Variant Creutzfeldt-Jakob disease

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1 CROSS-INFECTION CONTROL AND PRIMARY DENTAL CARE Variant Creutzfeldt-Jakob Disease A Problem for General Dental Practitioners? Christine L Whitworth Over a hundred deaths from variant Creutzfeldt-Jakob disease (vcjd) have now been recorded. The incubation period for vcjd may be up to 40 years and the number of asymptomatic carriers in the population could be as many as 100,000. Confirmed iatrogenic transmission of other human transmissible spongiform encephalopathies raises the possibility of cross-infection from apparently healthy persons who are incubating vcjd. Decontamination techniques routinely used in general dental practice are incapable of inactivating the infective protein responsible for transmitting the disease. So far, no evidence exists to preclude the risk of iatrogenic infection occurring during dental procedures. The author reviews present knowledge of the infective agent, its transmissibility, its origins, current guidelines and the implications for dental practice. KEY WORDS: CREUTZFELDT-JAKOB SYNDROME, PREVENTION AND CONTROL, TRANSMISSION; DENTAL INSTRUMENTS; PRIMARY DENTAL CARE 2002;9(3):95-99 HUMAN, INFECTION-CONTROL; DENTAL; RISK FACTORS; STERILISATION/METHODS Introduction Transmissible spongiform encephalopathies (TSEs) are a rare group of fatal degenerative conditions affecting the brains of mammals that may be acquired, inherited or occur sporadically. Decontamination techniques currently in use for dental instruments are unsuitable for patients with TSEs or their relatives, as the infective prion protein agent is so difficult to remove and inactivate. 1 The Infective Agent Infective prions are not microorganisms, but a type of protein that interacts with normal host tissue to produce disease. The prion protein, PrP c, is a normal glycoprotein encoded by a gene present in all nuclear cells of humans and other mammals. 2 PrP c is present on the surfaces of cells and is noninfective. The presence of PrP c is a prerequisite for prion replication 3 and any cell that expresses PrP c at its surface is vulnerable to infection. The infective prion (PrP sc ) is CL Whitworth BDS, DGDP(UK). General Dental Practitioner, Wirral. an isomer of PrP c, which when introduced by inoculation or dietary exposure, in some way modifies the normal prion to the pathological form. The highest levels of PrP c are found in the central nervous system and it is here that the pathological deposition of infective PrP sc causes characteristic neuronal damage. 4 Uniquely among human TSEs, PrP sc is detectable in the lymphoreticular systems of sufferers of variant Creutzfeldt-Jakob disease (vcjd). 5 Recent research found that extra-neural spread of scrapie occurs via the lymphatic system. 6 It is entirely possible that vcjd may be transmitted in a similar fashion. It would appear that the pathogenesis of vcjd differs from the other forms of human TSE, being more closely related to scrapie and bovine spongiform encephalopathy (BSE). Human Transmissible Spongiform Encephalopathies There are several recognised forms of human TSE that may be divided into groups according to their method of transmission, as shown in Table 1. It is possible to identify those at risk of development of most forms of TSE by taking a thorough medical history. Patients whose parents or other close blood relatives have had Gerstmann-Straussler- Table 1: Human transmissible spongiform encephalopathies showing types and conditions Type Inherited Sporadic Acquired Condition Fatal familial insomnia Gerstmann-Straussler-Scheinker syndrome Other autosomal dominant Creutzfeldt-Jakob disease Kuru Iatrogenic Variant Creutzfeldt-Jakob disease PRIMARY DENTAL CARE JULY

