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Causes
By Mayo Clinic staff
Creutzfeldt-Jakob disease and its variants belong to a broad group of human and animal diseases known as transmissible spongiform encephalopathies (TSEs). The name derives from the spongy holes, visible under a microscope, that develop in affected brain tissue.
The cause of Creutzfeldt-Jakob disease and other TSEs appears to be abnormal versions of a kind of protein called a prion.
The nature of prions
Prions are proteins that occur naturally in the brains of animals and people. Normally, these proteins are harmless, but when they're misshapen they can wreak havoc on normal biological processes.
All proteins start out as loose strings of amino acids. But proteins can't perform their intended function until the amino acids fold into a specific three-dimensional shape. The shape a particular protein assumes is determined by the sequence of its amino acids. Most proteins fold spontaneously during or just after they're synthesized inside cells.
But some prions don't fold as intended. Not only do they misfold, but also they may enter brain cells and force normal proteins to misfold as well. When the infected cells die, the defective prions are released into normal tissue and go on to infect more cells. Eventually, large clusters of cells die, leaving the brain riddled with holes. This is a prolonged process, and symptoms of disease may not appear for years.
Defective prions also seem capable of crossing species barriers, jumping, for example, from sheep to cows to humans. Although transmission may occur more readily when proteins have the same amino acid sequence and exist within a single species, evidence is strong that prions can move from one species to another.
How CJD is transmitted
Researchers have identified three basic ways that CJD may develop:
- Spontaneously. Most people with classic CJD develop the disease for no apparent reason. CJD that occurs without explanation is termed spontaneous CJD or sporadic CJD and accounts for the great majority of all cases.
- By genetic mutation. In the United States, about 5 percent to 10 percent of people with CJD have a family history of the disease or test positive for a genetic mutation associated with CJD. This type is referred to as familial CJD.
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By contamination. The risk of being contaminated with CJD-related prions is low. CJD can't be transmitted through airborne droplets of fluid (coughing or sneezing), casual contact, such as touching, or even through sexual contact.
A small number of people have developed the disease after being exposed to infected human tissue during a medical procedure, such as a cornea or skin transplant, or grafts of dura mater, the membrane that covers the brain. Some adults have developed CJD decades after receiving injections of contaminated human growth hormone (HGH) derived from human pituitary glands. Since 1985, HGH in the U.S. is made from genetically engineered synthetic materials, avoiding the risk of contaminated tissue. Cases of CJD related to medical procedures are referred to as iatrogenic CJD.
Misshapen prions aren't affected by standard sterilization methods, including heat, radiation, alcohol, benzene and formaldehyde. As a result, a few people have developed CJD after undergoing brain surgery with instruments contaminated by previous use in a person with CJD. Now, instruments used during brain surgery on a person with suspected CJD are destroyed.
Variant CJD has been linked primarily to the consumption of beef infected with bovine spongiform encephalopathy (BSE), the medical term for mad cow disease. Not everyone who becomes infected with the abnormal prion protein develops vCJD.
To date, four cases of vCJD have occurred in the United Kingdom as a result of blood transfusions. All four cases involved transfusions received in the 1990s.
