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From: TSS ()
Subject: Assessing the risk of vCJD transmission via surgery: an interim review 15/06/2005
Date: June 16, 2005 at 2:54 pm PST

Assessing the risk of vCJD transmission via surgery: an interim review
Document type:

Publication


Author:
Department of Health. Economics, Statistics and Operational Research Division


Published date:
15/06/2005


Primary audience:
Professionals


Gateway reference:
4742


Document size:
A4


Pages:
50, iiip


Electronic only:
Yes


Copyright holder:
Crown


This paper updates previous analysis of the risks of vCJD being transmitted via re-use of surgical instruments. As before, a scenario-building approach is used, reflecting continuing uncertainties around both the prevalence and transmissibility of the disease. Presents new evidence on key topics notably: the infectivity of various tissues in those incubating vCJD; the amount of tissue liable to remain on instruments after cleaning; the likely effect of autoclaving in reducing the infectivity of these residues; possible mechanisms of vCJD transmission and the possible prevalence of vCJD within the population.

i

ASSESSING THE RISK OF vCJD TRANSMISSION VIA SURGERY:

AN INTERIM REVIEW

Economics, Statistics and Operational Research

Department of Health

Skipton House, 80 London Road, London SE1 6LW

March 2005

SUMMARY

As is now well-known, variant Creutzfeldt-Jakob disease (vCJD) is a fatal, degenerative

neurological condition, generally thought to have spread to the human population via

consumption of cattle infected with bovine spongiform encephalopathy (Will et al, 1996;

Bruce et al, 1997). So far there have been over 150 known cases, the large majority in the

UK. Given that an unknown number of people may be incubating the disease – infected

but without showing symptoms – protection of public health requires consideration of the

possible risks of secondary (person-to-person) transmission.

This paper updates previous analysis of the risks of vCJD being transmitted via re-use of

surgical instruments. As before, a scenario-building approach is used, reflecting

continuing uncertainties around both the prevalence and transmissibility of the disease.

However new evidence is available on several key topics, notably:

- The infectivity of various tissues in those incubating vCJD,

- The amount of tissue liable to remain on instruments after cleaning,

- The likely effect of autoclaving in reducing the infectivity of these residues

- Possible mechanisms of vCJD transmission

- The possible prevalence of vCJD within the population

Where changes to previous inputs appear warranted, this paper sets out their implications.

ii

The main conclusions are as follows:

(i) The overall policy implications of the original analysis – which stressed the need

for a precautionary approach to reducing vCJD transmission risks - remain valid.

(ii) On the potential infectivity of key tissues, subsequent evidence supports the

values used before as regards the brain, the back of the eye, tonsil and spleen.

However experiments now nearing completion on tissues from vCJD patients

suggest that some other tissues are less infective than considered in previous

scenarios. These include tissue from the front of the eye, and some lymphoid

tissues (which appear to carry lower levels than tonsil or spleen). Nevertheless,

the previous assumption of infectivity being widely distributed through the body

prior to the onset of clinical symptoms still appears valid.

(iii) On instrument decontamination (cleaning and autoclaving), survey data show

improvements in practice within the NHS. However new scientific evidence on

protein residues left after cleaning:

- supports the previous emphasis on pessimistic scenarios

- indicates that these scenarios might not have been pessimistic enough to

allow for the effects of poor practice.

- confirms that the effectiveness of cleaning remains highly variable.

(iv) Cleaning of instruments may therefore have improved, but from a worse startingpoint

than previously thought. More research is needed to investigate why results

are so variable – e.g. whether allowing some instruments to dry out before

cleaning is making residues more difficult to remove.

(v) Preliminary experimental results also suggest that the infective agent for vCJD is

at least as resistant to autoclaving as previously thought, and perhaps more so.

Results from this research will be firmed-up over the coming months.

(vi) A separate line of research supports the hypothesis that the risk of vCJD

transmission may be heightened when instruments come into prolonged, rather

than transient contact with relevant tissues.

(vii) On the key question of how many people are already infected with vCJD, the

latest evidence tells against both the most optimistic and the most pessimistic

scenarios considered before. However there is still much uncertainty. More

definite evidence should come from a prospective survey of tonsil samples now

under way, but this will not be completed until about two years’ time.

Overall, the risks of vCJD transmission via surgery still appear significant. Though

achieving high standards of decontamination remains of critical importance, the

limitations of current technology mean that instruments used on an infective patient

could still be at significant risk of passing on vCJD.

iii

The highest risks would be associated with operations involving the brain or back of the

eye, followed by those involving the spinal cord. Risks would also attach to a wide range

of procedures encountering lymphoid tissue and/or peripheral nerves. New evidence

makes it possible to distinguish between procedures within this range, with lower

transmission risks attaching to other operations than to those involving tonsil or spleen.

Further research may allow this classification to be further refined. However, any

procedure encountering lymphoid tissue or peripheral nerve to a substantial extent is

likely to carry some risk of transmitting vCJD, if carried out on a patient incubating the

disease.

In the more pessimistic scenarios modelled here, secondary transmission would have a

major impact on the overall course of the vCJD outbreak. One possibility that cannot be

ruled out is that vCJD could become self-sustaining within the population, rather than

dying away. This risk is amplified when surgery is considered in combination with other

possible transmission routes, such as blood transfusion. It would also be heightened if –

as has been suggested – a significant proportion of those infected with vCJD may remain

in a "carrier state", not developing symptoms of the disease themselves, but acting as

possible sources of further onward infection.

