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From: TSS ()
SEAC Annual Report 2004 SEAC Spongiform Encephalopathy Advisory Committee ANNUAL REPORT 2004 SEAC Annual Report 2004 CONTENTS Foreword 1 About the Committee 3 Topics Considered 7 A. CJD and Public Health 7 B. Food Safety and the Protection of Animal Health 12 C. Safety of Animal By-Products 16 D. Research on TSEs 18 E. TSEs in Sheep 19 Recommendations 21 Annex 1 Abbreviations 23 Annex 2 Membership and Secretariat of SEAC 24 Annex 3 Members Indemnity 27 Annex 4 Register of Members Interests 29 Annex 5 Membership and Terms of Reference of SEAC/FSA Risk Assessment Group 33 Annex 6 Membership of the SEAC Sheep Subgroup 34 Annex 7 Summary of Discussion on Second Presumed Case of Blood Transfusion Associated Infection with vCJD 35 Annex 8 Position Statement on Maternal Transmission of vCJD 38 Annex 9 Summary of Discussion on RAG Conclusions on the Over Thirty Month Rule Review 41 Annex 10 Updated approach to assessing the future number of vCJD cases arising from relaxation of the OTM scheme 44 Annex 11 Summary of Discussion on BSE & Sheep: the FSA Contingency Policy 49 Annex 12 Position Statement on Chronic Wasting Disease in UK Deer 52 Annex 13 SEAC Sheep Subgroup Consideration of Options for the National Scrapie Plan 56 SEAC Annual Report 2004 1 Foreword I would like to introduce this report by thanking my predecessor as SEAC Chair, Professor Peter Smith, for his many years of distinguished service to the committee and wish him well for the future. In addition to Professor Smith’s retirement as Chair, the membership of the committee has changed considerably over the past year. Professors Adriano Aguzzi, Robin Carrell, Chris Bostock and Harriet Kimbell have stepped down from the committee having made many important contributions to SEAC during their periods of service. As well as a new Chair, ten new members were appointed to SEAC in 2004. I am sure that, like me, they are looking forward to the challenging and complex issues SEAC will be considering in the forthcoming years. In 2004, initially under Peter Smith’s Chairmanship and then under mine, SEAC discussed a wide range of issues relating to transmissible spongiform encephalopathies. The committee provided advice to the Department of Health on the implications of the first two presumed instances of blood transfusion associated transmission of the vCJD agent and on the potential risks of transmission of vCJD via surgical instruments. At the request of the Chief Medical Officer for England, the committee also considered the potential for transmission of vCJD from mother to child. The committee advised the Food Standards Agency on the impact of replacement of the Over Thirty Month Rule with a BSE testing regime and on possible risk reduction measures should BSE be found to be present in the national sheep flock. SEAC also advised the Agency on a survey of historic butchery practices and on the potential risks of chronic wasting disease in UK deer. Advice was provided to Defra on an assessment of the effects of tallow separation and solvent extraction on the BSE agent during the rendering of cattle carcases and on the safety of collagen sourced from the hides of UK cattle. The committee received presentations from a number of researchers including an overview of CJD surveillance in Switzerland and an investigation of possible atypical cases of BSE. The work of the committee continues to attract a great deal of interest from the public, the media and the scientific community. SEAC continues to hold open meetings to provide an opportunity for interested parties to see SEAC Annual Report 2004 how the committee’s advice is formulated. In addition, this year the committee took part in a trial of broadcasting meetings live over the internet. This initiative has opened up the committee’s deliberations to audiences throughout the UK and also internationally. The trial will continue into 2005 when it will be evaluated to determine whether it is worthwhile and sustainable in the longer term. In November 2004, SEAC held its first open meeting outside of London, which was hosted by the National Assembly Government of Wales in Cardiff. The committee continues to be reliant upon access to early research findings and I would like to thank the researchers who have kindly presented their work to the committee prior to publication. Finally, I would like to add my thanks to the work carried out by the committee’s secretariat without whose efforts the committee would not run so smoothly. In particular, I would like to thank Catherine Boyle who has moved to a new post at DEFRA and welcome her replacement as Secretary, Kate Richards. Professor Chris Higgins Chair of SEAC snip... SEAC Annual Report 2004 7 Topics Considered A. CJD and Public Health vCJD update 15. In February, April and September, SEAC received updates on the number of variant Creutzfeld-Jacob disease (vCJD) cases in the UK and worldwide from the National CJD Surveillance Unit. By 28th September 2004, the total number of definite and probable vCJD cases in the UK was 149. All the cases tested were of the same