What evidence is there that dentistry might constitute a risk of CJD transmission?
Clearly it is not possible to carry out experimental work in humans, and so the only direct evidence would be likely be epidemiological. A number of epidemiological studies have failed to demonstrate an association between CJD and dental treatment.
However there has been one report of a cluster of three cases in Japan, that appeared to be associated only with a shared dental practice, and CJD patients have also been shown to be more likely to have recieved dental surgery. Epidemiological evidence would not be expected to be found at all easily because of the long incubation period for CJD, the mass use of dentistry, the relatively lower use of dentistry in older people, and because there are normally so few CJD cases around.
In considering dental transmission of CJD, three questions logically arise. Firstly, are the oral tissues involved a potential source of infection? Secondly, can infection enter another patient via their oral tissue? And thirdly, could contaminated dental instruments carry sufficient material to transmit CJD?
There are a number of clinical and animal studies pointing to the presence of infectivity in peripheral nerves and other peripheral tissues, including tonsils. Since the gingiva contain peripheral nerves the possibility that these might carry significant infectivity late in the CJD incubation period can not be ruled out.
Furthermore, the membrane associated lymphoreticular tissues (MALT) present in the oral tissues might prove to be infectious, as they have been shown to be at other sites in animals. Animal studies have also shown that it was possible to transmit scrapie by inoculation onto dental scarification, suggesting that CJD infection may enter via the gingiva.
The remaining question is the degree to which dental instruments may harbour infective material. The resistance of TSEs to heat and chemical treatment has been well documented and surgical electrodes used on CJD patients have transmitted infection despite cleaning and sterilisation.
Thus instruments might reasonably be expected to retain some infectivity, though in small quantities (32). However, there do not appear to have been any studies on the amounts of tissue that can remain on dental instruments after cleaning and sterilisation.
Is there any need for action now?
In 2001, Bagg and Sweeney published a survey on the adequacy of cross infection control measures with regard to CJD in dental practice, which revealed gaps in procedures and variable standards of infection control measures in practices.
The general issue of risks from contaminated surgical instruments has also been examined in recent years by the UK government. In 2000, the Department of Health published a circular on decontamination of instruments, including those used in dentistry. The following year a comprehensive survey on the decontamination of surgical instruments in the NHS England was published, as was a risk assessment of transmission of vCJD via surgical instruments.
In January 2001 the department recommended the use of disposable instruments for tonsillectomy and adenoidectomy because of concerns about the possibility of transmission of vCJD, however this policy was later abandoned because of problems arising from their use.
Although withdrawn because of practical problems, the recommendation on single use instruments for tonsillectomy and adenoidectomy was made because of a theoretical risk. Similarly animal TSE models of blood transmission were seen as sufficient to justify action to minimise the possibility of vCJD transmission via blood and blood products.
vCJD is a fatal disease with, at present, no cure. The incubation period is very long and the number of people who might be incubating the disease is unknown, but in the UK at least, could be large. In 2000 Smith and Martin concluded that 'the current absence of definitive data for evidence to know precisely what action to take should be adequate to permit dentists to continue with universal precautions'.
Ultimately, their 'pragmatic approach' might not be adequate. The UK and other countries may soon have to confront the possibility of measures such as increased use of disposable instruments in dentistry, difficult and expensive though this might be. The latest data suggesting that there is in fact quite a large amount of infectivity in the oral tissues was largely ignored as being correct by the UK Government.
They picked 1000IU per gram as being more valid but the source was not given in their document. It seems that this figure has been taken as being useful for many tissues when the level is too low for demonstration using standard procedures. At the moment there appears to be no way to argue in that all that can be done is to limit the amount of infecitvity as 'much as would seem reasonable'.
The Government, with SEAC, agreed that this was the best way to go and as such, when taking into account the amount of washing that can take place of instruments and how much infectivity is likely to come off into a new patient, it is calculated by them (see Peter Bennet) as being useful. Their decision is that the best action will be to simply enforce adequately the infection control policies that are being carried out already against many other types of disease.