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Transmissible spongiform encephalopathies in both humans and animals. Scrapie is the most common form in animals, while in humans the most prevalent form is Creutzfeldt-Jakob disease. This group of disorders is characterized at a neuropathological level by vacuolation of the brain's gray matter (spongiform change). They were initially considered to be examples of slow virus infections. Experimental work has consistently failed to demonstrate detectable nucleic acids—both ribonucleic acid (RNA) and deoxyribonucleic acid (DNA)—as constituting part of the infectious agent. Contemporary understanding suggests that the infectious particles are composed predominantly, or perhaps even solely, of protein, and from this concept was derived the acronym prion (proteinaceous infectious particles). Also of interest is the apparent paradox of how these disorders can be simultaneously infectious and yet inherited in an autosomal dominant fashion (from a gene on a chromosome other than a sex chromosome).
Scrapie, which occurs naturally in sheep and goats, was the first of the spongiform encephalopathies to be described. An increasing range of animal species have been recognized as occasional natural hosts of this type of disease. Bovine spongioform encephalopathy, commonly known as mad cow disease, has been epidemic in British cattle. The first confirmed cases were reported in late 1986. By early 1995 it had been identified in almost 150,000 cattle and more than half of all British herds. Its exact origin is not known, but claims that it came from sheep are now discredited.
So far, animal models have indicated that only central nervous system tissue has been shown to transmit the disease after oral ingestion—a diverse range of other organs, including udder, skeletal muscle, lymph nodes, liver, and buffy coat of blood (white blood cells) proving noninfectious.
The currently recognized spectrum of human disorders encompasses kuru, Creutzfeldt-Jakob disease, Gerstmann-Straussler-Scheinker disease, and fatal familial insomnia. All, including familial cases, have been shown to be transmissible to animals and hence potentially infectious; all are invariably fatal with no effective treatments currently available.
A variety of mechanisms of human-to-human transmission have been described. Transmission is due in part to the ineffectiveness of conventional sterilization and disinfection procedures to control the infectivity of transmissible spongiform encephalopathies. Numerically, pituitary hormone–related Creutzfeldt-Jakob disease is the most important form of human-to-human transmission of disease. However, epidemiological evidence suggests that there is no increased risk of contracting Creutzfeldt-Jakob disease from exposure in the form of close personal contact during domestic and occupational activities. Incubation periods in cases involving human-to-human transmission appear to vary enormously, depending upon the mechanism of inoculation. Current evidence suggests that transmission of Creutzfeldt-Jakob disease from mother to child does not occur. Two important factors pertaining to transmissibility are the method of inoculation and the dose of infectious material administered. A high dose of infectious material administered by direct intracerebral inoculation is clearly the most effective method of transmissibility and generally provides the shortest incubation time. See Brain, Mutation, Nervous system disorders