Prion diseases, for which there are no known treatments or cures, are caused by misfolded proteins that cause the proteins around them to become misfolded as well.
A prion is a misfolded protein that causes the proteins around it to become misfolded as well. The proteins that misfold to give rise to prions are found natively in neurons, typically of the brain. The pathogenic, misfolded form of the protein can enter an animal through an infection or can result from a mutation in a protein-coding gene that causes the amino acid chain to fold incorrectly. Infectious prions induce the healthy, correctly folded forms of the same protein to refold into the pathogenic prion form, creating a chain reaction in which misfolded prions accumulate in neural cells. Transmissible spongiform encephalopathy (TSE) is a class of diseases caused by prions. In humans, TSEs are collectively known as Creutzfeldt-Jakob disease (CJD) and kuru. Worldwide, CJD occurs in about one person in a million. Kuru was once common in certain ethnic groups in New Guinea but was effectively contained in the 20th century. These diseases take other names in other animals, but all are variants of the same disease. Many people are familiar with the common name for the disease in cattle—mad cow disease—which is clinically called bovine spongiform encephalopathy. As spongiform encephalopathies progress, prion aggregations can break apart and seed additional transformation of benign proteins elsewhere in the brain. The scattered accumulations of prions destroy brain tissue, eventually resulting in holes in the neurologic tissue and giving rise to a spongelike pattern.
In addition to the various clinically visible symptoms, other indicators of prion diseases are an increase in the size of astrocytes (the cells that regulate the passage of materials between blood and neurons), an increase in levels of amyloid protein (associated with degenerative nerve disorders), and neuron loss. The word spongiform in the name of the diseases describes the sponge-like appearance of the brain tissue in affected individuals. Symptoms include difficulties in communication, personality changes, mood disorders such as depression, and lack of coordination. As the disease progresses, patients may experience confusion and memory problems, leading to dementia and loss of the ability to walk and speak. The disease is degenerative, meaning the patient will continue to get worse as the disease progresses. Many patients die from the disease or from complications arising from it, such as loss of the ability to eat or injuries suffered while attempting to walk. There is no current treatment or cure.
Prions are of great concern because they are not alive and thus cannot be killed. Moreover, unlike viruses, they contain no genetic material that can be rendered inactive. They are infectious simply by existing. The only way to render prions inert is to denature them—to alter their protein structure. This is very difficult to accomplish. Prions have proven resistant to sterilization by heat, irradiation, and formaldehyde treatments. Sterilization by sodium hypochlorite (bleach), sodium hydroxide, or very acidic detergent has been shown to be effective, however.
Chronic wasting disease, a form of TSE affecting elk and deer, may be transmitted more easily than other forms of the disease. It is unknown if this disease may spread to other animals, including humans.