Despite the remaining uncertainties, this update reinforces the importance of:

- accelerated development and introduction of new decontamination technology;

- encouraging the introduction of single-use instruments, particularly items in

prolonged contact with the highest-risk tissue (e.g. brain, posterior eye),

- investigating the variability in residues currently left on instruments;

- taking steps to avoid protein getting dried-onto instruments, ensuring that this is

not an unintended consequence of moves to improve decontamination by moving

to more centralised facilities,

- implementation of updated guidelines on decontamination of endoscopes (covered

by a separate Risk Assessment currently under way);

- research into the effects of repeated autoclaving on vCJD infectivity.

Several lines of research on novel decontamination methods are now showing promise,

and work is under way to explore how these might be rolled out across the NHS. In

parallel to this, further analysis of the cost-effectiveness of single-use instruments is

being undertaken by the National Institute for Clinical Excellence (NICE).

iv

Contents

1. Introduction

1.1 Background and Aim 1

1.2 The Modelling Approach 2

2. Risks per Operation: Review of Evidence

2.1 Initial Model 4

2.2 Evidence on Tissue Infectivity 6

2.3 Evidence on Decontamination 9

2.4 Summary of Input Revisions 15

3. Operations on an Infective Patient: Numerical Scenarios

3.1 Previous Scenarios 16

3.2 Initial Revised Scenarios 17

3.3 Scenarios with "Steady State" residues 19

3.4 Revisiting the Transmission Model 21

3.5 Transmission Risks from Endoscopy 23

4. Infection Rate in the Population: Review of Inputs

4.1 Overview 24

4.2 Prevalence of vCJD Infection 24

4.3 Numbers and Classification of Operations 26

5. "Snapshot" Infection Rates

5.1 Introduction 28

5.2 Numerical Scenarios 30

6. Longer-term Scenarios and Infection Dynamics

6.1 Introduction 33

6.2 Illustrative Scenarios 36

6.3 General Observations 41

6.4 Could vCJD become Self-Sustaining? 42

6.5 Further Analytical Issues 43

7. Implications of the Review 45

References 48

Annex A: "Steady State" Model for Residues on Instruments

Annex B: Previous Scenarios for "Snapshot Infection Rates"

1. INTRODUCTION

1.1 Background and aim.........

SNIP........

45

7. IMPLICATIONS OF THE REVIEW

7.1 This provisional review has emphasised the continued relevance of pessimistic

scenarios for surgical transmission of vCJD, despite recent efforts to improve

instrument decontamination. This does not mean that these efforts have been

wasted: it may well be that risks have been reduced from a worse starting-point

than was suggested by earlier evidence.

7.2 A good deal of empirical research is associated with new decontamination

technologies – much of it being directly supported by the Department of Health.

However, some aspects of current practice would still benefit from further

investigation. The continued variability in residues left on instruments remains

somewhat puzzling. The possibility of this being due to some instruments being

allowed to dry before cleaning has been noted.

46

7.3 Some of the previous gross uncertainties about the effects of decontamination have

now been reduced considerably. The new experimental results on both cleaning

and autoclaving should soon appear in the research literature. However these refer

only to the initial decontamination cycle. The need for further investigation of

repeated cleaning and autoclaving is suggested by:

- the likelihood that only a small proportion of material will come off

instruments during each re-use (implying that most of it will be autoclaved

several times)

- the implications of repeated decontamination cycles for the "contact"

model of transmission.

Meanwhile it is prudent to assume that these cycles may have little effect, and the

analysis set out here has done so.

7.4 Many uncertainties remain around the potential dynamics of secondary infection.

Natural extensions to the work outlined here will be to model scenarios in which

portions of the population would go into a "carrier" state if infected, and which

capture the potential compounding effects of both surgical and blood-borne

infection.

7.5 Such models will necessarily still be "what-if" exercises rather than attempts at

prediction, but may help identify broad steps to reduce the overall risk of

secondary transmission. One step already implemented is the exclusion of

previously-transfused blood donors. As from April 2004, the UK blood services no

longer accept blood from donors who have themselves received transfusions in the

UK after 1980. The main aim was to prevent the "recycling" of vCJD infection via

blood donation. However, a further benefit will be to diminish the potential

indirect impact of surgical transmission. Approximately half of all blood usage

takes place in the context of surgery, so deferring previously-transfused donors

will also reduce the risk of infection being passed on first via surgery and then

again via blood.

7.6 Despite the many caveats still surrounding the analysis, some practical conclusions

can also be drawn specifically with regard to surgical risks. These include the

47

need:

- to press forward with decontamination improvement, and use of disposable

instruments (or parts of instruments) where practicable;

- specifically, to consider the introduction of single-use items in prolonged

contact with the highest-risk (CNS, posterior eye) tissue during surgery;

- alongside this, to accelerate the development and introduction of new

decontamination technology, given even good use of current methods may

not ensure that risks are reduced sufficiently. Several new technologies are

starting to show promise in this respect (see e.g. Fichet et al, 2004;

Lemmer et al, 2004; Jackson et al, 2005).

- meanwhile, to consider specific measures to avoid protein getting driedonto

instrument, ensuring that this is not an unintended consequence of

moves toward more centralised reprocessing;

- to ensure implementation of updated guidelines on decontamination of

endoscopes

7.7 Finally, this risk assessment will be subject to further updating as and when

significant new evidence on vCJD and its transmissibility becomes available.

MAIN TEXT ENDS

FULL TEXT;

http://www.dh.gov.uk/assetRoot/04/11/35/42/04113542.pdf

The Decontamination of surgical instruments in the NHS in England update report: “A Step Change”

http://www.dh.gov.uk/PublicationsAndStatistics/Publications/PublicationsPolicyAndGuidance/PublicationsPolicyAndGuidanceArticle/fs/en?CONTENT_ID=4113543&chk=lclkSf

TSS




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