Phenolized formalin fixation may obscure early histological changes of Creutzfeldt-Jakob disease

Prions: Who Should Worry about Them?

Prion diseases, also called transmissible spongiform encephalopathies (TSEs), are a family of neurodegenerative disorders affecting both humans and animals. They are caused by the accumulation of an abnormal form of a protein known as prion that results in neuronal death and a characteristic spongiform appearance of the brain tissue. Human prion diseases can be sporadic, acquired or hereditary. Acquired prion diseases have been linked to entering contaminated food into the human food chain, failure to completely disinfect or sterilize contaminated surgical instruments, patients receiving tissues and organs from infected donors, recipients of blood and other biological contaminated products, and potentially to cross infection in dental procedures. At present, there is unfortunately no efficient therapy that can be administered to clinically infected patients with prion diseases. Moreover, there are no simple diagnostic tests that can be used to show the agent of transmissible spongiform encephalopathy during the preclinical phase of the disease. Therefore, to prevent the spread of this emerging infectious agent it is necessary to implement several health control strategies and maintain surveillance for subclinical infections.

Preventing accidental transmission of human transmissible spongifom encephalopathies

The mechanism by which humans became infected with the BSE agent is discussed, and the matter of potential person-to person transmission of TSEs through contagion or medical procedures is considered. There is some discussion regarding the current evidence relating to whether or not the blood of humans infected with TSEs is infectious. Considerable emphasis is placed on the fact that TSE agents are known to be relatively resistant to decontamination by procedures that are effective with conventional micro-organisms, including (under worst-case conditions) the autoclaving procedures used to sterilise surgical instruments. Methods for providing additional re-assurance with regard to the safety of instruments are described. Safety in the pathology laboratory is discussed extensively because TSE agents are not inactivated by the usual processes used to fix tissues, and such laboratories will receive fixed tissues that are still highly infectious as far as TSE agents are concerned.

The effect of formic acid on BSE and scrapie infectivity in fixed and unfixed brain-tissue

Brain-tissue infected with scrapie-like agents remains infectious after histological fixation, and represents a source of occupational exposure. Infectivity titres in formol or paraformaldehyde-lysine-periodate (PLP)-fixed mouse-brains infected with the 301V strain of BSE agent were reduced by around six logs following treatment with formic acid. After PLP fixation and formic acid treatment, no infectivity was detectable in mouse-brain infected with the 87V strain of scrapie agent. Similarly-treated mouse-brain infected with the ME7 strain of scrapie agent showed a titre loss of approximately 5 logs. No infectivity was detectable in PLP-fixed, BSE-infected bovine brain after formic acid treatment, but this was an unreliable result. With unfixed brain homogenates, formic acid reduced the titre of ME7 by > or = 5.0 logs; technical problems limited the measured loss of BSE infectivity to > or = 1.0 logs. These studies confirm that formic acid treatment during fixation of brain-tissue significantly reduces the infectivity titres of scrapie-like agents, thus reducing the level of any occupational exposure.