CJD is caused by an infectious protein in the brain called a prion.
Role of proteins in the brain
Proteins are molecules, made up of amino acids, which help the cells in our body to function.
Proteins begin as a string of amino acids which then fold themselves into a three-dimensional shape. This 'protein folding' allows them to perform useful functions within our cells.
Prion proteins (which are not the same as the infectious prions that cause CJD) are a type of protein found in the brain and several other tissues of the nervous system. The exact role of prion proteins in our brain is unknown, but it is thought they may have something to do with our long-term memory.
When protein folding goes wrong
Sometimes, mistakes happen during protein folding and the prion protein cannot be used by the body. These misfolded prion proteins are normally recycled by the body, but sometimes they can build up. This can cause problems, such as Alzheimer's disease.
Prions are misfolded prion proteins that enter brain cells and cause normal proteins to misfold as well. This causes the brain cell to die, releasing more prions to infect other brain cells.
Eventually, clusters of brain cells are killed and replaced with deposits of prions, called plaques. These plaques produce small holes in the brain, causing it to become sponge-like. The damage to the brain causes the mental and physical impairment and eventual death associated with CJD.
Prions can survive in nerve tissue, such as the brain or spinal cord, for a very long time, even after the person or animal has died.
Sporadic CJD (sCJD) is the most common type of CJD, although it is still very rare. It is not known what triggers sCJD, but it may be due to a normal protein spontaneously changing into a prion or a normal gene spontaneously changing into a faulty gene that produces prions. sCJD normally affects people over 40 years of age.
There is clear evidence that variant CJD (vCJD) is caused by the same strain of prions that causes bovine spongiform encephalopathy (BSE, also known as mad cow disease).
A government inquiry in 2000 concluded that the prion was spread through cattle that were fed meat-and-bone mix containing traces of infected brains or spinal cords. The prion then ended up in processed meat products, such as beefburgers, and entered the human food chain. Strict controls have been in place since 1996 to prevent BSE from entering the human food chain and the use of meat-and-bone mix has since been outlawed.
Cases of vCJD peaked in the year 2000, in which there were 28 deaths from vCJD. There were only two confirmed deaths in 2009. Some experts believe that the food controls have worked and that further cases of vCJD will continue to decline.
Other experts have warned that people who have died could have had a genetic trait that meant the vCJD affected them more quickly than normal. Other similar infections caused by prions normally take between 15 and 20 years before they become active. These experts argue that many other people could have vCJD, but the symptoms might not begin to show for many years to come.
There have been four cases of variant CJD infection associated with blood transfusion (see HPA: variant CJD and blood for details). As a result, people who have received a blood transfusion in the UK since 1980 are no longer able to give blood.
Iatrogenic CJD (iCJD) is the result of the infection being spread from someone with CJD through medical or surgical treatment.
Most iatrogenic CJD cases have happened through the use of human growth hormone, which is used to treat children who have restricted growth. Between 1958 and 1985, thousands of children were treated with the hormone which, at the time, was extracted from the pituitary glands (a gland that sits at the base of the skull) of human corpses. A tiny minority of those children developed CJD, as the hormones they received were taken from glands infected with CJD. Since 1985, all human growth hormone in the UK has been artificially manufactured, so there is now no risk.
A few other cases of iCJD occurred when people received transplants of infected tissue or came into contact with surgical instruments that were contaminated with CJD. This happened because prions are tougher than viruses or bacteria, so the normal process of sterilising surgical instruments had no effect.
Once the risk was recognised, the Department of Health tightened the guidelines on organ donation and the reuse of surgical equipment. As a result, cases of iCJD are now extremely rare.
Inherited prion disease
This very rare form of CJD is caused by an inherited mutation (fault) of the gene that produces normal proteins. The altered gene seems to produce prions that cause CJD.
Everyone has two copies of their genes, but the mutated gene is dominant. This means you only need to inherit one mutated gene to develop the illness.