National Institute for Health and Clinical Excellence

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1 National Institute for Health and Clinical Excellence Patient safety and reduction of risk of transmission of Creutzfeldt Jakob disease (CJD) via interventional procedures Systematic literature reviews Final Report ScHARR Dr Myfanwy Lloyd Jones Dr Matt Stevenson Ms Anthea Sutton April 2006

2 CONTENTS 1 Introduction 1 2 The UK incidence and prevalence of CJD and vcjd Screening studies 2 3 The secondary transmission of CJD/vCJD by invasive 4 diagnostic, surgical or dental procedures 3.1 Case reports Case-control studies Experimental studies of the transmission of prion disease to 5 animals 3.4 Secondary transmission: discussion 6 4 The incubation period of acquired human TSEs Kuru Kuru: mode of transmission Kuru and genotype Kuru: summary Iatrogenic CJD Intracerebral procedures Corneal transplants Dura mater grafts Growth hormone and gonadotrophin Iatrogenic CJD and genotype Iatrogenic CJD: summary vcjd Acquired human TSEs: discussion 17 5 The likely infectivity of infectious tissue 18 6 The infectious mass required to transmit CJD/vCJD 18 7 The decontamination of surgical, anaesthetic and diagnostic 19 instruments, scopes and implantable devices 7.1 Evidence for contamination of routinely decontaminated 19 instruments in clinical use 7.2 The decontamination cycle Cleaning Disinfection Sterilisation Disinfection plus sterilisation Discussion Shortcomings of the identified studies Practical issues 26 8 The extent to which surgical instruments remain in their 27 original sets following use and decontamination 9 The complication rates associated with the use of single-use 27 vs reusable anaesthetic, surgical or diagnostic instruments 9.1 Open surgery Open surgery: background Open surgery: systematic review of studies which compare 29 ii

3 complication rates with single-use and reusable instruments Open surgery: Single-use instruments which replicate 29 reusable instruments Open surgery: innovative single-use instruments Laparoscopic surgery Laparoscopic surgery: background Laparoscopic surgery: systematic review of studies which 33 compare complication rates with single-use and reusable instruments 9.3 Endoscopy Endoscopy: systematic review of studies which compare 34 complication rates with single-use and reusable instruments 9.4 Anaesthetic equipment Anaesthetic equipment: background Anaesthetic equipment: systematic review of studies which 35 compare complication rates with single-use and reusable instruments Laryngoscopes Laryngeal mask airways (LMAs) and laryngeal tubes Summary of conclusions Open surgery Laparoscopic surgery Endoscopy Anaesthetic equipment: laryngeal masks and laryngoscopes The risk of future surgery following surgery 38 References 114 Appendices Appendix 1 The UK incidence and prevalence of CJD and vcjd 39 Appendix 2 The secondary transmission of CJD/vCJD by invasive 42 diagnostic, surgical or dental procedures Appendix 3 The incubation period of acquired human TSEs 47 Appendix 4 Infectious mass required to transmit CJD/vCJD 49 Appendix 5 The decontamination of surgical, anaesthetic and diagnostic 51 instruments, scopes and implantable devices Appendix 6 The extent to which surgical instruments remain in their 82 original sets following use and decontamination Appendix 7 The complication rates associated with the use of single-use 84 vs reusable anaesthetic, diagnostic or surgical instruments Appendix 8 The risk of future surgery following surgery 112 Tables Table 1 Genotype frequencies in various populations 8 Table 2 PRNP codon 129 genotypes in Fore and non-fore 9 population groups during and after the kuru epidemic Table 3 Age at onset of kuru symptoms and duration of illness, by 9 PRNP codon 129 genotype Table 4 Iatrogenic CJD: incubation periods and modes of 11 iii

4 Table 5 Table 6 Table 7 Table 8 transmission Incubation periods of CJD attributed to contaminated hgh: cases identified in or before 2000 Patients with CJD, by codon 129 genotype, compared with the general population Patients with hgh-related icjd in France and the UK: distribution of codon 129 genotype Effectiveness of decontamination methods: number of test animals infected Figures Figure 1 Incidence of CJD in hgh recipients in France 15 iv

5 PATIENT SAFETY AND REDUCTION OF RISK OF TRANSMISSION OF CREUTZFELDT JAKOB DISEASE (CJD) VIA INTERVENTIONAL PROCEDURES: SYSTEMATIC LITERATURE REVIEWS 1 INTRODUCTION This report summarises the findings of the systematic reviews undertaken on the above topic. For all of the reviews, twelve electronic bibliographic databases were searched: Medline EMBASE Cochrane Database of Systematic Reviews (CDSR) Cochrane Central Register of Controlled Trials Database of Abstracts of Reviews of Effects (DARE) Science Citation Index NHS Economic Evaluation Database (NHS EED) Health Technology Assessment Database (NHS HTA) CINAHL AMED BIOSIS Health Economic Evaluations Database (HEED). No language, study/publication, or date restrictions were applied to the main searches. The Medline search strategies for each review are set out in the relevant Appendices. Search strategies for the other databases are available on request. In addition, the reference lists of relevant articles were handsearched. The references identified by the literature searches were sifted in three stages. All studies were first screened for relevance by title, and the abstracts of those which were not excluded at this stage were then read. Finally, all studies which seemed from their abstracts to be potentially relevant were obtained for a full reading; if studies did not provide abstracts, the full studies were screened. 2 THE UK INCIDENCE AND PREVALENCE OF CJD AND VCJD Recent and reliable data on CJD and vcjd mortality and morbidity are published regularly by the National CJD Surveillance Unit (NCJDSU). However, CJD and, in particular, vcjd are believed to have a long incubation period, and therefore these figures cannot indicate the full extent of both clinical and preclinical CJD/vCJD infection in the population. Studies have therefore been carried out with the aim of determining the prevalence of clinical and preclinical infection using: retrospective screening for abnormal PrP in anonymised appendix and tonsil samples prospective screening for abnormal PrP in anonymised tonsil samples. 1

