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From: TSS ()
Subject: SEAC Annual Report 2004
Date: June 22, 2005 at 7:39 am PST

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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