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Scientific and technical assistance on the minimum sample size to test should an annual BSE statistical testing regime be authorised in healthy slaughtered cattle

EFSA Journal 2012;10(10):2913 [90 pp.]. doi:10.2903/j.efsa.2012.2913
European Food Safety Authority Acknowledgment EFSA wishes to thank the members of the Working Group on sample size healthy slaughter cattle BSE monitoring: Olivier Andreoletti, Christian Ducrot, Aline de Koeijer, Giuseppe Ru, Thomas Selhorst and Emmanuel Vanopdenbosch for the preparatory work on this scientific output and EFSA staff: Ernesto Liebana Criado and Luis Vivas-Alegre for the support provided to this scientific output, and to José Cortiñas Abrahantes and Gabriele Zancanaro for having supported the contractor that provided the model C-TSEMM. Contact biohaz@efsa.europa.eu
Type: Scientific Report of EFSA On request from: European Commission Question number: EFSA-Q-2011-01175 Approved: 08 October 2012 Published: 15 October 2012 Last updated: 04 April 2014. This version replaces the previous one/s. Affiliation: European Food Safety Authority (EFSA) Parma Italy
Abstract

This Scientific Report of EFSA provides scientific and technical assistance to the European Commission on (i) the minimum sample size to test, should an annual Bovine Spongiform Encephalopathy (BSE) statistical regime be authorised in healthy slaughtered cattle in certain EU Member States (MSs), and (ii) on the added value of that sample size for monitoring the trend of Classical BSE, Atypical BSE, and the emergence of a hypothetical new type of cattle Transmissible Spongiform Encephalopathy (TSE). Firstly, an evaluation of the epidemiological trends of BSE in 25 EU MSs was carried out in groups based on historical BSE monitoring data. Secondly, and with the aid of a purpose-built model called Cattle TSE Monitoring Model (C-TSEMM) developed by an EFSA contractor, both the assessment of the design prevalence and of the sensitivity of different BSE monitoring scenarios were carried out. Among the assumptions made in the C-TSEMM, a key one is that for those EU MSs with no, or few, BSE cases post-2001 an alternative estimate of cohort-based prevalence is required. This is estimated based on the average prevalence of the group of MSs with BSE cases under which they were placed in previous EFSA Opinions. Also, the model estimates presented are based on the demographics of the adult cattle population and on the number of adult cattle removed from the population via the different streams in 2011 (i.e. healthy slaughter, animals showing clinical signs of disease during ante mortem inspection, emergency slaughtered animals and fallen stock). Therefore, future fluctuations in those numbers at EU level and in each of the MSs will impact on the validity of the estimates presented in this report. A series of recommendations are made on sampling strategies for BSE monitoring and on the future use of the C-TSEMM.  

© European Food Safety Authority, 2012

Summary

The European Commission has requested that the European Food Safety Authority (EFSA) provides scientific and technical assistance on the minimum sample size to test should an annual Bovine Spongiform Encephalopathy (BSE) statistical regime be authorised in healthy slaughtered cattle. In particular, in a scenario where the BSE testing of at-risk cattle would remain unchanged (i.e. testing of 100 % of at-risk cattle over 48 months), EFSA was asked: (i) to propose a minimum annual sample size in healthy slaughtered cattle above 72 months of age, that would allow the detection of BSE with a yearly design prevalence of at least 1 case per 100 000 in the adult population (i.e. older than 24 months of age) of the Member States (MSs), at a confidence level of 95% and both in the group of 25 EU MSs that are entitled for having the BSE monitoring system in healthy slaughtered cattle reviewed as a whole and in each Member State individually; and (ii) to advise on the added value of this minimum sample to the overall surveillance programme in terms of monitoring the trend of Classical BSE, Atypical BSE and the emergence of a hypothetical new type of cattle Transmissible Spongiform Encephalopathy (TSE).

Firstly, an evaluation of the epidemiological trends of BSE in the 25 EU MSs is presented in this report based on BSE monitoring data provided by the European Commission. For this purpose, MSs are grouped following a similar approach taken in former EFSA Opinions: EU17 (Austria, Belgium, Cyprus, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Luxembourg, Netherlands, Portugal, Slovenia, Spain, Sweden and United Kingdom) and EU8 (Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland and Slovakia). The EU8 group of MSs is further subdivided in EU5 (Estonia, Hungary, Latvia, Lithuania and Malta), where BSE has not been detected and in EU3 (Czech Republic, Poland and Slovakia), where BSE has been detected. It is to be noted that unless otherwise specified, the term BSE on its own refers to refers to all BSE types, including Classical BSE, Atypical BSE and “Unknown” type of BSE (i.e. a reported BSE case that has not been typed).