6 A systematic review was undertaken to identify all relevant studies. Details of the literature searches and inclusion criteria may be found in Appendix Screening studies Four papers were identified which sought to determine the prevalence of vcjd infection on the basis of samples of tonsil and appendix tissue. However, these four papers relate to only two independent studies: a prospective study of tonsil tissue taken from patients in the London area in a retrospective study of appendix and tonsil tissue taken from patients across the UK in , 4 two subsets of which were published separately. 5,6 Further details of these studies may be found in Appendix 1 Table 1. The prospective study of tonsil tissue identified no evidence of vcjd infection in 2000 patients. 3 The retrospective study identified three cases in 12,674 samples, equating to a prevalence of 237 per million (95% confidence intervals 49 to 692). 4 However, these results should be treated with caution for two fundamental reasons: Such studies can only provide reliable estimates of the true prevalence of preclinical infection in the population if the tests used to identify infection are highly sensitive and specific early in the incubation period. 7 On the basis of extremely small studies, the tests currently available (Western blot, immunohistochemistry) seem able to identify PrP Sc in 100% of tonsil samples from individuals known or suspected to be infected with vcjd, 8,9 in other words, patients with symptomatic disease. The Western blot test has only been demonstrated to identify PrP Sc in one out of four appendix samples from individuals with known vcjd, 10 but 19 out of 20 such samples have been identified by immunohistochemistry. 5 Thus, in symptomatic disease, the sensitivity of both Western blot and immunohistochemistry has been demonstrated in relation to tonsil samples, and the sensitivity of immunohistochemistry has also been demonstrated in relation to appendix samples. The specificity of immunohistochemistry has also been demonstrated: in the prospective tonsil study, none of the 2000 samples identified as negative using the Western blot tested positive by immunohistochemistry, 3 while in another study immunohistochemistry reliably identified known vcjd, but was negative for other conditions, including sporadic CJD. 11 However, it is not known how early in the incubation period these tests can detect PrP Sc in lymphoid tissue, and with what degree of sensitivity. 12 The available evidence rests on only three cases: in two of these, PrP indicative of vcjd infection was detected in appendices removed up to 2 years before symptom onset, while in the third case such PrP was not identified in an appendix removed 10 years prior to symptom onset. 4 It is generally assumed, on the basis of animal experimental data, that, in vcjd, infectivity (and thus presumably PrP) accumulates during the earlier part of the incubation period, and that it is therefore highly unlikely that immunohistochemistry will be able to detect PrP in lymphoid tissue throughout the entire incubation period: it may only be detectable once it has reached a peak level. 13 There is thus considerable uncertainty regarding the extent to which current tests can identify preclinical infection, and consequently it has been 2

7 suggested that immunohistochemistry may only have 50% sensitivity overall (ie across a lengthy incubation period). 13 Reliable estimates of the true prevalence of infection in the UK population can only be obtained if the study populations are fully representative of the general UK population. The generalisability of the study by Frosh et al is limited by its population: half of the subjects were under nine years of age 3 and therefore, if the bovine-offal bans of 1989 and 1996 provided adequate public protection from BSE-infested material, might never have been exposed to BSE prions. 14 Moreover, the population was drawn entirely from the London area, and this substantially limits its applicability to the remainder of the UK. In addition, the size of the sample affects the strength of the conclusions which can be drawn from it. In the study by Frosh et al, the sample size was too small to allow precise statements to be made about the prevalence of a rare disease. 14 Consequently, because Hilton et al studied a larger population which was drawn from across the UK, 4 their results are preferred to those of Frosh et al, 3 even though PrP indicative of vcjd infection may be less readily detected by currently available tests in appendix than in tonsil tissue. As noted above, three positive samples were identified in the 12,674 samples tested, equating to an overall prevalence of 237 per million. As 83% of the study population was drawn from the age group, the investigators suggested that the estimate essentially related to that age group, and that 3,808 individuals aged may therefore have been incubating vcjd (95% CI 785 to 11,128). Only one of the three positive samples was similar to that seen in previous cases of vcjd. It is therefore theoretically possible that the other two were false positives, although this is unlikely given the evidence summarised above for the specificity of the immunohistochemical test. However, because the samples were anonymised for ethical reasons, it will never be possible to either confirm whether the two anomalous positive samples were false positives or to determine the general sensitivity and specificity of the immunohistochemistry in identifying preclinical vcjd infection. If two of the three samples were false positives, the overall prevalence would be lower, at 79 infections per million population (95% CI 2 to 440). 4 In relation to a condition whose prevalence may be changing, such as vcjd, it is not appropriate to combine the results of studies which relate to different time periods, as do the studies by Frosh et al 3 and Hilton et al. 4 However, if the results were combined, this would give three positive cases in 14,674 samples, equating to an overall prevalence of 204 per million (95% confidence intervals 70 to 601). Both studies will eventually be superseded by the results of an ongoing study of 100,000 fresh tonsil samples from routine tonsillectomies. 15 However, the results of this study will not be available for at least another two years. 3

