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Breeding Programmes Needed to Eliminate Scrapie

15 August 2014

An eradication policy for Classical Scrapie that relies only on the detection and culling of infected flocks and does not include breeding programmes for resistance is unlikely to succeed, according to experts on EFSA’s Panel on Biological Hazards.

Following a request from the European Commission (EC), the EFSA Panel on Biological Hazards (BIOHAZ) was asked to deliver a scientific opinion on the scrapie situation in the European Union (EU) after 10 years of monitoring and control in sheep and goats.

In particular, in order to evaluate the measures in place and assess the progress accomplished, the EC consulted EFSA to gain a better understanding of the dynamics of the epidemiological situation of Classical scrapie (CS) and Atypical scrapie (AS), and a retrospective analysis of the effectiveness of the control tools encompassed by EU legislation.

The opinion provides background information in relation to CS and AS in sheep and goats, including information on the nature of the agents, genetic susceptibility of the hosts, transmission pathways and epidemiology of the diseases.

Based on information from the EU TSE surveillance database, CS and AS in sheep and goats in the period 2002-2012 have been analysed to provide a spatio-temporal description of the occurrence of the diseases, after appropriate statistical adjustment accounting for the main known confounding factors.

Due to the heterogeneity of the distribution of CS within the different EU Member States (MSs), and, to some extent, of the monitoring and control measures implemented, it was not considered meaningful to present an overall EU trend of the disease. Therefore, the temporal trends have been considered country-by-country.

CS in sheep was reported in 17 MSs, with an overall average prevalence of 8.7 cases per 10 000 rapid tests performed. Both the temporal trend and geographical distribution of CS showed great heterogeneity across the MSs. Among countries reporting a sufficient number of cases of CS in sheep over the years, six MSs showed a statistically significant decreasing trend, and seven showed a trend not statistically different from a flat one.

CS in goats was reported in eight MSs, with an overall prevalence of 9.8 cases per 10 000 rapid tests performed. This is mostly explained by the unique epidemic in one MS, while the overall prevalence in the remaining seven MSs was 2.2 cases per 10 000 rapid tests. A statistically decreasing trend was detected in one MS for the period 2002-2012, and in two more MSs for the period 2007-2012.

Although it was not possible to identify causes that can explain objectively the failure to improve the situation of CS in some MSs, the assessment of country-specific data, obtained through ad hoc surveys to MSs, and related to the implementation of surveillance and of genetic and non-genetic control measures, allowed the formulation of some hypotheses.

In the case of sheep, these included ineffective implementation of genetic and non-genetic measures for the control of the disease, whereas in goats these included the absence of genetic measures and the variability of the non-genetic measures applied.

AS in sheep was reported in 21 MSs, with an overall prevalence of 5.8 cases per 10 000 rapid tests performed. A similar prevalence over time and space was observed, with no large epidemics, and only sporadic detection in five MSs. Only two MSs show a statistically significant trend, decreasing in one case and increasing in the other case.

AS in goats was reported by five countries, at a very low prevalence and with no statistically significant trend in any of them.

The different control options available for CS in sheep and goats are discussed. Firstly, non-genetic control measures are described, with the support of data from two case-studies in non-EU countries applying exclusively non-genetic measures for a period of time.

Detection and eradication measures in affected flocks were effective in reducing the observed prevalence of CS in a population with a high prevalence of disease.

However, it is concluded that, due to the pathogenesis and the epidemiological characteristics of CS, and to the high persistence of the CS agent in the environment, a CS eradication policy that relies solely on detection of infected flocks by post-mortem testing and subsequent depopulation would be unlikely to succeed.
Secondly, the use of genetic measures, i.e. breeding programmes for resistance to CS (BP-CS), is discussed.

During the last 10 years, BP-SC have been implemented by 17 MSs as a strategy to control the disease. Based on information collected from MSs, the potential effectiveness of these BP-CS in the dissemination of resistance into the general sheep population was assessed for those countries for which the CS trend analysis was performed.

There was a clear heterogeneity in the characteristics of the BP-CSs implemented by the different MSs. There was also heterogeneity within MSs according to different geographical areas, ARR allele frequency in the general population prior to BP-CS implementation, breeds and production types.

Given the characteristics of each national BP-SC, a deterministic model was used to estimate the ARR/ARR frequency in the general sheep population over time. Subsequently, the outputs of the model were compared with the national CS situations. The results obtained suggest that, at national level, a favourable reduction in CS seems to be linked to better-achieving BP-CSs.

Given the very strong resistance to CS of sheep of the homozygote ARR genotype, one may expect that a minimum frequency of the ARR allele in a sheep population exists, above which CS may be expected to fade-out. Some case-studies are presented, showing that this hypothetical minimum frequency is not universal and that it is affected by parameters such as disease prevalence and the national characteristics of the sheep industry. In those cases studied, the required minimum frequency ranged between 53 % and close to 100 per cent, in a context where no additional control or eradication measures were applied.

Additional/alternative measures to control CS in sheep and goats are recommended. These focus on: i) the improvement of surveillance and control measures and their adaptation to the individual MSs, ii) the reinforcement and improvement of the policy of breeding for resistance in sheep, iii) the introduction of breeding policies in goats, and iv) knowledge transfer on scrapie.

Further Reading

You can view more on Scrapie on our Disease Guide by clicking here.

August 2014