Based on that first evaluation, it was concluded that a constant decline in the total number of detected BSE cases (i.e. coming from both Active and Passive surveillance) has been recorded in the EU17 group from 2 157 cases in 2001 to 27 cases in 2011.  In the EU3 group, the number of detected cases dropped down from 28 in 2005 (peak) to one in 2011. Moreover, the log10 transformed annual BSE prevalence and incidence (defined respectively as the number of positive BSE cases out of the tested population and out of the standing adult cattle population) in the EU17 and in the EU8 show a statistically significant decreasing trend. There has been a statistically significant increasing trend in the average age of the detected BSE cases per test year during the last 11 years and eight years in the EU17 and the EU8, respectively. At present, this average age exceeds 11 years in each of these MSs (where reported in 2011). Furthermore, and assuming that the age distribution of cattle within the EU25 has not changed substantially, the decreasing trend observed in the annual BSE occurrence and the increasing trend observed in the annual average age of the cases are the consequence of the implementation of the BSE control measures.

Regarding Atypical BSE, it is concluded that epidemiological data reported by the EU MSs indicate that over the last years the number of detected did not show any trend and that these cases were mainly identified in the fallen stock and healthy slaughtered animals older than eight years of age. However, it is also noted that the performance of the current BSE monitoring system, both in terms of its analytical sensitivity and earliness of the detection of animals infected with Atypical BSE is unknown.

Secondly, a model called Cattle TSE Monitoring Model (C-TSEMM) was developed by an EFSA contractor in order to provide a general frame for evaluating the design prevalence and the sensitivity of cattle TSE monitoring systems. The model was built considering available historical EU wide data on BSE monitoring, and contains assumptions, limitations and uncertainty that have to be considered when interpreting the different estimates that the model provides.

Among those assumptions, a key one is that for MSs with no, or few, BSE cases post-2001 an alternative estimate of cohort-based prevalence is required. This has been estimated for those MSs based on the average prevalence of the group of MSs with BSE cases under which they were placed in previous EFSA Opinions: the EU17 or the EU8 group. This results in an overestimate of prevalence for countries with no recorded cases as they are assumed to be a merged epidemiological unit with MSs where cases are observed.

Based on the estimates provided by the C-TSEMM model (that considered prevalence in the standing adult cattle population (i.e. period prevalence in a given year of detectable infected animals in the standing population) and the available historical EU wide data on BSE monitoring), it can be concluded that in the EU25 as a whole the current BSE monitoring regime enables the detection of one BSE case in 6 354 930 adult cattle with a confidence level of 95%. Moreover, if the current BSE monitoring regime would exclude testing of healthy slaughter cattle, it would be able to detect in the standing population one BSE case in 4 021 940 adult cattle with a confidence level of 95%. Therefore, no healthy slaughtered animals need to be tested in order to meet a design prevalence of 1 detectable case in 100 000 adult cattle, since testing of at risk animals (i.e. animals showing clinical signs during ante mortem inspection, emergency slaughter and fallen stock over 48 months of age and clinical suspects) is sufficient to meet the proposed design prevalence.

Furthermore and also based on C-TSEMM model estimates, it can also be concluded that at individual MS level, in eight MSs (Belgium, Denmark, France, Germany, Ireland, Netherlands, Spain and the UK) the testing of healthy slaughter animals is not needed in order to meet a 1 in 100 000 design prevalence with a confidence level of 95%, since testing of at risk animals is sufficient to meet the proposed design prevalence. On the other hand, in four MSs (Austria, Italy, Poland and Sweden) the testing of a fraction of healthy slaughtered animals older than 72 months of age (i.e. on the basis of the number tested in 2011) would be sufficient to meet a 1 in 100 000 design prevalence with a confidence level of 95%. Finally, in thirteen MSs (Cyprus, Czech Republic, Estonia, Finland, Greece, Hungary, Latvia, Lithuania, Luxembourg, Malta, Portugal, Slovakia and Slovenia) the number of tested animals in 2011 (i.e. including all the healthy slaughtered animals older than 72 months of age) did not allow to meet a 1 in 100 000 design prevalence with 95% confidence. However, fitting a sample size larger than the actually slaughtered cattle population of a MS is neither feasible nor realistic. Thus, the current testing of all animals of certain age categories that are slaughtered or dead may provide the most sensitive BSE monitoring system possible (i.e. that employs post mortem tests) under the current epidemiological scenario with the potential limitation on the impact of the age at testing as evaluated in former related EFSA Opinions.