8 3 THE SECONDARY TRANSMISSION OF CJD/VCJD BY INVASIVE DIAGNOSTIC, SURGICAL OR DENTAL PROCEDURES The aim of this review was to examine the evidence for the secondary transmission of CJD/vCJD by invasive diagnostic, surgical or dental procedures. The relevant data derive from three sources: case reports case-control studies experimental studies of the transmission of prion disease to animals using surgical instruments or surrogates. A systematic review was undertaken to identify all relevant studies. Details of the literature searches and inclusion criteria may be found in Appendix Case reports By July 2000, only seven cases of CJD worldwide were known to have been transmitted by invasive diagnostic or surgical procedures which did not involve transplantation or grafting. Two cases were transmitted in 1974 by stereotactic EEG using probes cleaned by a method which would no longer be considered adequate. Symptomatic neurological disease developed after 20 months in one patient and after 16 months in the other. 16 After sterilisation and storage in formaldehyde vapour for two years, the probes were implanted in the brain of a chimpanzee, who developed signs of encephalopathy 18 months after implantation. 17 A further five cases have been attributed to neurosurgery (median incubation period 17 months, range 12-28). 18 These presumably include the case infected by cranial surgery in 1965, with an incubation period of 26 months 19 and three cases infected in the UK in the 1950s in which the incubation period was months. 20 No cases of CJD attributable to invasive diagnostic or surgical procedures without transplantation or grafting have been identified more recently than the 1970s. 18 No cases of CJD have been securely attributed to dental procedures. However, in Japan two patients treated by the same dentist developed CJD, and it is possible that one or both were infected in this way Case-control studies Seven studies were identified which recorded histories of surgical and dental procedures in cases with definite or probable CJD and in matched controls 22,23,24,25,26,27,28 (for details, see Appendix 2). Two of these studies 27,28 used substantially the same cases, but different controls. An eighth study, the EUROSURGYCJD study, is ongoing. 29 Case-control studies are inevitably problematic as they depend on respondent recall of past events. Case-control studies of CJD are particularly problematic because the cases are not able to respond for themselves, and therefore past events are further filtered through the memory of another respondent (usually a close family member). Some of the included studies attempted to achieve comparability by collecting information on both cases and controls from similar respondents; others introduced a source of bias by using the controls themselves as respondents. 4

9 The validity of case-control studies also depends on the appropriate choice of controls. Some of the included studies used controls drawn from hospital settings. This may distort the results as such controls are likely to have higher than average exposure frequencies for surgical interventions. 30,23,24,25,26,27,28 Ideally, therefore, only those studies would have been included which both drew their control group from the community and used similar respondents as a source of data for cases and controls. However, no studies were identified which met these criteria. The best quality evidence available is therefore derived from five studies which used community controls who responded directly: these are the studies by Collins et al, 22 Davanipour et al, 23 Juan et al, 25 Kondo and Kuroiwa, 26 and Ward et al. 28 The studies by Collins et al 22 and Ward et al 28 found that a history of any surgery was associated with a significantly increased risk of CJD (odds ratio ), while Kondo and Kuroiwa found that any surgery in the five years preceding diagnosis was associated with a relative risk of developing CJD of 3.5 (p<0.01) 26 (for details, see Appendix 2 Table 1). 3.3 Experimental studies of the transmission of prion disease to animals It has been demonstrated that CJD can be transmitted from humans to non-human primates by intracerebral inoculation or, using larger doses, by peripheral inoculation. Oral transmission has been demonstrated using even larger doses. 31 However, because the focus of the current study is the surgical transmission of CJD/vCJD, the review was limited to studies which used surgical instruments or surrogates to transfer prion-infected material to laboratory animals; studies which inoculated homogenates of infected tissue were excluded. No study was found which used material infected with CJD/vCJD. The most relevant study which was identified was that of Flechsig et al 32,33 in which stainless steel wires were inserted for five minutes into the brains of scrapie-infected, but clinically still healthy, mice. The wires were then washed exhaustively before insertion into the brains of indicator mice. A contact time of 30 minutes was enough to transfer the infection to 100% of indicator mice, although the average incubation time was longer (94 days) in these mice than in mice in whom the wires were permanently inserted (100% infected, average incubation time 71 days). The wires which had been transiently inserted into the indicator mice were then permanently inserted (apparently without cleaning) into a further set of indicator mice, 100% of whom were infected. 33 No equivalent studies were identified which used materials other than stainless steel wires. However, it has been shown that gold wires also acquire 100% infectivity when exposed to the brain homogenate of scrapie-infected mice. 32 It has also been shown that plastic surfaces (eg polystyrene, polypropylene and polyethylene) can transmit scrapie infectivity to susceptible cultured cells. 33 5