2 VARIANT CREUTZFELDT-JAKOB DISEASE Number * Year *To 4 February 2002 Figure 1 Confirmed deaths from variant Creutzfeldt-Jakob disease in the UK. Scheinker syndrome, fatal familial insomnia or other form of familial Creutzfeldt-Jakob disease (CJD) are a potential source of infective prions. Iatrogenic CJD has occurred following prion transmission from poorly decontaminated neurosurgical instruments, dura mater homografting, corneal grafts, pituitary gonadotropins and cadaver-derived human growth hormone. 7 Where did variant Creutzfeldt-Jakob disease come from? Infective prions isolated from sufferers of vcjd are identical to the prion proteins responsible for BSE and the mode of transmission of the disease is thought to be via ingestion of infected beef products. 8,9 vcjd is a disease affecting predominantly young people, mainly teenagers and young adults. A recent report of a case of vcjd in an elderly patient 10 raises the possibility of greater spread within the population. The appendix removed from a patient, who eight months later went on to develop the symptoms of vcjd, was found to contain infective prions. 11 This raises the possibility that the tissues of other asymptomatic carriers of the disease may contain infective prions that may then be transmitted iatrogenically during surgical treatment. Recently, the presence of PrP sc in the tonsillar tissue of vcjd patients 12 prompted the Government s insistence that future tonsillectomies should be carried out using disposable instruments to protect patients from the risk of iatrogenic transmission of vcjd. 13 Patients who are incubating the disease will be unaware of their condition and are asymptomatic. It is not possible to identify asymptomatic carriers from their medical history. It is possible therefore that dental practitioners may have unknowingly treated patients incubating vcjd. Estimation of Numbers of Asymptomatic Carriers of vcjd in the Population The incidence of vcjd has not risen dramatically since the disease was first recognised. 14 Statistics for the numbers of UK deaths from vcjd are shown in Figure A large retrospective study is presently under way to examine archived surgically removed human tonsillar tissue for the presence of infective prions. 16 So far, no positive cases have been identified. 17 This research has important implications for the estimation of asymptomatic carriers within the population, though the point at which PrP sc becomes detectable within lymphoid tissues is unknown and the test process used in the survey may not identify the infective prion in some cases. There is no available evidence to identify how many people will develop vcjd following ingestion of infected beef products. 18 The incubation period of vcjd is unknown but may be as long as 40 years. This hypothesis is drawn from a comparison with kuru, another acquired human TSE. Kuru developed as a result of cannibalism within a single tribe in Papua New Guinea and although this practice ceased in the 1950s, new cases of the disease were still being identified up to 40 years later. There appears to be increased susceptibility to infection in those individuals with a particular host PrP genotype, as all recorded cases of vcjd have so far been found in patients with homozygous methionine (met/met) at codon The met/met genotype is also associated with an earlier age of onset and shorter disease duration in kuru, 20 however the development of kuru is not confined to those people with the met/met genotype. The absence of vcjd in individuals with other genotypes does not therefore mean that they may not be susceptible to infection. Estimates of asymptomatic carriers within the population accordingly range from hundreds 21 to hundreds of thousands PRIMARY DENTAL CARE JULY 2002

3 CL WHITWORTH Implications for dentistry The presence of PrP sc in circulating β-lymphocytes in patients with confirmed vcjd 23 should be of concern to dentists undertaking surgical treatment, or even placing a matrix band subgingivally. Any instrument contaminated with the blood of such a patient may carry the risk of iatrogenic transmission. Those practitioners who carry out endodontic treatment, periodontal or implant surgery have the additional concern that PrP sc may be present in the dental pulp and gingival tissues of patients with vcjd. Hamsters subjected to intraperitoneal inoculation with hamster-passaged scrapie strain 263K demonstrated high levels of infective PrP sc in the gingival tissues and dental pulp. When disease-free animals were subjected to inoculation of 263K into the dental pulp, all animals subsequently went on to develop scrapie. 24 PrP sc is undetectable in the dental pulp of patients with sporadic CJD. 25 There is no current evidence of infective prions in the oral tissues of humans with vcjd, but in all other tissues the distribution of PrP sc in humans infected with vcjd and animals infected with scrapie has been found to be identical. The presence of PrP sc in the oral tissues of undiagnosed carriers of vcjd is Table 2: Summary of the effect of decontamination techniques on PrP sc Decontamination technique Effect on PrP sc Irradiation Ionising Ultraviolet Microwave Dry heat 200 o C for 1 hour Autoclaving Non-vacuum 121 o C for 90 min 134 o C for 30 min 132 o C for 60 min Vacuum 134 o C for 18 min 138 o C for 18 min 121 o C for min in Inactivation the presence of 2 M sodium hydroxide Chemical agents Acids and bases 1 M and 2 M sodium hydroxide for 2 hr Boiling in 1 M sodium hydroxide for 1 min Formic acid Hydrochloric acid Inactivation Little effect Alkylating agents Formalin Glutaraldehyde β-propriolactone Ethylene oxide Detergents Sodium dodecyl sulfate Halogens Sodium hypochlorite Inactivation (25,000 ppm available chlorine) for 1 hour Dichloroisocyanurate Organic solvents Acetone, chloroform, Little effect ethanol, phenols Oxidising agents Chlorine dioxide, hydrogen Little effect peroxide, peracetic acid Salts Sodium metaperiodate, Little effect potassium permanganate Chaotropes Urea Little effect Guanidine hydrochloride Proteolytic enzymes Trypsin Little effect Pronase, Proteinase K, Quiagen Significant reduction in activation after prolonged digestion times PRIMARY DENTAL CARE JULY

