed carcass only would allow a greater reduction of the BSE exposure risk than the current SRM policy measures. The elimination of the ileum has a major impact on the relative reduction of the BSE infectivity load that might enter in the food chain from an animal aged below 12 months. The CNS (Central Nervous System) removal is the most efficient measure to reduce the relative infectivity load associated with a BSE infected small ruminant older than 12 months entering into the food chain.
It was finally indicated that a modification of the SRM list driven only by consideration about BSE will also impact on the dietary exposure to Classical scrapie and Atypical scrapie agents.
The BIOHAZ Panel recommended: i) to update the assessment once data from ongoing experiments will become available; ii) to develop a specific probabilistic model in order to provide more precise estimates of the impact of SRM removal on the infectious load of TSE agents entering into the food chain at EU 27 level; iii) to improve the quality of the data collected on the small ruminant population (e.g. age category and destination of the animal); and iv) to expand the current data collected in the context of the TSE surveillance activities by recording the tested animal age category and the type of rapid test used.
Bovine Spongiform Encephalopathy (BSE), Classical scrapie, Atypical scrapie, Transmissible Spongiform Encephalopathies (TSEs), Specified Risk Material (SRM), Small Ruminants