10 3.4 Secondary transmission: discussion The evidence summarised above suggests that CJD can be transmitted from human to human by contaminated instruments used in invasive diagnostic or surgical neurological procedures. Moreover, the evidence from case-control studies suggests that individuals who have undergone surgery of any kind are at increased risk of developing CJD. However, there is no clear evidence from either case reports or case-control studies of transmission by dental procedures. It has also been demonstrated experimentally that scrapie can be transmitted to laboratory animals by means of infected steel wires. It is probable that the risk of any individual becoming infected by CJD/vCJD is related to the dose of infectious material received. However, there is no evidence regarding the quantity of infective material required to transmit CJD/vCJD in humans, and it would be ethically impossible to conduct experiments which would provide such evidence. The relationship between the dose of infective material transferred to a given individual and the probability of subsequent infection in that individual must therefore remain uncertain. Two scenarios will therefore be analysed in the model. The first is that developed by ESOR. 34 Their hypotheses assume that: the risk of infection is proportional to the infectious dose received there is no lower threshold beneath which there is no risk of transmitting infection. As one ID 50 is defined as the dose which would produce a 50% risk of infection, then above 2 ID 50 s, infection is regarded by ESOR as certain. It is assumed that, up to a limit of 2 ID 50 s, the risk of being infected is proportional to the dose received. A second scenario is that the risk of infection is equal to = 1 0.5^(number of ID 50 s in dose). In this scenario 2 ID 50 s would result in a probability of infection of 75%. 4 THE INCUBATION PERIOD OF ACQUIRED HUMAN TSEs The aim of this review was to identify the best available evidence relating to the incubation periods of acquired human TSEs. Familial TSEs were excluded from the review because they are not acquired from an external source but arise from mutations in the prion gene. Sporadic CJD was also excluded because it is not clear whether its cause is acquired or endogenous. Moreover, as symptomatic disease is extremely rare under the age of 40, with peak incidence in the year age group, it is presumably the result of a slow neurodegenerative process. 35 Consequently, even if scjd is an acquired infection, the potential acquisition period may be so wide as to make the estimation of meaningful incubation periods impossible. The review therefore focuses on the incubation periods of the following TSEs: kuru iatrogenic CJD vcjd. Details of the literature searches, inclusion criteria and study selection may be found in Appendix 3. 6

11 4.1 Kuru Kuru: mode of transmission Kuru is a fatal TSE found primarily among the Fore people of the Eastern Highlands of Papua-New Guinea. Although found to a lesser extent among the immediately neighbouring peoples with whom the Fore intermarried, in these peoples it was usually seen in Fore women who had moved to marry non-fore men. 36 Kuru was spread through ritual cannibalistic funerary feasts in which deceased individuals were eaten by their close relatives and other members of the immediate community. It is assumed to have its origins in the consumption of an individual who had developed scjd. 37 It then spread as people who had died of kuru were consumed in turn. 38 The chance nature of its origin and development is illustrated by the fact that other neighbouring peoples also practised ritual cannibalism but did not develop kuru. 39 No-one born since ritual cannibalism ceased in the late 1950s has ever developed kuru. 38 However, the frequency of cannibalistic meals before the practice ended makes it difficult to determine exactly when individual kuru patients were exposed to infection. 37 Mean incubation periods have been estimated to be around 12 years, but the range is wide: in some cases the incubation period seems to be over 40 years, 37 while the youngest known patient was diagnosed at the age of 4, 36 suggesting a particularly short incubation period. The most precise estimates available are from later cases who had well-defined exposures; in these cases, the incubation periods were often as long as 25 to 30 years. Moreover, as kuru was recorded in very few children of five years of age or under, an incubation period of less than four or five years seems to have been unusual. 40 Because laboratory animals inoculated with infectious material have shorter mean incubation periods than those exposed to such material in their food, it has been suggested that cases of kuru with particularly short incubation periods may be due to inoculation with brain or other tissue, via cuts and sores or eye rubbing, rather than to oral transmission. 37 The possibility of a non-oral as well as an oral route for kuru is suggested by the fact that adult women were two to three times more likely to develop kuru than adult men, while children of both sexes were equally likely to be affected. 41 Women prepared the body for consumption, whilst children of both sexes also took part in the preparation of the meal. 41 The liquefying brain tissue has been found to be infectious at a dilution of The closest living female relatives and their youngest children were apparently the people most likely to have both handled and eaten the brain and other organs containing the highest titre of the infection agent, and the mourning custom of leaving the hands unwashed for three weeks following the funeral would have extended exposure to the infectious agent, enhancing the likelihood of non-oral transmission. 40 However, because of the uncertainty surrounding dates of birth and death predating the introduction of official monitoring in 1955, 40 it is not clear whether children too young to have eaten the infected material were infected. Klitzman et al refer to the case of Toman., a boy said to have been born in 1954, who died of kuru in 1960; he had been present at the funeral feast of an aunt who died of kuru in However, as Toman. is also said to 7

