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| Transmissible spongiform encephalopathy (TSE) | |
|---|---|
| Other names | Prion disease |
 | |
| Micrograph showing spongiform degeneration (vacuoles that appear as holes in tissue sections) in thecerebral cortex of a patient who had died ofCreutzfeldt–Jakob disease.H&E stain, scale bar = 30microns (0.03 mm). | |
| Specialty | Infectious diseases  |
| Symptoms | Dementia, seizures, tremors, insomnia, psychosis, delirium, confusion |
| Usual onset | Months to decades |
| Types | Bovine spongiform encephalopathy,Fatal familial insomnia,Creutzfeldt-Jakob disease,kuru,Huntington's disease-like 1,scrapie,variably protease-sensitive prionopathy,chronic wasting disease,Gerstmann-Sträussler-Scheinker syndrome,feline spongiform encephalopathy,transmissible mink encephalopathy,exotic ungulate encephalopathy,camel spongiform encephalopathy,PrP systemic amyloidosis,Familial Alzheimer-like prion disease |
| Causes | Prion |
| Risk factors | Contact with infected fluids, ingestion of infected flesh, having one or two parents that have the disease (in case of fatal familial insomnia) |
| Diagnostic method | Currently there is no way to reliably detect prions except at post-mortem |
| Prevention | Varies |
| Treatment | Palliative care |
| Prognosis | Invariably fatal |
| Frequency | Rare |
Transmissible spongiform encephalopathies (TSEs), also known asprion diseases,[1] are a group of progressive, incurable, and fatal conditions that are associated with theprion hypothesis and affect thebrain andnervous system of manyanimals, includinghumans,cattle, andsheep. According to the most widespread hypothesis, they are transmitted byprions, though some other data suggest an involvement of aSpiroplasma infection.[2] Mental and physical abilities deteriorate and many tiny holes appear in thecortex causing it to appear like a sponge when brain tissue obtained atautopsy is examined under amicroscope. The disorders cause impairment of brain function which may result in memory loss, personality changes, andabnormal or impaired movement which worsen over time.[3]
TSEs of humans includeCreutzfeldt–Jakob disease,Gerstmann–Sträussler–Scheinker syndrome,fatal familial insomnia, andkuru, as well as the recently discoveredvariably protease-sensitive prionopathy and familial spongiform encephalopathy. Creutzfeldt-Jakob disease itself has four main forms, the sporadic (sCJD), the hereditary/familial (fCJD), the iatrogenic (iCJD) and the variant form (vCJD). These conditions form a spectrum of diseases with overlapping signs and symptoms.
TSEs in non-human mammals includescrapie in sheep,bovine spongiform encephalopathy (BSE) in cattle – popularly known as "mad cow disease" – andchronic wasting disease (CWD) in deer and elk. Thevariant form of Creutzfeldt–Jakob disease in humans is caused by exposure tobovine spongiform encephalopathy prions.[4][5][6]
Unlike other kinds of infectious disease, which are spread by agents with aDNA orRNA genome (such asvirus orbacteria), the infectious agent in TSEs is believed to be aprion, thus being composed solely ofprotein material. Misfolded prion proteins carry the disease between individuals and cause deterioration of thebrain. TSEs are unique diseases in that theiraetiology may be genetic, sporadic, or infectious via ingestion of infected foodstuffs and viaiatrogenic means (e.g., blood transfusion).[7] Most TSEs are sporadic and occur in an animal with no prion protein mutation. Inherited TSE occurs in animals carrying a raremutant prionallele, which expresses prion proteins that contort by themselves into the disease-causingconformation. Transmission occurs when healthy animals consume tainted tissues from others with the disease. In the 1980s and 1990s, bovine spongiform encephalopathy spread incattle in an epidemic fashion. This occurred because cattle were fed theprocessed remains of other cattle, a practice now banned in many countries. In turn, consumption (by humans) of bovine-derived foodstuff which contained prion-contaminated tissues resulted in an outbreak of thevariant form of Creutzfeldt–Jakob disease in the 1990s and 2000s.[8]
Prions cannot be transmitted through the air, through touching, or most other forms of casual contact. However, they may be transmitted through contact with infected tissue, body fluids, or contaminated medical instruments. Normalsterilization procedures such as boiling or irradiating materials fail to render prions non-infective. However, treatment with strong, almost undiluted bleach and/or sodium hydroxide, or heating to a minimum of 134 °C, does destroy prions.[9]
Differences in shape between the different prion protein forms are poorly understood.
