Transmissible spongiform encephalopathies (TSE) are a group of rare, fatal, degenerative brain diseases believed to be caused by proteins called prions that become infectious. These diseases, which are also called prion diseases, typically have long incubation periods. Many years and even decades may pass between the time of infection and the first signs and symptoms of the disease. According to the National Institute of Neurological Disorders and Stroke, this incubation period may even reach as many as 40 years. However, once symptoms do occur, the diseases typically progress quickly, resulting in death after a span of months to a few years.

Proteins are long chains of amino acids that are folded into particular shapes. These shapes allow the proteins to perform their functions in the body. Prions, which are also called proteinaceous infectious particles, are a certain type of harmless protein – they are not viruses, bacteria or other cellular organisms. In their normal state, prions are noninfectious. The purpose of prions is not known, but they are present in a safe form on the membranes of many different types of cells, including brain cells. They may play a role in cell-to-cell communication or transporting minerals into cells.

However, prions can misfold, or fold into abnormal shapes and become infectious. When these infectious prions come into contact with other proteins, they cause them to misfold and become infectious, as well. Eventually, enough abnormal prions are present that symptoms of disease occur. It is believed that abnormal prions kill nerve tissues (e.g., brain cells) that they contact. This can cause tiny holes to form in the brain, giving it the appearance of a sponge when viewed under a microscope. Hence the name, “spongiform (sponge-like) encephalopathy (brain disease).”

Infectious prions are unusual because, unlike viruses or bacteria, they do not appear to have any genetic material (e.g., DNA, RNA). They are also extremely difficult to destroy and can survive many methods normally used to kill harmful bacteria and viruses (e.g., antibiotic medications and sterilization procedures such as boiling or irradiation).

The theory that infectious prions alone are responsible for TSEs is widely accepted, but it has not been definitively proven. One alternate theory claims that a slow-acting virus is the infectious agent responsible for TSE, and that the abnormal prions are a result of the infection, not the cause. However, no actual virus, bacterium or other cellular organism that may cause the disease has been identified.

According to the National Institutes of Health, TSE occurs in about one person per million each year, worldwide. In the United States, there are about 300 cases per year.