12 have been born within a few days of his cousin Pet., who is said to have been born in 1952, and both were said to have attended the funeral feast of another aunt who died of kuru in 1953, 40 it is possible that he was old enough to consume infected material. Kuru has not developed in any of the children born since 1957 to mothers who subsequently died of kuru, and thus it seems that it is not transmitted from mother to child prenatally, perinatally or neonatally via human milk Kuru and genotype Susceptibility to TSEs is affected by a genetic factor, the methionine/valine (M/V) variation at codon 129 of the PRNP gene on chromosome 20p. The genotype frequencies vary in different populations (see Table 1). 43 Table 1: Genotype frequencies in various populations 43 Population Genotype % M/M M/V V/V Eastern Highlands of Papua New Guinea (population unaffected by kuru) UK Japan Lee et al 43 studied genotype frequencies in kuru patients and survivors in Papua- New Guinea. Data were available on the following groups: A. Patients diagnosed with kuru in Fore villages during B. Unaffected individuals from the same villages who had the same risk of infection as group A and who were tested during the same period, none of whom subsequently developed kuru C. Controls studied in 1959 in three neighbouring non-fore populations which were assumed to be genetically homogeneous with the Fore population prior to the kuru outbreak D. Kuru patients diagnosed in Fore villages during E. Unaffected Fore individuals studied during , years after exposure to kuru ended. None of the Fore survivors of the kuru epidemic (group B) were M/M homozygotes. They differed significantly in this respect from both the earlier group of kuru patients (group A) on the one hand and the healthy population of neighbouring non-fore villages (group C) and the post-epidemic Fore population (group E) on the other (see Table 2). This suggests that M/M homozygotes were more susceptible to kuru, dying disproportionately in the earlier years of the epidemic. The group of kuru patients diagnosed between 1964 and 1988 (group D) was too small to yield statistically significant results in comparison with those diagnosed earlier (Group A). However, there was a trend towards an excess of V/V homozygotes compared with other genotypes, suggesting that the disease incubation period was longer in V/V homozygotes than in M/M homozygotes. 43 The similarity of the genotype distribution between the earlier group of kuru patients (group A) and both the non-fore controls (group C) and the postepidemic Fore controls (group E) (see Table 2) is not surprising: exposure to kuru did not begin to 8

13 decline until the late 1950s, and therefore the kuru patients will have included those with a shorter as well as those with a longer incubation period. Table 2: PRNP codon 129 genotypes in Fore and non-fore population groups during and after the kuru epidemic 43 Study period, population Total no of patients Number with genotype (%) P group or controls M/M M/V V/V A Fore kuru patients (30) 35 (44) 21 (26) B Fore survivors (62) 8 (38) B vs A C Non-Fore controls (29) 23 (48) 11 (23) C vs A C vs B D Fore kuru patients 13 4 (31) 2 (15) 7 (54) D vs A D vs B D vs C E Postepidemic Fore controls (38) 9 (35) 7 (27) E vs A E vs B E vs C The proposed difference in incubation period between homozygotes and heterozygotes is supported by a retrospective study of frozen specimens from 92 kuru patients who died in the late 1950s, at the height of the kuru epidemic. 44 Homozygosity was associated with an earlier age at symptom onset than heterozygosity: 50% of M/M homozygotes who developed kuru were affected before the age of 15, as were 48% of V/V homozygotes, but only 15% of heterozygotes (see Table 3). As it seems unlikely that patients with different genotypes were exposed to infection at different ages, the incubation period of kuru appears to be shorter in homozygotes than in heterozygotes. Similarly, the duration of symptomatic illness was shorter in homozygotes, with 32% of M/M homozygotes, and 58% of V/V homozygotes dying within 8 months of symptom onset, compared with 17% of heterozygotes (see Table 3). 44 Number with genotype (%) Table 3: Age at onset of kuru symptoms and duration of illness, by PRNP Total no codon 129 genotype 44 tested M/M M/V V/V Age at symptom onset (years) < (45) 6 (19) 11 (36) (28) 17 (47) 9 (25) > (16) 18 (72) 3 (12) Duration of illness* (months) < (29) 4 (19) 11 (52) (42) 10 (38) 5 (19) > (13) 10 (67) 3 (20) * Information about duration of illness was incomplete for 30 cases, who were excluded from the analysis 9

14 4.1.3 Kuru: summary Kuru is a TSE spread by the consumption and handling of infected material. Although the mean incubation period is estimated to be around 12 years, the range appears to be from under 4 to over 40 years. People who are homozygous at codon 129 of the PRNP gene, and in particular those who are homozygous for methionine, appear to be more susceptible to infection than those who are heterozygous. Following infection, the incubation period appears to be shorter in homozygotes than in heterozygotes. 4.2 Iatrogenic CJD The evidence relating to iatrogenic CJD has been well reviewed by Brown et al. 18 It is known to have been transmitted by: neurosurgical procedures corneal and dura mater grafts intramuscular growth hormone and gonadotrophin. Table 4 summarises those cases of icjd where the incubation periods are known or can be estimated; while based on the review by Brown et al., 18 it includes some more recent cases identified by the literature searches. As may be seen, the incubation periods associated with the different modes of transmission CJD vary considerably. 10