genotype (methionine homozygous) at codon 129 of the prion protein (PrP) gene. 16. As in previous years statistical analysis of the number of deaths from vCJD continued to show evidence that the epidemic is no longer increasing exponentially, suggesting that at least in the short term, the epidemic may have peaked. By September, six vCJD cases had been reported in France, and single cases in Ireland, Italy, Canada and the USA. The vCJD cases reported in France and Italy did not have a history of residence in the UK but the cases reported in Ireland, Canada and the USA had a history of UK residence during the late 1980s. SEAC welcomed the evidence of a decline in the epidemic but expressed caution as it considered that future peaks in the epidemic could not be discounted. First probable case of blood transfusion associated transmission of vCJD 17. In February, SEAC was updated about the Transfusion Medicine Epidemiology Review (TMER) funded by the Department of Health (DH) to examine possible links between vCJD and blood transfusion. In the course of the study, a case of possible transmission of vCJD by blood transfusion had been identified. The recipient received blood in 1996 from a donor who, at the time of donation, was free of clinical signs of vCJD, although went on to develop vCJD in 1999. The blood recipient died of vCJD in 2003. Although the possibility could not be ruled out that the blood recipient was infected as a consequence of consuming meat products contaminated with the BSE agent, statistical analysis suggested that it was unlikely that the association between the blood recipient and the blood donor was due to chance. 18. SEAC agreed that the case raised the possibility that the infection may have been transmitted by the blood transfusion. However, it was SEAC Annual Report 2004 8 noted that this was a single case. The recipient had received blood before leucodepletion of blood donations had been adopted in the UK in 1998, as a precautionary measure. The measure was introduced as it was considered that leucodepletion of blood would significantly reduce the risk of blood transfusion associated transmission of vCJD. SEAC noted the importance of the TMER in identifying potential transfusion associated vCJD cases. CJD surveillance in Switzerland 19. In February, SEAC was informed about the epidemiology of sporadic CJD (sCJD) in Switzerland. In recent years, an increased incidence of sCJD had been noted that could not be linked to patient gender, age, geographical location, surgery or occupation. In common with other countries, the increased incidence had been noted following the introduction of testing for the protein marker 14-3-3 as an aid to differential diagnosis of sCJD. The committee agreed with the Swiss investigators that improved surveillance and an increased awareness of the disease rather than a real rise in the number of cases may explain the rise in disease incidence. An analysis of Western Blot profiles of the cases was inconsistent with a diagnosis of vCJD. However, strain typing bioassays of tissues collected from the cases together with material collected from vCJD cases and animals with BSE, scrapie or chronic wasting disease were planned to confirm the diagnosis of sCJD. The committee agreed that strain typing experiments were important and that incubation times and lesion profiles would be likely to provide a clearer distinction between different prion disease strains compared with Western Blot profiles. Survey of human tonsil and appendix samples 20. In February, SEAC considered the results from an unlinked and anonymised retrospective survey of human appendix and tonsil samples. This was funded by DH, to determine the prevalence of detectable abnormal PrP in these tissues. Of 12 674 samples tested, detectable levels of abnormal PrP were found in three appendix samples. One sample showed an immunohistochemical staining pattern similar to that observed with vCJD cases. In the other two samples, the immunostaining was dissimilar to that typically observed with vCJD cases. The committee considered it was possible that the atypical immunostaining could be an experimental artefact. Alternatively, it could represent infection at an early stage of the incubation period or signify the existence of a carrier state or different SEAC Annual Report 2004 9 infection phenotype possibly with a longer incubation period than had been observed with the vCJD cases reported to date. 21. Assuming all three positive samples represented real infections, the data indicated that the estimated prevalence of vCJD could be around 237 infections per million of the population (95% confidence interval 49-692 per million). If it was assumed that this prevalence related to people aged 10-30 years (83% of the samples were from people within this age range) then around 3800 individuals (95% confidence interval 785-11128) aged 10-30 years could be infected with vCJD in the UK. The committee noted that this prevalence of infection with the vCJD agent was appreciably higher than that predicted from the number of vCJD cases recorded to date. To date all of the clinical cases of vCJD have been the same PrP genotype (PrP codon 129 methionine homozygotes). However, it is possible that other PrP genotypes may also be infected but develop the disease after a longer incubation period. 