Transmissible spongiform encephalopathies (TSEs), also known as prion diseases, are a group of progressive, incurable, and invariably fatal conditions that are associated with the degeneration of the nervous system in many animals, including humans, cattle, and sheep. Strong evidence supports the once unorthodox hypothesis that prion diseases are transmitted by abnormally shaped protein molecules known as prions. Prions consist of a protein called the prion protein (PrP). Misshapen PrP (often referred to as PrP) conveys its abnormal structure to naive PrP molecules by a crystallization-like seeding process. Because the abnormal proteins stick to each other, and because PrP is continuously produced by cells, PrP accumulates in the brain, harming neurons and eventually causing clinical disease.
Prion diseases are marked by mental and physical deterioration that worsens over time. A defining pathologic characteristic of prion diseases is the appearance of small vacuoles in various parts of the central nervous system that create a sponge-like appearance when brain tissue obtained at autopsy is examined under a microscope. Other changes in affected regions include the buildup of PrP, gliosis, and the loss of neurons.