15 Table 4: Iatrogenic CJD: incubation periods and modes of transmission (after Brown et al 2000, 18 updated where more recent evidence is available) Original study No of cases Mode of transmission Route of transmission Time to appearance of Comments symptoms Approx 21 months, approx 23 months* 2 Stereotactic Bernouilli et al Intracerebral EEG Brown et al Neurosurgery Intracerebral months (median 17 months) Duffy et al Corneal transplant Uchiyama et al 46 1 Corneal transplant Heckmann et al 47 1 Corneal transplant Brown et al Dura mater graft Matsui et al Dura mater graft Kretzschmar et al 1 Dura mater graft Brown et al Growth hormone Caboclo et al 1 Growth hormone Croes et al 2002, 52 1 Growth Roks et al hormone Optic nerve or bloodstream Optic nerve or bloodstream Optic nerve or bloodstream Intracranial (cerebral surface)** Intracranial (cerebral surface) Intracranial (cerebral surface)** Peripheral (intramuscular injection) Peripheral (intramuscular injection) Peripheral (intramuscular injection) Brown et al Gonadotropin Peripheral (intramuscular injection) Approx 18 months 16 months These apparently include the French case infected in 1965, with an incubation period of 26 months, 19 and three cases infected in the UK in the 1950s with incubation periods of months; 20 the identity of the fifth case is not clear. Over 30 years Although Brown et al report the incubation time as 320 months, the original report 47 indicates that it was slightly over 30 years years These apparently include the 2 cases with incubation periods of (median 6 years) 10 and 14 years reported by Croes et al in years Presumably includes the 67 Japanese patients included in the (median 9.2 years) analysis by Brown et al years Although the patient may have been exposed to BSE, the authors argue that this is icjd, not vcjd, because the PrP Sc type seems different from that observed in either vcjd or scjd years (median 12 years) years (midpoint 28 years) 38 years years (median 15 years) * Case I: contact with contaminated electrodes , symptoms first recorded July 1976; case II contact with contaminated electrodes , symptoms first recorded April 1976 ** In 2 cases, dura mater was not used intracranially but was grafted onto vessels of non-cns tissues 11

16 4.2.1 Intracerebral procedures Only seven cases of icjd worldwide have been attributed to intracerebral procedures. 18 In each case, the incubation period was relatively short, ranging between 12 and 28 months (see Table 4), indicating that this is a very efficient route of infection Corneal transplants Only three cases of icjd worldwide have been attributed to corneal transplants. The incubation period varied widely, being under two years in two cases, 18 and 30 years in the third. 47 No reason was identified for the extended incubation period in the last case Dura mater grafts Iatrogenic CJD attributed to dura mater grafting has been reported from 16 countries. However, more than half of the known cases have occurred in Japan, where the use of dura mater grafts was particularly high (estimated at about 20,000 per year, compared with an estimate of fewer than 4,000 per year in the USA). Most cases can be traced back to grafts produced by a German company before Brown et al found a median incubation time of 6 years (range 1.5 to 18 years) in 114 patients infected by dura mater grafts. 18 However, a more recent analysis found a median incubation period of 9.2 years in 76 Japanese patients infected by dura mater grafts. 49 These Japanese cases could be divided into three clusters: those with short ( years), medium ( years) and long ( years) incubation times. The median age at transplantation was 49.3, 48.7 and 39.1 years for the clusters with short, medium and long incubation periods respectively (p=0.02). Incubation was significantly shorter in women than in men (p=0.03). Some host factors appeared to influence the incubation period. Thus, patients who had dura mater transplants for facial spasms had a median incubation period of 4.2 years, and were significantly more likely to have a short incubation period than those who received transplants for other indications (p=0.01) Growth hormone and gonadotropin The recipients of contaminated cadaveric human growth hormone (hgh) form the largest group of patients with icjd. The treatment of short stature with hgh began in the late 1950s. 54 In the USA, the use of cadaveric hgh was stopped in April 1985 after the identification there of three cases of CJD attributed to hgh. 55 Cadaveric hgh was subsequently replaced in the USA, UK and New Zealand by recombinant hormone, 18 while in France inactivation by 8 M urea was introduced in March Virtually all cases of iatrogenic CJD infection attributed to hgh will therefore have occurred before mid Indeed, in the USA, all known cases of CJD associated with hgh occurred in patients who began treatment before 1977, when a column chromatography step was introduced into the purification protocol. 18 In France, CJD infection 12

17 has been reported only in people who received hgh between 1983 and 1985; no cases have been associated with urea-treated hgh. 56 Because most patients treated with cadaveric hgh will have received multiple exposures, commonly over a period of several years, it is generally not possible to determine the precise date of exposure to the infectious agent. 37 Brown et al therefore calculated median incubation periods on the assumption that infection occurred at the midpoint of treatment with cadaveric hgh. 18 They found that most cases of CJD associated with hgh had comparatively long incubation periods (see Table 5), and suggested that this might be due, at least in part, to the small size of the infecting dose and the peripheral route of administration. 18 Indeed, the patient for whom the date of infection can be most closely identified, a patient in the Netherlands who received a particularly low dose (6 IU) of hgh over a five-day period as part of a diagnostic procedure, did not develop symptomatic CJD until 38 years after this exposure, 52 an incubation period substantially longer than any reported by Brown et al. Brown et al also suggested that the shorter incubation periods reported from France compared with other countries might result from comparatively severe contamination leading to higher infecting doses; this is supported by the high incidence of CJD in French hgh recipients compared with those in the UK and USA. 18 As the French patients all seem to have been infected between 1983 and 1985, while the US patients may all have been infected before 1977, 18 cases with longer incubation periods may yet be reported from France. Table 5: Incubation periods of CJD attributed to contaminated hgh: cases identified in or before Country No of cases Median incubation period (range)* France years (6-16 years) New Zealand** 5 26 years (14-30 years) UK years (8-22 years) USA years (10-30 years) Worldwide years (5-30 years) * Calculated from midpoint of treatment to disease onset ** All patients received hgh processed in the USA Includes 1 patient in Brazil who received hgh processed in the USA The most recent published analysis of hgh-related CJD in the UK 54 followed up to the end of 2000 a cohort of patients treated with hgh in the UK between 1959 and patients were found to have developed CJD. Because the window of time within which patients may have received infected hgh is considerably wider in the UK than in France, it was only possible to state with certainty that the shortest incubation period was between 2 years 3 months and 11 years 9 months, and the longest incubation period so far identified was between 20 years 7 months and 32 years 1 month, depending on when during treatment the patients were infected. The peak risk of infection occurred at around years from start of treatment (16-18 years from midpoint of treatment). These figures suggest incubation periods closer to those found in the US than to those seen in France (see Table 5). However, the authors of 13