22. The committee was also informed about an unlinked and anonymised prospective survey of tonsil samples. In this study, samples would be collected and analysed in batches of 5000 until approximately 100 000 had been collected in total. The committee endorsed the decision to batch analyse samples and recommended that ethical approval be obtained to allow the PrP genotype of positive samples to be determined. Bone risk assessment 23. In February, SEAC considered an updated risk assessment, produced by the National Blood Service (NBS) and DH to examine the risk of transmission of vCJD via implantation of human bone. The assessment compared the risks associated with different bone products (processed or unprocessed, pooled or unpooled) under different scenarios of infectivity. The risk assessment showed that the theoretical risk of transmission of vCJD via bone transplantation is increased by pooling, because of the possibility of sourcing infectious material from any one of multiple donors. Processing of bone reduced the amount of blood and marrow, lowering the risk of infection. 24. The committee agreed with the general approach used in the risk assessment, noting that the risk of infection would also depend on the prevalence of vCJD in the general population. The committee concluded that the use of unpooled bone, and removal of blood and SEAC Annual Report 2004 10 marrow from bone, minimises the risk of vCJD transmission via the implantation of human bone. Tissue risk assessment 25. In February, SEAC considered a draft risk assessment of vCJD transmission via tissue transplantation, produced by NBS and DH to identify critical procedures to minimise the potential for transmission. The assessment indicated that factors affecting the level of risk of infection by transplantation included the type of tissue, pooling of tissues, mass of tissue transplanted, age of donor and recipient and the site of transplant. A lack of scientific data on the potential levels of infectivity in different tissues precluded the production of a quantitative risk assessment. 26. The committee agreed that the scientific uncertainty relating to tissue infectivity meant that it was difficult to conduct a quantitative risk assessment but concluded that the approach adopted in the assessment was reasonable. Systems, such as the TMER to trace the use of human tissues in medical procedures, were considered to be an important surveillance mechanism to identify iatrogenic transmission of vCJD. Second probable case of blood transfusion associated transmission of the vCJD agent 27. In June, DH asked the committee to advise on the public health implications of a second probable case of blood transfusion associated transmission of the vCJD agent. The patient who died in 2004, from a cause unrelated to vCJD, showing no clinical signs of vCJD, had received a single unit of blood in 1999. This blood was donated by an individual who subsequently developed vCJD in 2000 and died in 2001. Post mortem findings in the blood transfusion recipient suggested a possible preclinical case of iatrogenic vCJD associated with blood transfusion. However, the patient resided in the UK, and oral exposure to the BSE infectious agent could not be excluded as a possible cause of infection. 28. On the basis of a statistical analysis of the possible causes of infection, the committee agreed that the case was much more likely to be attributable to blood transfusion, than food borne infection. The report strengthened the evidence for the transmission of vCJD via blood. However, the committee noted that in this instance, although vCJD infection appeared to have been transmitted, it was not known if clinical vCJD would have developed if the patient had lived longer. SEAC Annual Report 2004 11 29. The committee agreed that the new case strengthened its opinion, first stated in October 1997, that human blood from persons incubating vCJD may be infective. Additionally, it was a public health priority for all recipients of blood transfusions from donors incubating vCJD to be subject to post mortem investigation, to help quantify the nature and magnitude of the risks of transmission of vCJD via blood. A statement summarising the committee’s consideration of the case is given at Annex 7. Transmission of vCJD via surgical instruments 30. In September, SEAC considered an updated risk assessment by DH on the transmission of vCJD via surgical instruments. This assessment examined the consequences of an operation on an infected patient, in terms of the number of subsequent infections that could possibly arise from use of potentially contaminated instruments. The implications of such infections for the spread of vCJD in the UK population were modelled. The assessment identified key variables that influenced the risks. Due to the paucity of data, particularly on tissue infectivity and the effectiveness of decontamination practices, there were multiple uncertainties in the assessment. 