The most common prion disease is scrapie, found in sheep and goats.  Victims of scrapie tend to become irritable and develop an intense itch, causing them to scrape off their wool or hair (leading to the term "scrapie").  As a result of neurodegeneration, sheep infected with scrapie also exhibit a gradual loss of muscular coordination to the point they can no longer stand.  A closely related disease is BSE or Mad Cow Disease, in which cows become increasingly uncoordinated and apprehensive.  Post mortem examination of afflicted bovine reveals large holes in the brain, similar to those found in sheep afflicted with scrapie.  An outbreak of BSE occurred in Great Britain in the late 1980s, which was linked to a food supplement that included meat and bone meal from dead sheep.  As a result, the British government banned the use of animal-derived feed supplements to prevent any further outbreaks.

     In humans, several prion diseases have been positively identified, including Kuru, Creutzfeld-Jacob Disease (CJD), Gerstmann-Straussler-Scheinker Syndrome(GSC), Fatal Familial Insomnia(FFI) and Alpers Syndrome.  Each is characterized by gradual loss of motor control, dementia, paralysis, and eventual death (typically following pneumonia).  In post mortem examinations, people afflicted with prion diseases exhibit non-inflammatory lesions, vacuoles in brain tissue, amyloid protein deposits and astrogliosis.  Unlike viral or bacterial infections, people afflicted with prion diseases exhibit any sign of inflammation or fever, indicating the immune system is not activated by prion infection.  Each of these ailments has similar symtoms to animal prion diseases and each other, particularly the spongiform appearance of the brain, suggesting they are "closely related".

     Kuru is the most isolated human prion disease, limited to the Fore people of Papua New Guinea.  It was first described in 1957 by Vincent Zigas and D. Carleton Gaidusek who noted that many people in this geographically isolated tribe became afflicted by a fatal disease marked by the loss of muscular coordinaion and dementia.  Kuru has a slow onset period and is characterized by cerbral difficulties, shaking ataxia, congestion of the blood vessels and cortical atrophy, leading to death within 18-24 months.  The cause of the strange disease was traced to ritual cannibalism within the tribe (the Fore people honored their dead by eating a soupprepared from the deceased's brain).  Since then, the practice has ceased and kuru has rapidly declined, almost to the point of extinction.  Interestingly, none of the other tribes in the region who practice the same ritual are afflicted with kuru, leading researchers to believe that at some point in the tribe's history, one person developed a prion disease which was passed on to the tribe when the afflicted person's brain was consumed.

     A similar disease is Creutzfeld-Jacob Disease(CJD), which occurs worldwide in people around age 60.  Approximately 1 per million people become afflicted with CJD, although an estimated 1 in 10,000 people are infected at the time of death (thesenumbers are approximations because CJD maybe confused with other neurological disorders).  Approximately 10 to 15 percent of the cases result from inheritance, while a small number of cases occur via intragonic infection (ie, the disease is passed on directly through corneal transplantation, contaminated surgical instruments, and injection of growth hormone derived from human pituitaries).  The remaining cases occur from sporadic infection.  The most consistent sign of CJD is the spongiform appearance of the brain due to vacuoles arising from the neuronal cytoplasm which range from 20 to 200 microns in diameter.  This photograph shows the spongiform appearance of brain of a CJD victim.  Note the large vacuoles which occur both singly and in groups.

    Gerstmann-Straussler-Scheinker Syndrome(GSS) occurs earlier than CJD, typically in the 4th to 5th decade.  The outward symptoms are slightly different than CJD:  patients exhibit cerebral ataxia compounded with motor problems.  Unlike CJD, dememtia is less common and victims can survive several years before death.  Other human prion diseases include Fatal Familial Insomnia (characterized by severe atrophy of the thalamus) and Alpers Syndrome (a prion disease in infants).

     Although degeneration of the central nervous system is the primary characteristic of prion diseases, it is crucial to note that "spongiosis can result from any neurological disorder which results in widespread neuronal death and collapse of the cerebral cortical architecture".  For example, neural vacuoles are also common in patients afflicted with Pick's disease, Alzheimer's and diffuse Lewy body disease, none of which are currently believed to be caused by prions.  Specific sites of degradation within the brain help distinguish prion disease from other neurodegenerative diseases.  For this reason, the terminology has been changed in recent years from "spongiform encephalopathy" to "prion diseases" to indicate the distinction.