4 VARIANT CREUTZFELDT-JAKOB DISEASE therefore a possibility. Without evidence to the contrary, is it possible for dental practitioners to assure their patients that routine dental treatment carries no risk of transmission of vcjd? 26 Problems of Inactivation of Infective Prions It is impossible to inactivate prions using techniques of sterilisation currently available in general dental practice. PrP c, the normal cellular form of prion protein, is proteasesensitive, but the infective PrP sc prion is resistant to proteolytic enzymes. 1 Though chemically identical, the structures of PrP c and PrP sc are very different. PrP sc has a greater ratio of β-sheets and less α-helical structures than PrP c. 27 The structural nature of PrP sc is the reason for its resistance to inactivation. An extensive review of methods that have been used in an attempt to inactivate TSE agents has been undertaken 28 and a summary is shown in Table 2. The results are from tests carried out on various strains of infective prion protein that have different degrees of thermostability and susceptibility to inactivation. Some of the techniques were used for inactivation of PrP sc in histological samples and would be unsuitable for sterilisation of surgical or dental instruments. The only effective methods so far discovered appear to be immersion in sodium hypochlorite (25,000 ppm available chlorine) for one hour, boiling in 1 M sodium hydroxide for one hour or vacuum autoclaving at 121 o C for minutes in the presence of 2 M sodium hydroxide. These methods are unsuitable for most dental instruments and would cause immense damage to delicate items such as handpieces. No decontamination technique currently used for the sterilisation of surgical or dental instruments has been shown to reliably inactivate PrP sc. The importance of universal infection-control measures, particularly thorough precleaning of instruments, cannot be over-emphasised. It has been demonstrated that removal of contamination decreases the infectivity of instruments by a factor of In addition, it has been demonstrated that PrP sc adsorbs to stainless steel surfaces, resists removal by immersion in 2 M sodium hydroxide and remains infective. 30 This has serious implications for practitioners, as many dental instruments are constructed from stainless steel. There is an argument for single use of those instruments that become most heavily contaminated with potentially infective material, such as burs, 31 endodontic instruments 32 and matrix bands. 33 This course of action may be sustainable for the small numbers of patients with TSEs, but have great cost implications for provision of instruments and their subsequent safe disposal. Current Guidelines The decontamination technique recommended for inactivation of PrP sc -infected instruments by the Spongiform Encephalopathy Advisory Committee (SEAC) is one 18-minute, or six consecutive threeminute cycles at 134 o C in a vacuum autoclave, with the caveat that the method does not completely inactivate the infective agent. 34 A study of decontamination methods used in general dental practice found that only 3% of dentists surveyed possessed a vacuum autoclave. 35 Current guidelines advise general dental practitioners to refer those patients suffering from and at risk of TSEs to secondary care centres, where the instruments used for their treatment may be effectively quarantined, in the case of at-risk patients, or destroyed in those patients known to have TSE. 36 All instruments used on both these categories of patients must be traceable. Guidelines for treatment of patients suffering from TSEs include destruction by incineration of all instruments including handpieces, not connecting handpieces to waterlines and not using a fixed aspirator or spittoon. Instruments used on at-risk patients are required to be kept in a leak-proof container until their disease status is known. The Department of Health advises that any accident involving sharps or contamination of abrasions with blood or body fluid should be encouraged to bleed under a running tap for several minutes, washed gently with soapy water, rinsed, dried and covered with a waterproof dressing. 37 The incident should be recorded. The Future It is important that a reproducible method be found for inactivation of infective prions that is suitable for use in general dental practice. The Department of Health has instructed Health Authorities to review and validate the decontamination techniques currently in use in general dental practice. 38 Guidelines for decontamination are due to be issued later this year and these may have medicolegal and cost implications for dentists. The draconian nature of current guidelines for treatment of vcjd patients makes following them in general dental practice impossible. Research is also required into the tissues where vcjd PrP sc is present, so that potentially infected instruments may be identified. A new test method has recently been described that is able to detect PrP sc more reliably, 39 though oral tissues were excluded from this research. There remains a problem in detection of PrP sc in tissues where large quantities of PrP c are present. 40 No diagnostic test exists to identify carriers of vcjd and until such a method can be found, all patients that have consumed potentially infected beef products may be considered a source of infective prions. Identification of asymptomatic vcjd patients would present an enormous ethical dilemma, as the disease is invariably fatal, though trials of quinacrine and chlorpromazine in the treatment of CJD and vcjd appear to offer some hope to sufferers PRIMARY DENTAL CARE JULY 2002

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London: Department of Health, Wadsworth JD, Joiner S, Hill AF, Campbell TA, Desbruslais M, Luthert PJ, et al. Tissue distribution of protease resistant prion protein in variant Creutzfeldt-Jakob disease using a highly sensitive immunoblotting assay. Lancet 2001;358: MacGregor I. Prion protein and developments in its detection. Transfus Med 2001;11: Josefson D. Drugs for malaria and psychosis may offer hope to people with CJD. BMJ 2001;323:416B. Correspondence: CL Whitworth, 9 Rake Lane, Upton, Wirral CH49 0US. blackadder@clara.co.uk PRIMARY DENTAL CARE JULY