It is further concluded that in the event of a re-emergence of Classical BSE, stopping the testing of healthy slaughtered cattle would lower the sensitivity of its detection by the TSE monitoring system. As an example, based on a theoretical scenario of an annual 10% increase in detectable cases in the tested population (prevalence), the C-TSEMM model estimates that: (i) In the EU25 as a whole, where testing healthy slaughtered cattle above and age of 72 months is not needed in order to meet the proposed design prevalence, the time to detection of the supposed 10% yearly increase in detectable cases would increase from six to 11 years ( i.e. five extra years to detect the supposed 10% yearly increase in prevalence of detectable cases) should testing of healthy slaughtered cattle be stopped compared to the current testing regime; (ii) In those MSs where testing healthy slaughtered cattle above the age of 72 months is not needed in order to meet the proposed design prevalence (Belgium, Denmark, France, Germany, France, Ireland, Netherlands, Spain and the UK), it would take between three and eight extra years (depending on the MS) to detect that yearly increase in prevalence should testing of healthy slaughtered cattle be stopped compared to the current testing regime; (iii) In those MSs where testing healthy slaughtered cattle could be reduced in order to meet the proposed design prevalence (Austria, Italy, Poland and Sweden), it would take between six and 16 extra years (depending on the MS) to detect that yearly increase in prevalence should testing of healthy slaughtered cattle older than 72 months of age be reduced to the number needed to meet the proposed design prevalence compared to the current testing regime; (iv) In those MSs where testing healthy slaughtered cattle older than 72 months of age as per the current BSE monitoring regime is not sufficient to meet the proposed design prevalence(Cyprus, Czech Republic, Estonia, Finland, Greece, Hungary, Latvia, Lithuania, Luxembourg, Malta, Portugal, Slovakia and Slovenia), it would take between three and 25 extra years (depending on the MS) to detect that yearly increase in prevalence should testing of healthy slaughtered cattle be stopped compared to the current testing regime.

When considering Atypical BSE, at EU25 as a whole there is not sufficient data (i.e. number of detected cases annually) to reliably estimate with the C-TSEMM model the impact of the stopping/continuation of testing healthy slaughtered animals older than 72 months. However, when using France as an example (i.e. country with a large population and sufficient number of detected Atypical cases) the C-TSEMM model indicates that, based on a theoretical scenario of an annual 10% increase of detectable prevalence of Atypical BSE in the tested population, it would take an extra 13 years to detect that yearly increase in prevalence should testing of healthy slaughtered cattle be stopped compared to the current testing regime.

Considering the timeframe available for this mandate, carrying out simulation studies for hypothetical new types of cattle TSEs was not possible. However, it was concluded that the C-TSEMM model can be considered as a useful tool in order to simulate future ad hoc epidemiological scenarios of hypothetical new types of cattle TSEs.

It is highlighted that when interpreting the estimates presented above or those obtained in future simulations performed with the C-TSEMM model, consideration has to be given to the assumptions, limitations and uncertainty in the model. Moreover, the models estimates presented in this report are based on the demographics of the adult cattle population in 2011 and on the number of adult cattle removed from the population via the different streams (i.e. healthy slaughter, animals showing clinical signs of disease during ante mortem inspection, emergency slaughtered animals and fallen stock). Therefore, future fluctuations in those numbers at EU level and in each MSs will impact the validity of current estimates.

A series of recommendations are made in this report including considerations on the sampling strategy should monitoring of BSE in healthy slaughtered cattle remain based on a sample of animals over certain age, and considerations on future potential needs for the assessment of the impact of changes to current EU BSE control measures in the sensitivity of the EU surveillance system.

It is finally recommended that if the C-TSEMM model will be employed in future years for the review of the BSE monitoring regime in the EU, updated yearly data including BSE testing data have to be considered as these drive the results estimated by the model.

Keywords

cattle, TSE, BSE, monitoring, design prevalence