A prion disease (also called transmissible spongiform encephalopathy) is a disease which is caused by prions. Prions are structurally altered versions of small proteins that are normally expressed in cells. Unlike diseases that are caused by a mutation of the gene resulting in the expression of a mutant protein, prions are able to replicate and transmit diseases through physical contact with the normal proteins resulting in a structural change from the normal state to the prion state.
Unlike bacteria, prions are not considered to be alive because they do not have their own metabolism, they do not possess genes and cannot naturally reproduce outside a host cell. Prion diseases are very rare, and no treatment is available for most of them.
Almost all of the known prion diseases are neurologic diseases. There are two common signs which are seen in typical prion diseases:
Few types of prion diseases are known. The most important ones are:
- Creutzfeldt-Jakob disease - A person can get the disease by eating of BSE-infected beef, having a blood tranfusion from a person with the disease, injection of Human Growth Hormone extracted from bodies with the disease, or through infected surgical instruments. Patients suffer from ataxia, dementia and usually die after one year.
- Kuru disease - This disease is seen in people living in New Guinea who eat the brains of dead people. Patients present with ataxia, dementia and inability in moving their eyes. They will usually die within two years.
- Gerstmann-Straussler-Scheinker syndrome - This is a very rare syndrome which presents with ataxia and dementia. The patients usually die after one year.
Prion diseases are very rare. With Creutzfeldt-Jakob disease, which makes up about 85% of the cases, there are about 1 to 1.5 cases per one million people per year.