Infection Control Guidelines for TSEs in Hospitals and Home-Care Settings

Maura N. Ricketts

World Health Organization, Geneva, Switzerland

BACKGROUND

Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are fatal degenerative brain diseases that occur in humans and certain animal species. They are characterized by microscopic vacuoles and the deposition of amyloid (prion) protein in the grey matter of the brain. All forms of TSE are experimentally transmissible.

TSE agents exhibit an unusual resistance to conventional chemical and physical decontamination methods. They are not adequately inactivated by most common disinfectants, or by most tissue fixatives, and some infectivity may persist under standard hospital or healthcare facility autoclaving conditions (e.g. 121°C for 15 minutes). They are also extremely resistant to high doses of ionizing and ultra-violet irradiation and some residual activity has been shown to survive for long periods in the environment. The unconventional nature of these agents, together with the appearance in the United Kingdom, Republic of Ireland and France of a new variant of CJD (vCJD) since the mid- 1990s, has stimulated interest in an updated guidance on safe practices for patient care and infection control. The World Health Organization guideline on the prevention of iatrogenic and nosocomial exposure to TSE agents was prepared following the WHO Consultation on Caring for Patients and Hospital Infection Control in Relation to Human Transmissible Spongiform Encephalopathies, held in Geneva from 24 to 26 March 1999. The meeting was chaired by Dr Paul Brown. Dr Martin Zeidler and Dr Maurizio Pocchiari kindly agreed to be Rapporteurs.

WHO Infection Control Guidelines for Transmissible Spongiform Encephalopathies (pdf, 194kb)

HAZARD IDENTIFICATION AND RISK REDUCTION

TSEs are not known to spread by contact from person to person, but transmission can occur during invasive medical interventions. Exposure to infectious material through the use of human cadaveric-derived pituitary hormones, dural and cornea homografts, and contaminated neurosurgical instruments have caused human TSEs.

When considering measures to prevent the transmission of TSE from patients to other individuals (patients, healthcare workers, or other care providers), it is important to base the measures upon the known and limited ways in which TSEs are transmitted between humans. Risk is dependent upon three considerations:

• the probability that an individual has or will develop TSE;
• the level of infectivity in tissues or fluids of these individuals;
• the nature or route of the exposure to these tissues.

From these considerations it is possible to make decisions about whether any special precautions are needed.

Is the patient at risk of TSE?

Is a high or low risk tissue involved?

Route of exposure to infectivity?

Specific precautions

 EVALUATING RISK IN PATIENT POPULATIONS

Where a clinician diagnoses or suspects the diagnosis of CJD or another TSE, the patient must be considered to be a risk for transmitting infection. Where a person has a history of exposure to dura mater, cornea or human pituitary hormones, they are considered to be ‘at risk’ for transmitting infection, however the risk is dependant upon the risk level of the tissues, as discussed next. Where persons have a family history of TSEs, or carry a genetic marker of inheritable TSEs, without any clinical signs of TSEs, it proved impossible to be infinitive – alternative approaches can be taken, also discussed in the following section. Finally, vCJD was not the subject of the consultation; the published guideline discusses this issue in an appendix.

EVALUATING RISK OF TISSUES

Infectivity levels in tissue were classified in one of three categories – high, low or no detectable infectivity - as per Table 1, below.

Table 1:   Distribution of infectivity in the human body¹

EVALUATING RISK OF PROCEDURES

Not all clinical or medical procedures carry a risk of transmission of TSE. Routine patient activities do not require any sort of special precautions for TSEs. However, neurosurgical and ocular surgical procedures are the highest risk procedures and it is essential that appropriate precautions be taken for any person known, suspect or at risk of a TSE if they undergo such procedures. If a person is at risk for familial TSEs, as was noted earlier, there was no consensus on whether precautions should be taken for neurosurgical or ocular surgical procedures. Regarding other surgical procedures, special precautions are recommended for persons known or suspected of TSEs, however, it was acknowledged that the less rigorous methods could be used and that special precautions were unnecessary for persons at risk. Regarding dental procedures, no consensus was reached, however some potential interventions can be taken and are described in the full document.