18 the UK study observe that both the French and the US data underestimate incubation periods as they do not censor follow-up at death or loss to followup. 54 Huillard d Aignaux et al identified 55 individuals with icjd in the cohort of patients who had received hgh treatment in France between January 1982 and mid On the assumption that infection occurred at the midpoint of hgh therapy, the median time to symptom onset was 10.4 years. No correlation was found between the date of symptom onset and the patient s age at the start of hgh therapy, the duration of therapy, or the total hgh dose Iatrogenic CJD and genotype As with kuru, the genotype on codon 129 of the prion protein gene has been demonstrated to influence susceptibility to, and/or the incubation period of, CJD. Brown et al found that people who were homozygous for either methionine or valine were over-represented amongst patients with icjd compared with a normal white population (see Table 6). Iatrogenic CJD patients with a CNS route of infection were mainly homozygous for methionine, while valine homozygotes were disproportionately represented in those with a peripheral route of infection (via hgh injections). 18 The preponderance of methionine homozygotes among those infected by a CNS route may be influenced by the relative popularity of dura mater grafting in Japan, where around 90% of the population are homozygous for methionine. 43 Table 6: Patients with CJD, by codon 129 genotype, compared with the general population, after Brown et al (UK growth hormone figures updated from Swerdlow et al ) Category Number Codon 129 genotype % tested M/M M/V V/V homozygotes Normal white subjects (39%) 198 (50%) 44 (11%) 50 Iatrogenic CJD (59%) 26 (20%) 29 (23%) 80 Corneal transplant Stereotactic EEG Dura mater graft (74%) 8 (19%) 3 (7%) 81 Growth hormone (48%) 17 (21%) 26 (32%) 78 US 9 5 (56%) 2 (22%) 2 (22%) 78 UK 27 1 (4%) 12 (44%) 14 (52%) 56 France (52%) 7 (13%) 13 (25%) 87 scjd (71%) 112 (13%) 133 (16%) 87 vcjd (100%) More recent figures from nine European countries, and Australia and Canada indicate that, of 113 deaths attributed to definite or probable iatrogenic CJD between 1993 and 2002, 73.5% were in homozygotes (M/M 54.0%, V/V 19.5%) and 26.5% in heterozygotes. 57 Genotype data were available for 95% (77/81) of French patients with hgh-transmitted CJD, and for 73% (27/37) of those in the UK, the two countries with the highest numbers of cases. The distribution of genotypes in infected patients in these two countries was very 14

19 different: 62% of those in France were M/M homozygotes compared with 4% in the UK (see Table 7). This difference, which is not explained by differences in the distribution of the codon 129 genotype in the unaffected populations of the two countries, has yet to be explained. 58 Genotyping has been undertaken in too few US patients to allow reliable comparison. 58 Table 7: Patients with hgh-related icjd in France and the UK: distribution of codon 129 genotype, after Brandel et al Number (%) with codon 129 genotype M/M M/V V/V 38 (41%) 45 (49%) 9 (10%) French normal population French hgh CJD 48 (62%) 12 (16)% 17 (22%) UK normal population 39 (37%) 54 (51%) 13 (12%) UK hgh CJD 1 (4%) 12 (44%) 14 (52 %) Huillard d Aignaux et al analysed data relating to 55 individuals in France with icjd. Genetic data were available for 52 of the 55: 46 (88%) were homozygous for either methionine or valine at codon 129. Symptoms developed significantly later in heterozygous than in homozygous patients (p=0.02), but there was no significant difference in incubation time between patients who were homozygous for methionine and those homozygous for valine (see Figure 1). 56 Figure 1: Number of cases Incidence of CJD in hgh recipients in France (after Huillard d Aignaux et al ) VAL-VAL MET-MET MET-VAL UNKNOWN mid-1997 Date of onset of CJD symptoms In the UK and USA, where the window of potential exposure to the infectious agent was wider than in France, no statistically significant relationship between incubation time and genotype was identified when incubation times were estimated taking the midpoint of the total treatment time as the point of infection. However if, as the French data suggest, homozygotes are more easily infected than heterozygotes, the choice of the midpoint of therapy as 15

