Prions and Prion Disease

Prion diseases, also known as transmissible spongiform encephalopathies (TSEs) are a group of progressive neurodegenerative conditions.  These illnesses exist in both animals and humans.  Scrapie, a disease affecting sheep and goats, was the first prion disease to be identified in the 1730s.  In more recent years other prion diseases have been seen in animals, including bovine spongiform encephalopathy (BSE) in cattle and chronic wasting disease (CWD) in deer.  Various forms of the disease have been identified since Creutzfeldt and Jakob first described the illness later known as CJD (Creutzfeldt-Jakob disease) in the 1920s.  These diseases in humans are now grouped together according to whether they are sporadic, inherited, or acquired.

Most cases of prion disease are sporadic; that is, they arise spontaneously for no known reason. More rarely prion disease is inherited due to a faulty gene, or acquired by medical procedures, transfusions, or contaminated food.  Sporadic and inherited prion disease occurs worldwide in all populations.  The incidence of sporadic CJD is around 1-2 per million of the population per annum resulting in a lifetime risk of 1 in 5000; males and females are equally affected.  The incidence of the various acquired prion diseases, however, is more localised to specific groups and populations.

What Causes Prion Disease?

Prion diseases are associated with the build up in the brain (and some other organs) of an abnormal or ‘rogue’ form of a naturally occurring cellular protein, known as the prion protein. The rogue protein results from a change in shape of the normal prion protein.  Once formed in the body these rogue proteins recruit and convert more of the normal prion protein into the abnormal form, setting off a kind of chain reaction which leads to a progressive accumulation of the rogue protein.  In the normal course of events, once they have served their purpose, prion proteins are broken down by enzymes in the body.  The abnormal prions however are more resistant to this process; so they accumulate and cause damage in the brain, which interferes with normal brain functioning.  All forms of the disease are thought to be associated with an incubation period.  This is a clinically 'silent phase' during which replication of the rogue protein is thought to be taking place.

The function of the normal form of the protein remains unclear, though it is thought they may possibly play a role in the transport of messages between specific brain cells (synaptic transmission), or the maintenance of cables around nerves.

Genetic Susceptibility

At present the most important and well defined genetic factor which influences the susceptibility of an individual to developing prion disease relates to a common variation in the prion protein gene itself.  At a particular position in the prion gene known as codon 129, there are two possible genetic types, which in turn specify the body to produce different amino acids at this position.  These amino acids are called methionine and valine, or M and V for short.  In most countries, MM and MV frequencies in the population are roughly equal (40-50%).  It has been known for some years that individuals, who are MV, are at much less risk of developing prion disease than are MM or VV individuals.  Nearly all definite cases of vCJD have been MM.  One definite patient and one suspected patient who did not have a post mortem, were MV.

Prion Disease and Infection Control

Prion disease is not contagious; there is no evidence to suggest it can be spread from person to person by close contact.  Once a person has developed prion disease, central nervous system tissues (brain, spinal cord and eye tissue) are thought to be extremely infectious.  However this is only relevant for those handling infected tissue directly, which does not include carers looking after a person with the disease.

Infectivity in the rest of the body varies in different types of prion disease but is generally much less than in brain tissue.  People with any form of prion disease are requested not to be blood or organ donors, and are requested to inform their doctor and dentist prior to any invasive medical procedures or dentistry.

As prions cannot be completely destroyed by conventional sterilisation procedures, transmission has also occurred inadvertently through the use of surgical instruments previously used during neurosurgery on a person with sporadic prion disease. Current Department of Health guidelines are that all surgical instruments used on medium or high infectivity tissues in a patient with suspected prion disease are quarantined and not re-used unless an alternative diagnosis is confirmed. Instruments used on patients with known prion disease are not reused.

Guidelines about infection control can be found at the Advisory Committee for Dangerous Pathogens website.