No special precautions are needed for routine laboratory procedures, with the exception of where CSF is being handled. Similarly, when high and low infectivity tissues are being examined (i.e. pathology), special handling is required. It is best if high risk tissues are handled only in specialized facilities with experience, training and specialized equipment

Autopsy requires special precautions, however can be conducted without undue risk. Mortuary handling of bodies may require alterations if the brain pan is open or if an autopsy was conducted, otherwise the body is handled as per routine procedures.

SPECIFIC PRECAUTIONS FOR TSEs

The safest and most unambiguous method for ensuring that there is no risk of residual infectivity on contaminated materials is to discard and destroy them by incineration. While this strategy should be universally applied to those devices and materials that are designed to be disposable, it was also recognized that this may not be feasible for many devices and materials that were not designed for single use. For these situations, the autoclave/chemical methods recommended below appear to remove most and possibly all infectivity under the widest range of conditions. Incineration remains the most suitable method for disposing of all waste and of contaminated tissues.

Those surgical instruments that are going to be re-used may be mechanically cleaned in advance of subjecting them to decontamination. Mechanical cleaning will reduce the bio-load and protect the instrument from damage caused by adherent tissues. If instruments are cleaned before decontamination, the cleaning materials must be treated as infectious waste, and the cleaning station must be decontaminated by one of the methods listed below. The instruments are then treated by one of the decontamination methods recommended below before reintroduction into the general instrument sterilization processes.

Incineration

• Use for all disposable instruments, materials, and wastes
• Preferred method for all instruments exposed to high infectivity tissues

Autoclave/chemical methods for heat-resistant instruments

1. Immerse in sodium hydroxide (NaOH)³ and heat in a gravity displacement autoclave at 121°C for 30 min; clean; rinse in water and subject to routine sterilization.

2. Immerse in NaOH or sodium hypochlorite⁴ for 1 hr; transfer instruments to water; heat in a gravity displacement autoclave at 121°C for 1 hr; clean and subject to routine sterilization.

3. Immerse in NaOH or sodium hypochlorite for 1 hr.; remove and rinse in water, then transfer to open pan and heat in a gravity displacement (121°C) or porous load (134°C) autoclave for 1 hr.; clean and subject to routine sterilization.

4. Immerse in NaOH and boil for 10 min at atmospheric pressure; clean, rinse in water and subject to routine sterilization.

5. Immerse in sodium hypochlorite (preferred) or NaOH (alternative) at ambient temperature for 1 hr; clean; rinse in water and subject to routine sterilization. Autoclave at 134°C for 18 minutes.

6. Autoclave at 134°C for 18 minutes.⁵

Chemical methods for surfaces and heat sensitive instruments

1. Flood with 2N NaOH or undiluted sodium hypochlorite; let stand for 1 hr.; mop up and rinse with water.

2. Where surfaces cannot tolerate NaOH or hypochlorite, thorough cleaning
   will remove most infectivity by dilution and some additional benefit may be derived from the use of one or another of the partially effective methods listed in the guideline.

Autoclave/chemical methods for dry goods

1. Small dry goods that can withstand either NaOH or sodium hypochlorite should first be immersed in one or the other solution (as described above) and then heated in a porous load autoclave at ≥ 121°C for 1 hr.

2. Bulky dry goods or dry goods of any size that cannot withstand exposure to NaOH or sodium hypochlorite should be heated in a porous load autoclave
   at 134°C for 1 hr.

SUMMARY

The principal recommendations of the guideline are contained in the following table.

Table 2:   Decontamination levels for different risk categories

Footnotes

1. Assignment of different organs and tissues to categories of high and low infectivity is chiefly based upon the frequency with which infectivity has been detectable, rather than upon quantitative assays of the level of infectivity, for which data are incomplete. Experimental data include primates inoculated with tissues from human cases of CJD, but have been supplemented in some categories by data obtained from naturally occurring animal TSEs. Actual infectivity titres in the various human tissues other than the brain are extremely limited, but data from experimentally-infected animals generally corroborate the grouping shown in the table.

2. Experimental results investigating the infectivity of blood have been conflicting, however even when infectivity has been detectable, it is present in very low amounts and there are no known transfusion transmissions of CJD.

3. Unless otherwise noted, the recommended concentration is 1N NaOH.

4. Unless otherwise noted, the recommended concentration is 20 000 ppm available chlorine.

5. In worse-case scenarios (brain tissue bake-dried on to surfaces) infectivity will be largely but not completely removed.

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