31. The committee was generally content with the approach taken in the risk assessment. It concluded that effective cleaning and decontamination of instruments was the most important measure to reduce the risks of transmission of vCJD via the use of surgical instruments. Maternal transmission of vCJD 32. In November, at the request of the Chief Medical Officer for England, SEAC considered current evidence on the potential transmission of vCJD from mother to child, via human breast milk. In utero transmission was also considered. The committee considered the limited published epidemiological and experimental research on maternal transmission of prion diseases, together with unpublished surveillance data of children born to vCJD cases from the National CJD Surveillance Unit and the PIND surveillance of neurological illness in UK children1. The committee also commented on a modelling work by DH on the effect of pooling breast milk in breast 1 Devereux et al. (2004) Variations in neurodegenerative disease across the UK: findings from the national study of Progressive Intellectual and Neurological Deterioration (PIND). Arch Dis Child. 89, 8-12. SEAC Annual Report 2004 milk banks on the possible transmission of vCJD via the use of such milk. 33. The committee noted that the modelling clearly showed that the practice of pooling breast milk increased the potential risk because it increased the number of donors to which a recipient is exposed. This therefore increased the potential risk of an infant receiving milk contaminated with vCJD infectivity. The committee concluded that the theoretical risk of infection could be minimised by not pooling the milk, by the use of individual hand operated breast milk pumps for single donors, and by the use of single use sterilised bottles for collection. As the available evidence suggested that infection/inflammation of the breast results in increased lymphocytes in milk, therefore increasing the risk of infectivity, risk would be minimised if milk from donors showing signs of infection was not used. The committee suggested that, if practicable, milk might be stored for an appropriate period of time to allow the health status of donors to be monitored, before it is released. 34. The committee concluded that there was no epidemiological evidence for maternal transmission of vCJD, including transmission via breast milk, however a hypothetical risk exists. Although evidence was limited and mostly indirect rather than direct, the risk, if any, appeared to be low. However, a watching brief should be maintained. The committee recommended that undiagnosed neurological diseases be carefully monitored and monitoring of neurological illnesses, such as the PIND surveillance of children, continue. A position statement summarising the committee’s consideration is given at Annex 8. B. Food Safety and the Protection of Animal Health American BSE case 35. In February, SEAC was informed that a case of BSE had been identified in an animal in the United States of America (USA). UK imports of beef from third countries are strictly controlled and very little US beef was imported because of other trade restrictions. SEAC asked to be kept up to date on the USA surveillance programme for BSE. snip... 15 FSA contingency policy for possible BSE in sheep 46. In September, SEAC was asked to advise on the underlying scientific assumptions and approaches adopted in two modelling studies carried out by the Veterinary Laboratory Agency (VLA) / Institute of Animal Health and the University of Oxford for the FSA. These would inform its contingency policy should BSE ever be found in sheep. The modelling studies, based upon discriminatory testing of almost 2400 samples from scrapie suspects by the VLA together with published and unpublished data on the pathogenesis of BSE and scrapie in sheep, examined the possible prevalence of BSE in sheep and the likely impact of different risk reduction strategies if BSE was found in sheep. The committee also considered the analytical methods used to discriminate BSE and scrapie in sheep. 47. The committee considered that tests to distinguish BSE from scrapie had limitations but were becoming more robust. Studies to date showed no evidence for BSE in UK sheep. The committee acknowledged the theoretical nature of the modelling work and that modelling the possible impact of BSE in sheep if it entered the national flock was complex. Due to the very limited data available, the models had necessarily relied heavily on many assumptions. In particular, the modelling assumed that BSE and scrapie would behave similarly in all types of sheep, which is largely unknown. The committee noted that the modelling indicated that a single BSE infected sheep entering the food supply could present a significantly SEAC Annual Report 2004 greater risk compared to the current risk from a single infected bovine. They also noted that the model suggests that a risk reduction strategy based on the PrP genotype of sheep would be the most effective, if BSE were found in the national flock. A summary of the committee’s