Prion diseases are rare progressive, fatal, and currently untreatable degenerative disorders of the nervous system (primarily the brain) that result when a protein changes into an abnormal form called prion.
Before prions were identified, diseases such as Creutzfeldt-Jakob disease and other transmissible spongiform encephalopathies were thought to be caused by viruses. Prions are much smaller than viruses and differ from viruses, bacteria, and all living cells because they do not contain any genetic material.
In prion diseases, a normal protein called cellular prion protein (PrPC) changes shape (misfolds) and becomes abnormal. This abnormal prion protein is called scrapie prion protein (PrPSc), or prion. Scrapie refers to the prion disease first observed in sheep. Scrapie is so named because the sheep scrape themselves against trees, fence posts, or other structures and tear their wool off. The disease causes sheep to behave in other bizarre ways, and it is fatal.
Some of the newly formed prions resist being broken down by enzymes in the brain. Thus, they slowly accumulate. Prions also trigger other nearby PrPC to change into prions, and the process continues. When prions reach a certain number, disease results. Prions never change back into normal PrPC.
PrPC is present in all cells of the body but has a high concentration in the brain. Consequently, most prion diseases affect the nervous system predominantly or exclusively. The most common change caused by prions is the formation of tiny bubbles in brain cells, and the brain becomes filled with microscopic holes. When samples of brain tissue are viewed through the microscope, they somewhat resemble a sponge (hence, the term spongiform). After a period of time (which can vary), the affected cells cease to function and die.
Prion diseases may be
Sporadic: Occurring spontaneously, without any known reason (most common)
Familial: Occurring in families
Acquired: Acquired from contaminated material (uncommon)
Sporadic prion diseases
Sporadic prion diseases are by far the most common of all human prion diseases.
There are 3 types of sporadic prion disease:
Sporadic Creutzfeldt-Jakob disease is the most common type of the sporadic prion diseases. Worldwide, sporadic Creutzfeldt-Jakob disease occurs at a rate of about 1 or 2 new cases per million people each year.
In sporadic prion diseases, how the first prion forms is unknown. But it is unlikely to be due to a gene mutation.
Familial prion diseases
Familial prion diseases involve a mutation in the gene that controls production of PrPC, which can be inherited. The mutation makes PrPC more likely to change into prions. More than 50 mutations exist. Different mutations can cause different prion diseases. Familial prion diseases are almost always inherited as an autosomal dominant disorder. That means that the mutation is not on the sex (X or Y) chromosomes and that only 1 mutated gene for the disease, 1 from either parent, is required for the disease to develop.
There are several groups of familial prion diseases:
Another familial prion disease has been discovered that differs from other prion diseases because it causes diarrhea and affects nerves throughout the body years before symptoms of brain malfunction develop. It is described as prion disease associated with diarrhea and autonomic neuropathy.
Acquired prion diseases
Acquired prion diseases are rare and are not transmitted through casual person-to-person contact. They occur
When people eat beef from prion-infected cattle—as is the case in variant Creutzfeldt-Jakob disease (sometimes called the human version of mad cow disease)
When a prion-contaminated organ or tissue generally associated with the brain is transplanted or grafted
When a prion-contaminated substance (such as a hormone) that is generally associated with the brain is given by injection
When brain surgery is done with prion-contaminated instruments
Rarely, when people receive a prion-contaminated blood transfusion, though there have only been 4 documented cases to date and this is no longer thought to be a relevant risk.
Kuru is also an acquired prion disease. It used to occur in natives of Papua New Guinea who practiced ritual cannibalism.
There are no reports of prion disease having been spread through casual contact with people who have the disease.
Prion Diseases in Animals and Humans
Prion diseases occur in sheep, goats, cattle, elk, deer, minks, cats, and camels. Depending on the disease, they can be transmitted from one species to another when an animal does the following:
Like people with Creutzfeldt-Jakob disease, affected animals gradually become uncoordinated and behave in abnormal ways as brain function deteriorates. Scrapie, the prion disease in sheep, is so named because the sheep tend to scrape themselves against fence posts or other structures and tear their wool off. In elk and deer, prion disease is called chronic wasting disease. In most of the United States, South Korea, Canada, Norway, Finland, and Sweden, there is concern that chronic wasting disease may be transmitted to people who hunt, butcher, or eat affected animals. However, there have been no reports of chronic wasting disease or sheep scrapie causing prion disease in people. Researchers are studying whether chronic wasting disease can be transmitted to people. Mad cow disease (bovine spongiform encephalopathy) is so named because the cattle become noticeably agitated. The disease was likely initially transmitted from sheep to cattle by feeding cattle scrapie-infected sheep parts. Eating contaminated beef or beef products causes a form of Creutzfeldt-Jakob disease in a small percentage of people. First described in 1996, this form is called variant Creutzfeldt-Jakob disease (sometimes also called the human version of mad cow disease). Variant Creutzfeldt-Jakob disease differs in many ways from other forms of Creutzfeldt-Jakob disease:
Variant Creutzfeldt-Jakob disease was very rare, even at its peak. As of May 7, 2022, there have been 178 cases in the United Kingdom, and there have been 55 in other countries as of 2026. In the United Kingdom, the number of new cases occurring each year peaked in 2000. Since then, the number has steadily declined, with only 2 cases after 2011. Since then, the disease has been diagnosed in 4 people in the United States and 2 in Canada. All of them probably acquired the disease in a foreign country. |
Treatment of Prion Diseases
Symptom relief and comfort measures
There is no cure for prion diseases, which are all fatal, usually within months to a few years after symptoms appear. Treatment focuses on symptom relief and comfort measures.
A number of strategies can help caregivers of people with a prion disease cope with the dementia caused by the disease (see sidebar ).
If possible, people who have a prion disease should establish advance directives about what kind of medical care they want at the end of life. Hospice care is recommended.
Family members of people who develop the hereditary form of the disease may benefit from genetic counseling.
Prevention of Prion Diseases
People should not eat the meat of animals that are known or suspected to be infected with prion disease. Prion disease screening for chronic wasting disease in deer and elk may be offered on a state-by-state basis.
Health care professionals can prevent the transmission of prion diseases in medical and surgical facilities by using appropriate techniques to clean contaminated instruments and protecting those who handle contaminated tissues or instruments.
