TSEs, or prion diseases, are infectious neurodegenerative diseases of mammals that include bovine spongiform encephalopathy,  chronic wasting disease of deer and elk, scrapie in sheep, and Creutzfeldt–Jakob disease (CJD) in humans. TSEs may be passed from host to host by ingestion of infected tissues or blood transfusions. Clinical symptoms of TSEs include loss of movement and coordination and dementia in humans. They have incubation periods of months to years, but after the appearance of clinical signs, they progress rapidly, are untreatable and invariably are fatal. Attempts at TSE risk-reduction have led to significant changes in the production and trade of agricultural goods, medicines, cosmetics, blood and tissue donations, and biotechnology products. Post mortem neuropathological examination of brain tissue from an animal or human has remained the ‘gold standard’ of TSE diagnosis and is very specific, but not as sensitive as other techniques.
To improve food safety, it would be  levels are very low in presymptomatic hosts. In addition, PrPs are generally unevenly distributed in body tissues, with highest concentration consistently found in nervous system tissues and very low concentrations in easily accessible body fluids such as blood or urine. Therefore, any such test would be required to detect extremely small amounts of PrP and would have to differentiate PrP and PrP.
Current PrP detection methods are time-consuming and employ post mortem analysis after suspicious animals manifest one or more symptoms of the disease. Current diagnostic methods are based mainly on detection of physiochemical differences between PrP and PrP which, to date, are the only reliable markers for TSEs. For example, the most widely used diagnostic tests exploit the relative protease resistance of PrP in brain samples to discriminate between PrP and PrP, in combination with antibody-based detection of the PK-resistant portion of PrP. It has as yet not been possible to detect prion diseases by using conventional methods, such as polymerase chain reaction, serology, or cell culture assays. An agent-specific nucleic acid has not yet been identified, and the infected host does not elicit an antibody response.
The conformationally altered form of PrP is PrP. Some groups believe PrP is the infectious agent (prion agent) in TSEs, while other groups do not. PrP could be a neuropathological product of the disease process, a component of the infectious agent, the infectious agent itself, or something else altogether. Regardless of what its actual function in the disease state is, PrP is clearly specifically associated with the disease process, and detection of it indicates infection with the agent causing prion diseases.