consideration that was provided to the FSA is given at Annex 11. Chronic wasting disease in UK deer 48. In November, SEAC was asked by the FSA to consider the possible public health implications of chronic wasting disease (CWD). CWD is an endemic TSE in certain captive and free-ranging species of deer in some areas of North America. It has not been found in the UK, or elsewhere in Europe. The committee also considered the possibility that BSE may also be present in UK deer. 49. The committee concluded that there is no evidence of CWD (or BSE) in the UK deer population. However, because limited surveillance is conducted, a low level prevalence of CWD could not be ruled out. Further surveillance of TSEs in UK deer was recommended. The committee considered that there was no evidence of transmission of CWD to humans from consumption of meat from infected deer (or to cattle, sheep or goats by natural means), although the data are limited. Thus, the committee concluded that CWD currently poses relatively little risk to human health but a watching brief should be maintained. A position statement summarising the committee’s consideration is given at Annex 12. snip... Atypical BSE cases 55. In February, SEAC considered the findings of a research paper (Casalone et al. 2004)6 suggesting that a second bovine amyloidotic spongiform encephalopathy had been identified with a molecular signature similar to that of a subtype of sCJD. The committee agreed that the results were very interesting, but without information on the transmissibility of the disease in bioassays, it was premature at this stage to conclude it was a new strain of BSE. SEAC agreed it would follow further research on these samples with interest. (same as they did with the BSE agent, and the agent spread while they floundered for decades. ...TSS) snip... Sheep subgroup 61. In March, Defra consulted the SEAC sheep subgroup following reports that abnormal PrP had been detected in the brains of ARR/ARR sheep, a PrP genotype thought to be naturally resistant to scrapie. The subgroup was asked to consider if this development had any implications for the scientific basis of the National Scrapie Plan (NSP). The sheep subgroup endorsed their previous opinion of December 2002, that the NSP strategy to increase resistant genotypes and decrease susceptible genotypes remained scientifically justified. It considered that, although the new evidence suggested that the ARR/ARR genotype may not be completely protective against natural TSE infection, the relative protection, compared to other genotypes, remained very large. However, the basis for the strategy should be kept under review in the light of emerging scientific findings with respect to the possible detection of scrapie infections in animals of genotypes currently thought to be most resistant to infection. snip... SEAC Annual Report 2004 21 Recommendations 64. In the course of the year the committee provided the following advice • removal of blood and marrow from bone and use of unpooled bone minimises the risk of vCJD transmission via medical procedures involving implantation of human bone (paragraph 24), • all blood transfusion recipients from donors incubating vCJD should be subject to post mortem investigation, to help quantify the nature and magnitude of the risks of transmission of the vCJD agent through blood (paragraph 29), • effective cleaning and decontamination of surgical instruments is the most important measure to reduce the risks of possible transmission of vCJD, via the use of surgical instruments (paragraph 31), • the theoretical risk of infection via breast milk banks could be minimised by not pooling milk, by the use of individual hand operated breast milk pumps for single donors and by the use of single use sterilised bottles for collection. Additionally, risk would be minimised if milk from donors showing signs of infection was not used and, if practicable, milk could be stored for an appropriate period of time to allow the health status of donors to be monitored, before it is released (paragraph 33), • inclusion of casualty animals increased the level of risk disproportionately to the number of casualty animals entering the food supply and that this should be noted when assessing risk management options for changes to the OTMR. (paragraph 42), • that the modelling presented to the committee suggests that a risk reduction strategy, based on PrP genotype of sheep, would be the most effective if BSE were found in the national sheep flock (paragraph 47), • the NSP strategy remains appropriate but should be kept under review in the light of emerging scientific findings with respect to the possible detection of scrapie infections in animals of SEAC Annual Report 2004 genotypes currently thought to be most resistant to infection (paragraph 61), • the NSP strategy of compulsory ram genotyping scheme with removal from use and sale of some scrapie-susceptible genotypes is the most scientifically desirable but a combination of mandatory and voluntary genotyping of certain sheep could also be considered (paragraph 62). 