The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [87]
The prion protein (PrP) is a normally occurring protein found on the surface of some cells (PrPC). The disease-causing form of the protein (PrPCJD) appears to have an identical amino acid sequence but has a different three-dimensional shape. When the normal protein PrPC is exposed to the disease-causing form, PrPCJD, it changes its conformation to that of PrPCJD. PrPCJD appears to progressively accumulate and be deposited in the brain, resulting in the characteristic ‘plaques’. This process is slow compared to the replication of most micro-organisms and ‘classic’ CJD normally appears in older people.
One of the characteristics of vCJD is that it affects a much younger age group, although the incubation period still appears to be a number of years. There are currently no reliable tests to identify infection before the onset of symptoms, which has led to the worry that there could be a large pool of asymptomatic carriers of the vCJD infectious agent who may act as a reservoir for onward transmission via healthcare procedures. Routes of transmission already confirmed for CJD and vCJD include dura mater and corneal grafts, treatment with human-derived growth hormone, blood transfusion and surgical instruments. The infectious agent does not appear to be affected by decontamination processes such as autoclaving and chemical disinfectants to the same extent as more familiar micro-organisms such as bacteria or viruses. This has led to extensive reviews of decontamination procedures in the UK and has resulted in an increased emphasis on effective washing to remove any residual organic material that may harbour the infectious agent, and on the tracking of instruments to individual patients to facilitate any look-back exercise should any patient be identified as suffering from CJD or vCJD at a later date.
Creuzfeldt–Jacob disease is a sporadic illness that affects around one person in every million and probably arises from a spontaneous genetic mutation. It should also be noted that only a very small number of people have developed vCJD. It appears that a combination of exposure to the infectious agent and genetic susceptibility is necessary for progression to the disease (related TSE have a very strong genetic component), and there are numerous measures in place to prevent both the infectious agent entering the food chain and onward transmission through healthcare interventions. These appear to be the only routes of infection – there is no evidence of transmission via any other route. However, there is much that is unclear about the disease and the causative agent.
Mechanisms of infection
Whether or not a particular infectious agent will cause an infection in any given circumstance is dependent on many different factors, including how easily that agent can be transmitted and its pathogenicity or virulence – its ability to cause disease and the severity of the infection produced. However, it is generally accepted that for infection to occur, certain linked requirements need to be met. These links are often referred to as the chain of infection (Damani 2003). While the chain of infection will not be strictly accurate in every case – some ‘links’ may be missing or will overlap – it is an extremely useful model to consider how infection can be prevented, by breaking the ‘links’ in this chain. Some links are easier to break than others – for example, it is often easier to prevent an infectious agent entering a susceptible person than it is to prevent it leaving an infected one.
The chain of infection is illustrated in Figure 3.4 and the links are listed, with examples of how infection can be prevented at each link, in Table 3.3.
Figure 3.4 The chain of infection. A useful tool for seeing how to prevent transmission. How would you break each of the links in the chain?