Scientific Opinion on the results of the EU survey for Chronic Wasting Disease (CWD) in cervids

Following a request from the European Commission, the Panel on Biological Hazards (BIOHAZ Panel) was asked to deliver a scientific opinion on the results of the EU survey for Chronic Wasting Disease (CWD) in cervids. Following from past recommendations from the Scientific Steering Committee (SSC) and EFSA, the European Commission set up a survey aimed at detecting the possible presence of CWD and other TSEs in wild and farmed cervids in the EU. The survey was carried out mainly during the years 2007, 2008, 2009, with some samples collected during 2006 and 2010. The BIOHAZ Panel was asked to provide a scientific opinion drawing conclusions on the occurrence of CWD in the cervid population in the EU.

The survey was designed taking into account recommendations from an earlier 2004 EFSA opinion and established the minimum sample size to be collected from wild and farmed red deer (Cervus elaphus elaphus) from a number of Member States and from wild white-tailed deer (Odocoileus virginianus) from Finland. In addition, it required all Member States to collect as many samples as possible from some categories of all cervids species.

The opinion outlines the results of the survey, including animal species tested, age and risk category of the animals tested and the diagnostic tests used. Overall, approximately 13,000 brain stem samples were collected from cervids of different species in 21 Member States and Norway. No TSE positive results were found. The sampling performed did not always allow reaching the required sample size and no data at all were collected from certain Member States.

The opinion analyses and discusses the results of the survey and explains the uncertainties involved. The survey and its results have some limitations, including the limited testing performed in cervid species potentially susceptible to CWD, the lack of representativeness of samples according to the size and distribution of the EU cervid population, the sensitivity of TSE testing when carried out using obex as a target tissue and the lack of data on the PrP gene polymorphisms frequency and diversity in the EU cervid population and in the tested animals.

It is concluded that the lack of one positive TSE test in the farmed and wild red deer and wild white-tailed deer which were sampled indicates that there is not a cervid TSE epidemic in the EU. The opinion also analyses two scenarios, calculating the maximum expected prevalence of TSEs in those cervid species in the EU as a whole and in some selected EU regions. However, since the assumption of a random sampling is not fulfilled, a quantitative estimate of the true prevalence with confidence intervals has limitations and needs to be interpreted with care. This is because, with regard to TSEs in cervids, the true prevalence may be different in presently unsampled areas. The opinion also highlights that considering the practical issues inherent to collection of samples in EU wild cervids, achieving a survey that would allow a quantitative estimate of the true prevalence of TSEs in these species in the EU would remain extremely difficult. Finally, considering the spreading of CWD within and from clusters in North America, the limitations of the sampling performed in the EU CWD/TSEs survey and the known susceptibility of certain cervid species to CWD, occurrence of cases of TSEs, especially in remote and presently unsampled geographic areas, may not be excluded in cervids in the EU.

The opinion also recommends further experimental studies investigating the susceptibility of the various European cervid species to CWD/TSEs and the genetic diversity of EU cervid species in comparison with North American cervid populations. Finally, inclusion of testing for TSEs as part of wildlife disease monitoring programmes for cervids in EU Member States is recommended, and the opinion advises that the design of monitoring programmes for TSEs in cervids should take into account both the objectives to be achieved and the new scientific knowledge available.