Part 1
Background and Introduction
1.1 Transmissible spongiform encephalopathies (TSEs), sometimes known as prion diseases, are fatal degenerative brain diseases which occur in humans and certain other animal species. A common feature of all TSEs is the appearance of microscopic vacuoles (holes) in the grey matter of the brain, giving a sponge-like appearance, from which the conditions derive their name. All the diseases are experimentally transmissible by inoculation and in some cases by oral challenge.
1.2 There are several recognised TSEs, including Creutzfeldt Jakob Disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep. Other related conditions are summarised in Box 1.
1.3 TSEs are in many ways unique, and exhibit biological properties that are different from those of other microbiological diseases. A useful summary about these diseases has been published previously by the Spongiform Encephalopathy Advisory Committee (SEAC) (see bibliography). Some of the important features relevant to occupational exposure are summarised below:
- TSEs are caused by unconventional infectious agents currently thought to be infectious proteins (possibly without nucleic acid) known as prions which do not share the normal properties of viruses or bacteria;
- some forms of human TSEs appear to have a solely genetic origin, whereas in other human TSEs, and in animal TSEs, there is a complex relationship between the infectious agent, host genetic factors and cellular proteins;
- TSE agents are not uniformly distributed in the tissues of affected individuals and infectivity levels vary at different stages of incubation. In general, and late in an infection, neural tissues pose the highest risk, spinal fluid and lymphoreticular tissue a lower risk and blood, other body fluids and most other tissues negligible risk;
- TSE agents exhibit an unusual resistance to conventional chemical and physical decontamination methods. They are not significantly affected by disinfectants like formalin and ethylene oxide, and infectivity persists after autoclaving at conventional times and temperatures (e.g. 121°C for 15 minutes). They are also extremely resistant to high doses of ionising and UV irradiation and some residual activity has been shown to survive for long periods in the environment;
- all TSEs are invariably fatal once clinical signs appear; there is no known treatment or prophylaxis;
- TSEs are not highly contagious and, other than sheep with scrapie, do not seem to spread from an infected host to an uninfected one by normal contact. There are documented cases of CJD being transmitted accidentally to patients via contaminated medical instruments or contaminated pituitary hormones prepared from human cadavers (these are known as iatrogenic infections);
- there have been no confirmed cases of transmission of TSE to humans as a result of occupation. If TSEs could be transmitted in the occupational setting this would be most likely to occur from exposure to infected tissues or materials by direct inoculation (e.g. puncture wounds, ‘sharps’ injuries or contamination of broken skin), by splashing of the mucous membranes or, exceptionally, by swallowing.
1.4 The unconventional nature of the agent, together with the appearance of BSE in the mid 1980s and a new distinct variant CJD (nvCJD) in the mid 1990s, has led to a considerable amount of scientific research. This in turn means that there is a need for updated guidance on safe work practices in laboratories and animal accommodation. Recent strain typing and other transmission studies have indicated that the agent responsible for nvCJD is identical to the BSE agent. It is too early to predict whether the cases of nvCJD seen to date herald a widespread epidemic. In any case, there is a need to provide guidance for health practitioners on the risks from humans infected with TSE agents.
Scope of this guidance
1.5 Health and safety law sets out a series of general duties on employers, employees and self employed people. There are specific regulations which cover work with biological agents such as TSEs, notably the Control of Substances Hazardous to Health Regulations 1994 (COSHH). These require employers to assess the risks in all cases where there may be exposure to biological agents. But the necessary containment and control measures will differ depending on whether there is a deliberate intention to work with the agent (such as in a research laboratory) or whether exposure is incidental to the work (such as in a hospital ward or operating theatre).
1.6 This guidance is therefore divided into three main sections as follows:
- hazards and risk associated with workplace exposure to TSE agents (including information on health and safety law);
- containment and control measures for experimental laboratory work with TSE agents, materials and infected animals (i.e. where there is deliberate intention to work with the agent);
- infection control of CJD and related disorders in healthcare settings (i.e. where any exposure to the agent is incidental to the work).
1.7 The purpose of this document is to provide guidance to employers on the precautions to minimise the exposure of employees and others to TSE agents from work activities. The guidance applies to many occupations that involve contact with people or animals infected with TSE agents, or potentially contaminated material. It should also be drawn to the attention of those responsible for advising others who may come into contact with TSE during the course of their work. Included in these two broad groups are:
- laboratory staff (including experimental animal house staff);
- healthcare workers (including infection control staff; medical and nursing staff particularly in neurology, ophthalmology, neuro- or ENT-surgery, oral and maxillofacial surgery, and dentistry; sterile services supply staff; and medical engineers);
- staff involved in hospice and community care;
- pathologists (including veterinary pathologists), histologists and post mortem technicians;
- funeral, cemetery and crematorium workers;
- local Consultants in Communicable Disease Control (CCDCs).
