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Overview of Prion Diseases

(Transmissible Spongiform Encephalopathies)

By Pierluigi Gambetti, MD

Prion diseases are rare progressive, fatal, and currently untreatable degenerative disorders of the brain (and rarely of other organs) that result when a protein changes into an abnormal form called prion.

Before prions were identified, diseases such as Creutzfeldt-Jakob disease and other 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.

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 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. Gradually, the affected cells die, and the brain becomes filled with microscopic holes. When samples of brain tissue are viewed through a microscope, they somewhat resemble Swiss cheese or a sponge (hence, the term spongiform).

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 the most common of all human prion diseases, accounting for 85 to 90% of all cases.

There are three 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 in about 1 in a million people each year.

Familial prion diseases

Familial prion diseases involve a mutation in the gene for PrPC, which can be inherited. The mutation makes PrPC more likely to change into prions. More than 50 mutations exist. Each mutation typically causes a different prion disease. There are three main groups of familial prion diseases:

Another familial prion disease has been recently discovered. It 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. 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 is transplanted

  • When a prion-contaminated substance (such as a hormone) is given by injection

  • When brain surgery is done with prion-contaminated instruments

  • Rarely, when people receive a prion-contaminated blood transfusion

Kuru is also an acquired prion disease. It used to occur in natives of Papua New Guinea who practiced ritual cannibalism.

Treatment

  • 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 Creating a Beneficial Environment for People With Dementia).

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.

Family members of people who develop the hereditary form of the disease may benefit from genetic counseling.

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