65. The committee also made the following recommendations in relation to research and surveillance: • systems such as the TMER to trace the use of human tissues in medical procedures are an important surveillance mechanism tool for identifying iatrogenic transmission of vCJD (paragraph 18), • ethical approval be obtained to allow the PrP genotype of positive samples to be determined in the survey of abnormal PrP in tonsil samples (paragraph 22), • undiagnosed neurological diseases should be carefully monitored, and monitoring of neurological illnesses, such as the PIND surveillance of children, should continue (paragraph 34), • research on methods to allow differential diagnosis of clinical cases of BSE is important in view of evidence on the phenotypic differences in infection in humans and sheep (paragraph 45), • further surveillance of TSEs in UK deer should be conducted (paragraph 49), • when terminating cattle bioassays tissues should be archived to allow for further study of these samples in the future, if required (paragraph 58). snip... SEAC Annual Report 2004 52 Annex 12 Position Statement on Chronic Wasting Disease in UK Deer Introduction 1. The Food Standards Agency asked SEAC to consider the possible public and animal health implications of chronic wasting disease (CWD), in particular the level of risk posed to consumers of meat from infected animals. The committee also considered the possibility that BSE may be present in UK deer. Background 2. CWD has emerged as an endemic transmissible spongiform encephalopathy (TSE) in certain captive and free-ranging species of cervid (deer) in some areas of North America. The disease is characterised by weight loss and behavioural changes in infected animals, usually over a period of weeks or months leading to death. CWD has not been found in the UK or elsewhere in Europe. No definitive or suspected cases of transmission of CWD to humans have been reported. 3. SEAC considered a review of the published, and some unpublished, research on CWD, together with surveillance data on TSEs in European cervids and information on UK cervid populations15. Origins 4. The origins of CWD are unknown. On the basis of epidemiological data, it is highly improbable that CWD originated from the recycling of mammalian protein in processed feed. It has been suggested that CWD may have arisen from transmission and adaptation of scrapie from sheep to cervids, as a result of a spontaneous change of endogenous prion protein (PrP) to an abnormal diseaseassociated form, or from an unknown source. 5. Data supporting any of these possible origins of CWD are either absent or equivocal. Although CWD could have originated from scrapie, the differing properties of the two prion diseases in strain typing bioassays, whilst limited, do not support this hypothesis. Evidence for multiple strains of CWD is equivocal. It seems most likely that CWD arose from a spontaneous change of endogenous PrP resulting in a disease-associated and laterally-transmissible form of PrP, although direct data to support this hypothesis are lacking. Host range 6. The known natural hosts for CWD are mule deer (Odocoileus hemionus hemionus), black-tailed deer (Odocoileus hemionus columbianus), white-tailed deer (Odocoileus virginianus) and Rocky Mountain elk (Cervus elaphus nelsoni). The prevalence and geographical distribution of CWD in these species appears to be 15 The information considered by the committee is available at: http://www.seac.gov.uk/agenda/agen301104.htm SEAC Annual Report 2004 53 increasing in North America in a manner which is unlikely to be due simply to increased surveillance. 7. There are no direct data relating to the transmissibility of CWD to UK cervid species. However, comparison of a limited number of PrP codons indicates some homology in the endogenous PrP gene of European and North American cervid species. Thus, the possibility that UK cervids may be susceptible to CWD cannot be excluded, in particular red deer (Cervus elaphus elaphus) which are closely related to elk. 8. There is no evidence to suggest that CWD is present in UK cervids. However, because surveillance in the UK is very limited, a low level prevalence of CWD cannot be ruled out. The committee endorsed the opinion of the European Food Safety Authority on CWD surveillance in the European Union (2004)16. 9. Transmission studies using parenteral routes of administration to cattle, sheep and a single goat, together with data from in vitro PrP conversion experiments, suggest that a significant barrier to CWD transmission to these species may exist. No transmission has been evident so far in an on-going oral transmission study in cattle after six years. Furthermore, no signs of infection have been observed from monitoring of cattle co-habiting areas with infected cervids, or in cattle, sheep or goats in close contact with infected cervids in research facilities. Thus, although the data are limited, there is currently no evidence to suggest that CWD can be transmitted naturally to cows, sheep or goats, and it is likely that there is a strong species barrier to such transmission. Routes of transmission 10. Epidemiological data indicate that lateral transmission between infected and susceptible cervids occurring naturally is sufficiently effective to maintain epidemics in both captive and free-living populations. There is good evidence from studies of cervids inhabiting paddocks previously inhabited by infected animals or contaminated with infected carcases, that CWD can be transmitted laterally between animals via the environment. The precise mechanism of transmission is unclear. It is possible that the infectious agent is shed in the saliva, faeces or urine or as a result of decomposition of infected carcases and transferred to other cervids grazing the contaminated areas. It is also possible that some maternal transmission occurs. 