20 the supposed moment of infection could lead to underestimation of the incubation period for homozygotes, and overestimation for heterozygotes. 18 In the UK, the mean incubation periods for the 27 patients of known genotype, calculated from the start of therapy, were 13.4 years for the one methionine homozygote, 17.1 (SD 2.8) years for valine homozygotes, and 19.1 (SD 3.5) years for heterozygotes. 54 A particularly long incubation period of 38 years in a patient who was homozygous for methionine has been attributed, at least in part, to the very low dose of hgh which was used for diagnostic purposes. 52 In patients infected by dura mater grafts, no statistically significant difference in incubation period was found between homozygous and heterozygous individuals: the median incubation period for heterozygotes was 94 months (range months) and for homozygotes 108 months (range months). 18 The one corneal graft recipient whose genotype was known had an unusually long incubation period of over 30 years, despite being homozygous for methionine Iatrogenic CJD: summary CJD has been transmitted by a number of healthcare interventions. The direct introduction of infectious material into the central nervous system through intracerebral procedures appears to be generally associated with shorter incubation times than peripheral inoculation of such material: incubation times related to neurosurgical procedures are in the region of one to two years, while those related to intramuscular injections of hgh, where the time of infection cannot be as precisely established, may range from five to nearly 40 years. It seems likely that in humans, as in experimental animals, higher infectious doses generally result in shorter incubation times, but this is impossible to demonstrate as it is impossible to quantify the infectious doses received by individual patients who then developed icjd. As with kuru, people who are heterozygous at codon 129 of the PRNP gene appear to be less susceptible to icjd than those who are homozygous and, if infected, may have longer incubation periods. 4.3 vcjd Variant CJD (vcjd) appears to have been originally transmitted to humans by the consumption of material from cattle infected with BSE; the absence of occupational clusters suggests that peripheral inoculation is not involved. 37 It may subsequently have been transmitted from human to human by blood transfusion. 59 There is considerable uncertainty concerning the incubation period of vcjd acquired from infected foodstuffs. The first clinical case of BSE in the UK was recognised in In 1989, specified bovine offals were banned from human consumption, and the exposure of the UK population to BSE may have peaked around If it is assumed that all cases of vcjd in the UK were infected near that peak period, the shortest incubation periods would seem to be 5 years or less. 37 The number of deaths attributed to definite or 16

21 probable vcjd peaked in 2000, years after the likely peak exposure period. However, the diagnosis of vcjd in a patient who was a strict vegetarian from 1985 onwards suggests that exposure of the human population to BSE may have predated 1986 unless the patient suffered occult exposure to BSE through processed foods, pharmaceuticals or cosmetics in or after It is also possible that some people were exposed to BSE after 1990 if the statutory bans designed to minimise exposure to BSE were less than totally effective. 37 The peak of deaths from vcjd years after the likely peak exposure to BSE suggests an incubation period comparable with the estimated mean incubation period of 12 years seen in kuru. 37 All individuals in the UK who had been diagnosed with vcjd by 1999 were methionine homozygotes; as none had a history of unusual dietary or occupational exposure to BSE, they may therefore have been particularly susceptible to infection because of their genetic makeup. 37 If, as seen in the case of kuru, methionine homozygotes have an increased, but not exclusive, susceptibility to infection, vcjd may occur after longer incubation periods in individuals with other codon 129 genotypes. The experience with kuru suggests that the incubation period in some individuals may be as long as 40 years. 36 It is likely that the mean incubation period will be shorter if vcjd is transmitted from human to human than if it is acquired from infected foodstuffs. Two cases have been reported in which vcjd was probably transmitted by blood transfusion. One individual, who was homozygous for methionine, developed clinical disease 6.5 years after receiving blood from a donor who developed symptoms of vcjd over 3 years after giving blood. 59 This incubation period is comparable with that seen in icjd transmitted by infected hgh. Preclinical vcjd was identified in a second, heterozygous (M/V), individual who died from a non-neurological disorder five years after receiving blood from a donor who developed symptoms of vcjd 18 months after donation Acquired human TSEs: discussion The evidence summarised above suggests that the incubation time of acquired human TSEs is variable. Influencing factors include the route of transmission, genotype and, probably, infectious dose. 17

22 5 THE LIKELY INFECTIVITY OF INFECTIOUS TISSUE In the absence of new information, we have assumed the duration and infectivity associated with tissue types to be that assumed by ESOR for preclinical human-to-human infectivity. 34 These are subdivided into those tissues assumed to be infectious only during the last 40% of the incubation period, and those tissues assumed to be infectious throughout the incubation period. (a) For the last 40% of the incubation period brain and posterior eye: 10 8 ID 50 /g spinal cord, trigeminal or dorsal root ganglia: 10 6 ID 50 /g anterior eye: ID 50 /g (b) Throughout the incubation period: tonsil or spleen into peripheral site: ID 50 /g other lymphoid tissue or peripheral nerve into peripheral site: ID 50 /g. The values reported above are for intracerebral transmission, where transmission is at a peripheral site, the infectivity would experience a log reduction. 6 THE INFECTIOUS MASS REQUIRED TO TRANSMIT CJD/VCJD A systematic review was undertaken to identify all studies which sought to identify the infectious mass required to transmit CJD or vcjd. Details of the literature searches and inclusion criteria may be found in Appendix 4. The electronic literature searches identified no relevant studies (see Appendix 4). However, a study identified by the secondary transmission search has some relevance in this context: this was the study by Flechsig et al 32,33 discussed in section 2 above. The authors of this study attempted to elute PrP from steel wires known to be infected with scrapie, using 2M NaOH: they failed to detect either protein (detection limit, 50 ng per wire) or PrP (detection limit 15 pg per wire), although PrP immunoreactivity was detected by chemoluminescence. This suggests that either: the infectious mass required to transmit scrapie is below the detection limits of that experiment; or the infectious mass required to transmit scrapie is above those limits, but binds so tightly to steel wires that it cannot be removed by exhaustive washing. If this is true of scrapie, it may also be true of other TSEs, including CJD/vCJD. 18