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Spongiform Encephalopathies

 

Creutzfeldt-Jacob Disease is a transmissible spongiform encephalopathy (TME). In recent years, there has been an increase in iatrogenic cases through injections of human cadaver derived growth hormone, cornea grafts, dura mater grafts for eardrums, and contaminated neurosurgical instruments. TME agents are highly resistant to heat, radiation and chemical inactivation. The sterilization and disinfection procedures for TME agents are far more stringent than those usually applied in hospital practice for the control of conventional bacterial, viral, and other infectious disease agents. Contaminated reusable instruments should be sterilized at 134-138oC for not less than 18 minutes (hold time) or for six cycles of 3 minutes. It is thought that the downward displacement autoclave which is commonly used in laboratory work may be less effective even at these high temperatures than the surgical sterilizer operating with a pulsed vacuum cycle. Scrapie-infected tissue stored in 10% formalin has been shown to be still infectious. The application of sodium hypochlorite containing 20000 ppm appears to be effective when used in decontaminating surfaces. Phenolic disinfectants and B -propiolactone are regarded as unreliable. There is practical difficulty in disinfecting safety cabinets of TME agents because of their resistance to formaldehyde and other fumigants.

To date, there are no confirmed cases of laboratory-acquired infection. However, the long incubation period makes the cause and effect difficult to relate and two recently reported cases of CJD in ex-laboratory workers who had handled neural tissues extensively are a matter of concern. Therefore, particular caution is advised in dealing with tissue from the CNS, especially from suspected cases of CJD and GSS. Equipment used in the laboratory and post-mortem room should be subjected to stringent disinfection procedures. Work practices should be reviewed to avoid puncture wounds and cuts. Eye protection and gloves should be worn. The skull of a CJD or GSS patients should only opened in a bag. At present, level 2 containment is advised for TME agents with additional measures to guard against puncture wounds and cuts and contamination of broken skin and eyes.

Bovine Spongiform Encephalopathy

A new variant of CJD was described in the UK in 1996 characterized by a much younger age of onset (mean of 26 years as opposed to 65 years for the classical disease), a longer duration of illness, and a distinct histopathological pattern for which the only common risk factor was the consumption of beef. A WHO Consultation was carried out in April 1996 which reviewed the public health issues related to BSE and the emergence of the new variant of CJD. The group concluded that there is no definite link between BSE and the variant CJD, but the circumstantial evidence suggests that exposure to BSE is the most likely hypothesis. The recommendations made by the group was as follows;-

  1. No part of product of any animal which has shown signs of transmissible spongiform encephalopathy should enter any food, whether human or animal.
  2. All countries should establish continuous surveillance and compulsory notification for BSE.
  3. Tissues that are likely to contain the BSE agent (specified bovine offals) such as brain, spinal cord, tonsil, spleen, thymus, and intestines should not be permitted to enter any food chain, whether human or animal.
  4. The use of ruminant tissues in ruminant feed should be prohibited.
  5. Milk and milk products are considered to be safe. Gelatin and tallow are considered to be safe provided that effective rendering procedures are in place.
  6. Manufacturers of medicinal products containing bovine material should obtain the material only from countries effective surveillance and control programmes.    

The group also recommends increased surveillance and research on BSE and CJD worldwide. There is concern that specific occupational groups such as abattoir workers, butchers and vets have a higher risk of exposure to TME agents. Specific recommendations include;-

  1. Taking care to avoid injuries especially with instruments which have been in contact with specified bovine offals.
  2. Covering open cuts and grazes with waterproof dressings
  3. Wearing suitable protective equipment and clothing
  4. Minimizing direct contact with specified bovine offals  

The infectivity of the BSE agent is substantially but not completely reduced when subjected to porous load autoclave at 134-138oC for up to 60 minutes. There is no detectable infectivity after exposure to hypochlorite 16500 ppm for 120 minutes

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Isolation of Patients