11. There have also been suggestions that the lateral transmission of CWD may be influenced by environmental factors. Pathogenesis 12. Information on the pathogenesis of CWD is limited. The data show that, following oral challenge, PrPCWD is first detected in the oral and gut-associated lymphoid tissues before spreading more widely within the lymphoid system and then to the brain. Involvement of the retropharyngeal lymph nodes or tonsils in the 16 http://www.efsa.eu.int/science/biohaz/biohaz_opinions/501_en.html SEAC Annual Report 2004 54 pathogenesis may not occur in some elk. At the microscopic level, the nature and distribution of the tissue lesions are similar to those found for scrapie. The available data suggest the pathogenesis of CWD is similar to scrapie. BSE in UK deer 13. Both captive and free-ranging cervids in the UK may have been exposed to contaminated feed prior to the reinforced mammalian meat and bone meal ban instituted in 1996. A study to look at the potential susceptibility of red deer to BSE has shown no signs of transmission of the disease by the oral route, but it is at a very preliminary stage. Although a theoretical possibility exists, there is no evidence from the very limited surveillance data to suggest that BSE is present in the UK cervid population. Human health implications 14. Epidemiological data on possible CWD infection of humans are very limited. The possibility that clinical symptoms of CWD in humans differ from those of Creutzfeldt-Jakob Disease (CJD) cannot be excluded. There is no significant difference between the prevalence of CJD in CWD endemic areas and other areas of the world. However, because CJD surveillance in the USA is relatively recent, not all CJD cases may have been identified. Additionally, detection of a small increase in prevalence of such a rare disease is very difficult. Investigation of six cases of prion disease in young people (< 30 years of age) in the USA found no definite causal link with consumption of venison from known CWD endemic areas. The disease characteristics in these cases were indistinguishable from sporadic CJD or Gerstmann-Sträussler-Scheinker syndrome. Likewise, in a study of three hunters (> 54 years of age) diagnosed with sporadic CJD, no link with consumption of venison from CWD endemic areas was found. No causal link was found in an investigation of three men with neurological illnesses who were known to partake in "wild game feasts". Only one of these subjects was found to have a prion disease and this was also indistinguishable from sporadic CJD. 15. Preliminary results from transmission experiments in transgenic mice expressing human PrP suggest the presence of a significant species barrier to transmission of CWD to humans. However, these findings must be interpreted with caution as they may not accurately predict the human situation. Data from in vitro experiments on conversion of human PrP by disease-associated forms of PrP, including PrPCWD, are equivocal. 16. The committee concluded there is no evidence of transmission of CWD to humans from consumption of venison, and that there may be significant barriers to transmission. Nevertheless, as the data are extremely limited a risk cannot be ruled out should CWD enter UK herds. Conclusions 17. There is no evidence that CWD (or BSE) is present in the UK cervid population. However, because only limited surveillance is conducted in the cervid population, a SEAC Annual Report 2004 low level prevalence of CWD cannot be ruled out. It is recommended that further surveillance of TSEs in UK cervids is conducted. 18. There is no evidence of transmission of CWD to humans from consumption of meat from infected cervids. Although epidemiological and experimental data on potential transmission of CWD are extremely limited, they suggest that there may be a significant species barrier. It would be helpful if further studies were available assessing the potential species barrier for transmission to humans. 19. Although limited, there is no evidence CWD can be transmitted to cattle, sheep or goats by natural means. 20. In summary, it appears that CWD currently poses relatively little risk to human health, or to the health of cattle, sheep or goats in the UK. Nevertheless, as a risk cannot be excluded a watching brief should be maintained. snip... http://www.seac.gov.uk/publicats/annualreport2004.pdf TSS
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