The hidden danger of prions

Pathogens, defined as microorganisms capable of causing disease[i], can be considered as the hidden enemies of dentistry.

In the dental environment, these infectious agents can be transmitted through direct contact (person to person) via inhalation, injection, ingestion, or contact with blood, saliva and secretions, or indirect contact through contaminated instruments, equipment or surfaces[ii], making infection prevention and control measures essential to prevent the transmission of pathogens between patients and between patients and dental healthcare workers.

What are pathogens?

For those responsible for infection control within dental practices, the most familiar pathogens include bacteria, fungi and viruses.

• Bacteria are single cell microorganisms, visible only under a microscope, that vary in size and shape and which multiply by cell division. While the vast majority are either harmless or beneficial to their host, such as those found in the human gastrointestinal tract to aid digestion, a small percentage are pathogenic and can cause infectious diseases such as salmonella, pneumonia, or meningitis[iii].
• Fungi are larger than bacteria and can even be seen by the naked eye. Fungi are eukaryotic (any cell or organism that possesses a clearly defined nucleus[iv]) organisms and can come in the form of a mushroom, yeast or moulds[v]. The majority of oral infections are caused by yeasts belonging to the genus Candida. The most common fungal infection of the oral cavity is candidiasis, commonly known as oral thrush.
• Viruses present a significant challenge in healthcare settings due to their resilience and ability to spread. Much smaller than bacteria, and visible only under an electron microscope, viruses have no metabolism and cannot reproduce on their own. Instead they survive and multiply by being parasites that hijack living hosts and are responsible for diseases such as influenza, hepatitis and HIV[vi].

The problem of prions

There is another pathogen to contend with in dentistry that is the least understood, but potentially the most dangerous, – this is known as a prion.

The word prion was first used in 1982 by Stanley B. Prusiner from the University of California, to describe an infectious agent that causes transmissible spongiform encephalopathies[vii] (TSEs), which are rare but fatal neurodegenerative diseases affecting humans and animals. Prusiner was awarded the Nobel Prize in Physiology or Medicine 1997 “for his discovery of Prions – a new biological principle of infection”[viii].

Prion denotes a small proteinaceous infectious particle which is resistant to inactivation by most procedures that modify nucleic acids[ix]. Prions are not living organisms but are naturally occurring protein molecules that change shape through abnormal folding.

When normal prion proteins misfold in the brain, they cause a chain reaction of protein misfolding that form toxic clumps that kill brain cells and eventually lead to death[x]. Prions are smaller still than viruses, and even through an electron microscope only clusters, not individual prions, can be seen, making them extremely difficult to identify and study.

What are TSEs?

TSEs are a group of rare degenerative brain disorders characterised by tiny holes that give the brain a “spongy” appearance. They include bovine spongiform encephalopathy (BSE), commonly known as ‘mad cow disease’ in cattle, and the fatal, degenerative disease of the nervous system, scrapie, in sheep and goats. Creutzfeldt-Jakob Disease (CJD) is the most well-known of the human TSEs[xi].

TSEs are characterised by a long incubation period, anything from 18 months to up to 40 years, with carriers generally showing no visible symptoms of disease during that time. And because prions are an abnormal form of a normal genetically encoded protein they do not produce an immune response in their host to fight prion disease, as would any other foreign infectious agent, such as a bacterial infection or virus.

Prions in dentistry

The problem for those responsible for infection control in dental practices is that unlike bacteria and viruses, prions are highly resistant to autoclave and other methods of sterilisation and disinfection[xii] and regulations make reference to the potential danger.

SHTM 01-05 Part C 4.1 states: “Prion proteins are not fully deactivated by the sterilisation process. Therefore, effective instrument cleaning is particularly important to physically remove contamination, including prion proteins, prior to sterilisation and reduce the risk of cross infection”.

 

With regards to the cleaning of dental instruments, HTM 01-05 1.15 states: “Recent research has indicated that a low level of prion contamination may theoretically be present on some instruments following contact with dental tissues. Section 1.16 continues: “there is a risk of prion transmission through protein contamination of instruments”.

However, it should be made clear that to date, there have been no reported or confirmed cases of prion disease transmission from dental procedures. So although there is a theoretical risk of prions being passed on from contaminated dental instruments, by following established cleaning and sterilisation procedures practices will be doing enough to protect patients and staff.

HTM 01-05 suggests that instruments cleaned as soon as possible after use may be more easily cleaned than those left for a number of hours before reprocessing[xiii]. Where this is not possible, immersing instruments in water, or using a foam spray intended to maintain a moist environment, is considered useful in aiding subsequent decontamination and removal of protein and debris.

In a paper published in 2007, Lipscombe et al found that pre-soaking instruments significantly reduced (by up to 96%) prion-infected tissue contamination[xiv]. The same study concluded that allowing a contaminant to dry onto an instrument surface for any length of time could severely hamper its removal and thereby compromise the effectiveness of subsequent sterilisation processes.

A ready-to-use, pH-neutral foam spray applied directly onto soiled instruments after use immediately starts the decontamination process, helping loosen debris, and for products containing an integral non-drying component, prevents biological matter from becoming dried onto the instruments’ surface. Once instruments are ready to be cleaned, the holding solution is simply rinsed away using water and cleaning (by either a manual or automated process), and sterilisation takes place in the normal way.

Conclusion

The advice given in HTM 01-05 1.17 to reduce the risk of prion contamination of instruments is as follows: “Currently there is no recognised process that can fully deactivate prion protein in the sense of removing any foreseeable level of contamination. In this Health Technical Memorandum, the cleaning process and its ability to remove protein in tandem with a validated steam sterilization procedure is emphasised, as this is known to at least reduce the risk of prion transmission through dental instruments”.

Understanding the potential danger of prions in addition to other infectious agents is something that everyone working in the dental environment should be aware of. It is the responsibility of every decontamination lead to ensure everyone on the dental team is given up to date training in decontamination and infection control, ensuring this crucial knowledge is disseminated and helping to ensure that prion disease transmission does not transform from a theoretical to a reality for those working in the field of dentistry.