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From: TSS ()
Subject: Opinion of the Scientific Panel BIOHAZ on the Breeding programme for TSE resistance in sheep [1]
Date: October 1, 2006 at 6:27 pm PST

Opinion of the Scientific Panel BIOHAZ on the Breeding programme for TSE resistance in sheep [1]
Last updated: 26 September 2006 Publication Date: 27 July 2006

Adopted on 13 July 2006. (Question N° EFSA-Q-2005-291)

Annex I
Annex II
Annex III

Decision 2003/100/EC (EU 2003) lays down a breeding programme in sheep with compulsory minimum requirements for all flocks of sheep of high genetic merit from 1 April 2005 on but has been applied also before that date in some Member States (MS). This breeding programme aims, within a sheep flock, to increase the frequency of the ARR alleles, which have been shown to contribute to TSE resistance, and to reduce the prevalence of alleles (e.g. VRQ) that have been shown to contribute to TSE susceptibility. The programme has been contested by MS and in scientific publications, mainly because of the emerge of atypical cases. The European Commission (EC) therefore, requested the European Food Safety Authority (EFSA) and its Scientific Panel on Biological Hazards (BioHaz Panel) for an opinion on this breeding programme for TSE resistance in sheep. More in particular EFSA was asked to evaluate if the current breeding programme in sheep is considered appropriate from a scientific point of view taking into account possible adverse effects on resistance against other disease, risks of inbreeding, the emerging of atypical cases and the current prevalence of the different TSEs and this in the framework of the eradication programmes of BSE and of classical scrapie. If EFSA did not consider the current breeding programme appropriate then it is to give scientific guidance for possible modifications or an alternative strategy. EFSA was further asked to indicate if human exposure risk from BSE infectivity in ARR/ARR sheep is still considered unlikely as previously indicated (EFSA, 2003). Lastly, EFSA was asked to indicate if the determination of genotypes in accordance with point 8.2 to Chapter A in Annex III of Regulation 999/2001 was appropriate to compare the prevalence of genotypes in TSE infected sheep with the prevalence of genotypes in the population and to follow the evolution of genotypes in time e.g. as a consequence of the breeding programme. EFSA was also requested to give scientific guidance for possible modifications if it did not find appropriate the system for determination of genotype.
In replying to the mandate, the BioHaz Panel concluded that on the general risks associated with current breeding strategy up to date no adverse effects have been stated.

Regarding the risks of “counter” selection of TSE strains in resistant genotypes EFSA and its BioHaz Panel concluded that today there is no evidence that the current breeding programme has in any way led to the counter-selection of TSE strains (including atypical scrapie) so far. On the risks of negative effects of the ARR allele on production traits or susceptibility to other diseases the BioHaz Panel concluded that no effects are proven at this stage.

Regarding the appropriateness for eradication of atypical scrapie the current breeding programme may be expected to reduce the occurrence of this TSE. However, the time scale for its reduction will be very long because AHQ and AFRQ, the most susceptible alleles to the atypical scrapie, are not specifically targeted and their decrease in frequency is only a consequence of the ARR increase.

Given the low frequency of multiple cases of atypical scrapie within flocks and the level of susceptibility of the ARR genotype there is a very low risk of disease in the remaining ARR/ARR animals in a flock that had atypical scrapie. The BioHaz Panel therefore recommends continuing the current breeding programme. It also recommends, in order to face potential identified negative effects of selecting for resistance genes, a rapid eradication of the most susceptible alleles and the increase of the frequency of the ARR allele in order to control efficiently the current scrapie epidemics and a preservation of the PrP variability as to be able to face possible new type of TSE in sheep for which alleles others than ARR could be a source of resistance. In terms of further research it is recommended organizing experimental research besides monitoring to exclude those genes controlling any production and health traits are located on the same chromosome as the PrP locus.

In respect of the question if there is a risk of excretion of BSE in sheep, the BioHaz panel concluded that the proportion of sheep being infected by BSE is likely to be extremely small. In connection to this conclusion the BioHaz Panel recommends regularly updating of the BSE estimation in the European sheep population.

In respect of the last question of the mandate on the determination of genotypes in accordance with point 8.2 to Chapter A in Annex III of Regulation 999/2001, the BioHaz panel concludes that for the purpose of estimating the risk for the different genotypes, the genotyping results for the selective culling can be used. In cases the whole flock is culled, genotyping at least 50 sheep per flock seems a minimal number. For the genetic screening and the monitoring of genetic progress in the general population the current strategy is inappropriate.

The Biohaz Panel recommends increasing the sample size to evaluate the genetic progress in sheep breeds, to request epidemiological relevant data, to genotype at least codons 136, 141, 154 and 171 and lastly to sequence the whole open reading frame of the PrP gene rather than investigate only mutations in the 3 currently tested codons especially in TSE cases in order to be prepared for the possible emergence of new scrapie types with a preference for genotypes with mutations in other codons.


[1] For citation purposes: Opinion of the Scientific Panel on Biological Hazards on “the breeding programme for TSE resistance in sheep”, The EFSA Journal (2006), 382, 1-46

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