1.8 Additional advice for veterinary surgeons and those involved in the transportation, slaughtering and processing of cattle and cattle products can be found in a separate publication (“BSE Background and general occupational guidance”. ACDP. 1996). Guidance on handling meat and bone meal (MBM) material and an information sheet on occupational health risks from cattle, which will be of interest to farmers and others involved in animal husbandry, have also been published by the Health and Safety Executive. Details of these publications are given in the bibliography.
Box 1 Human and Animal TSEs
The human TSEs are:
- Creutzfeldt Jakob disease (CJD) including classical sporadic; familial; iatrogenic and new variant;
- Gerstmann Sträussler Scheinker syndrome (GSS);
- fatal familial insomnia (FFI);
- kuru.
The human TSEs have a pre clinical phase that lasts for years. This is followed by rapidly progressive dementia, loss of memory and intellect, personality changes, or progressive unsteadiness and clumsiness. In CJD, sudden involuntary muscular jerking is frequently seen. In most cases, death occurs within a few months of onset of symptoms and the patient is usually mute and immobile in the terminal stages.
All human TSEs are very rare; the world wide incidence of CJD is about 1 per million people each year. The usual age of onset for classical sporadic CJD is late middle age (average age 63 years). About 85% of cases are classical sporadic; most of the rest are familial in origin. Some cases are the result of medical treatment (iatrogenic transmission). In March 1996, a new variant of CJD (nvCJD) was reported in 10 persons of unusually young age. It is characterised by behavioural changes, poor co ordination (ataxia), progressive cognitive impairment and often a relatively long duration of illness (up to 2 years). There are also characteristic changes in the brain with large accumulations of prion protein. Research findings published since March 1996 indicate that nvCJD and BSE are caused by the same infectious agent.
The other human TSEs (GSS and FFI) are exceptionally rare, affecting around 1 person in 10 100 million per year. Both appear to be inherited diseases and thus related members of a family may be affected. In GSS and FFI the disease is usually more prolonged than in CJD, and generally starts at an earlier age. GSS patients suffer predominantly from problems of balance and incoordination; FFI is characterised by abnormal sleeping patterns.
Kuru was found in the Fore tribe of New Guinea and was first reported in 1957. It was associated with funeral rites involving ritual contact with, preparation of, and consumption of the entire body including the brains of previous kuru victims. The establishment of a link between contact with infected tissue and the subsequent development of kuru first suggested an infective basis for all human TSEs. Occurrence of kuru has been markedly reduced following the abolition of cannibalism coupled with health education, although some cases may still arise from historical exposure.
There have been no confirmed cases of transmission of TSE by virtue of occupation. There have been a small number of reports of classical sporadic CJD in healthcare workers (including retired laboratory workers and a pathologist) but the link with their occupation is speculative. An excess of classical sporadic CJD has been found in dairy workers, but this is no higher in the UK than in other European countries which have not suffered the BSE epidemic. None of the cases of nvCJD have any obvious link with occupational exposure to BSE.
The animal TSEs are:
- scrapie in sheep, goats and moufflon;
- bovine spongiform encephalopathy (BSE) in cattle;
- transmissible mink encephalopathy (TME) in farmed mink;
- chronic wasting disease (CWD) in deer species;
- feline spongiform encephalopathy (FSE) in domestic cat and captive exotic felines;
- spongiform encephalopathy in captive exotic ungulates.
BSE was first confirmed in Great Britain in 1986. Up to December 1997 it has affected about 170,000 cattle in the UK and much smaller numbers in native-born cattle in Belgium, France, Luxembourg, Netherlands, Portugal, Republic of Ireland and Switzerland. A few cases have occurred in these or other countries following export of live cattle from the UK. Affected animals become unsteady on their feet, lose weight and become nervous hence ‘mad cow disease’. The BSE epidemic is in sharp decline as a result of the ban on feeding ruminant derived protein to ruminants. All animals suspected of having BSE are compulsorily slaughtered and completely destroyed.
Scrapie occurs in sheep, goats and moufflon, and has been recognised for more than 250 years. Affected animals often scrape themselves against objects to alleviate itching, become unsteady on their feet and lose condition. It is endemic in flocks in many countries, but there is no evidence that it can be transmitted to humans.
A disease similar to scrapie, TME has also occurred sporadically in farmed mink. CWD in Rocky Mountain elk, mule deer and some other deer species is also considered to be similar to scrapie. Neither have been reported in the UK. TSE has been recognised also in small numbers of domestic cats and captive, exotic felines and ungulates, most of which were born in the UK.
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