| ICTVdb Code | Disease name | Natural host | Prion name | PrPisoform | Ruminant |
|---|---|---|---|---|---|
| Non-humanmammals | |||||
| 90.001.0.01.001. | Scrapie | Sheep andgoats | Scrapie prion | PrPSc | Yes |
| 90.001.0.01.002. | Transmissible mink encephalopathy (TME) | Mink | TME prion | PrPTME | No |
| 90.001.0.01.003. | Chronic wasting disease (CWD) | Elk,white-tailed deer,mule deer andred deer | CWD prion | PrPCWD | Yes |
| 90.001.0.01.004. | Bovine spongiform encephalopathy (BSE) commonly known as "mad cow disease" | Cattle | BSE prion | PrPBSE | Yes |
| 90.001.0.01.005. | Feline spongiform encephalopathy (FSE) | Cats | FSE prion | PrPFSE | No |
| 90.001.0.01.006. | Exotic ungulate encephalopathy (EUE) | Nyala andgreater kudu | EUE prion | PrPEUE | Yes |
| Camel spongiform encephalopathy (CSE)[10] | Camel | PrPCSE | Yes | ||
| Human diseases | |||||
| 90.001.0.01.007. | Kuru | Humans | Kuru prion | PrPKuru | No |
| 90.001.0.01.008. | Creutzfeldt–Jakob disease (CJD) | CJD prion | PrPsCJD | No | |
| Variant Creutzfeldt–Jakob disease (vCJD, nvCJD) | vCJD prion[11] | PrPvCJD | |||
| 90.001.0.01.009. | Gerstmann-Sträussler-Scheinker syndrome (GSS) | GSS prion | PrPGSS | No | |
| 90.001.0.01.010. | Fatal familial insomnia (FFI) | FFI prion | PrPFFI | No | |
| Familial spongiform encephalopathy[12] | |||||
The degenerative tissue damage caused by human prion diseases (CJD, GSS, and kuru) is characterised by four features: spongiform change (the presence of many small holes), the death ofneurons,astrocytosis (abnormal increase in the number ofastrocytes due to the destruction of nearby neurons), andamyloid plaque formation. These features are shared with prion diseases in animals, and the recognition of these similarities prompted the first attempts to transmit a human prion disease (kuru) to aprimate in 1966, followed by CJD in 1968 and GSS in 1981. These neuropathological features have formed the basis of thehistological diagnosis of human prion diseases for many years, although it was recognized that these changes are enormously variable both from case to case and within thecentral nervous system in individual cases.[13]
The clinical signs in humans vary, but commonly include personality changes, psychiatric problems such asdepression, lack of coordination, and/or an unsteady gait (ataxia). Patients also may experience involuntary jerking movements calledmyoclonus, unusual sensations,insomnia, confusion, or memory problems. In the later stages of the disease, patients have severe mental impairment (dementia) and lose the ability to move or speak.[14]
Early neuropathological reports on human prion diseases suffered from a confusion of nomenclature, in which the significance of the diagnostic feature of spongiform change was occasionally overlooked. The subsequent demonstration that human prion diseases were transmissible reinforced the importance of spongiform change as a diagnostic feature, reflected in the use of the term "spongiform encephalopathy" for this group of disorders.
Prions appear to be most infectious when in direct contact with affected tissues. For example, Creutzfeldt–Jakob disease has been transmitted to patients taking injections ofgrowth hormone harvested from humanpituitary glands, from cadaverdura allografts and from instruments used forbrain surgery (Brown, 2000) (prions can survive the "autoclave" sterilization process used for most surgical instruments). Dietary consumption of affected animals can cause prions to accumulate slowly, especially whencannibalism or similar practices allow the proteins to accumulate over more than one generation. An example iskuru, which reached epidemic proportions in the mid-20th century in theFore people ofPapua New Guinea, who used to consume their dead as a funerary ritual.[15] Laws in developed countries now ban the use ofrenderedruminant proteins in ruminant feed as a precaution against the spread of prion infection in cattle and other ruminants.[citation needed]
Note that not allencephalopathies are caused by prions, as in the cases ofPML (caused by theJC virus),CADASIL (caused by abnormalNOTCH3 protein activity), andKrabbe disease (caused by a deficiency of theenzyme galactosylceramidase).Progressive Spongiform Leukoencephalopathy (PSL)—which is a spongiform encephalopathy—is also probably not caused by a prion, although the adulterant that causes it amongheroin smokers has not yet been identified.[16][17][18][19] This, combined with the highly variable nature of prion disease pathology, is why a prion disease cannot be diagnosed based solely on a patient's symptoms.
Familial forms of prion disease are caused by inherited mutations in the PRNP gene. However, only a small percentage of prion disease cases are familial; most occur sporadically, without known genetic mutations or risk factors.[20] In rare instances, prion diseases can be transmitted through exposure to prion-contaminated tissues or biological materials from affected individuals.[21]
The PRNP gene encodes the prion protein (PrP), which under normal conditions may play a role in transporting copper into cells and protecting neurons.[22] Misfolding of the prion protein leads to the accumulation of pathogenic PrP^Sc, the hallmark of prion diseases, causing progressive neurodegeneration.[23][14]
Epidemiological surveillance has identified cases of atypical bovine spongiform encephalopathy (BSE) and scrapie in livestock, as well as chronic wasting disease (CWD) in cervids, highlighting the zoonotic potential of prion diseases and their impact on animal and human health.[24]
Protein could be the infectious agent, inducing its own replication by causing conformational change of normal cellular PrPC into PrPSc. Evidence for this hypothesis:
While not containing a nucleic acid genome, prions may be composed of more than just a protein. Purified PrPC appears unable to convert to the infectious PrPSc form, unless other components are added, such as RNA and lipids.[29] These other components, termed cofactors, may form part of the infectious prion, or they may serve as catalysts for the replication of a protein-only prion.
This hypothesis postulates that a yet undiscovered infectious viral agent is the cause of the disease. The strongest evidence for viral replication in TSE infected brains is that long double stranded RNA is detected in 22L scrapie infected mouse brains[30]Other evidence for this hypothesis is as follows:
There continues to be a very practical problem with diagnosis of prion diseases, including BSE and CJD. They have an incubation period of months to decades during which there are no symptoms, even though the pathway of converting the normal brain PrP protein into the toxic, disease-related PrPSc form has started. At present, there is virtually no way to detect PrPSc reliably except by examining the brain using neuropathological and immunohistochemical methods after death. Accumulation of the abnormally folded PrPSc form of the PrP protein is a characteristic of the disease, but it is present at very low levels in easily accessible body fluids like blood or urine. Researchers have tried to develop methods to measure PrPSc, but there are still no fully accepted methods for use in materials such as blood.[citation needed]
In 2010, a team from New York described detection of PrPSc even when initially present at only one part in a hundred billion (10−11) in brain tissue. The method combines amplification with a novel technology calledSurround Optical Fiber Immunoassay (SOFIA) and some specific antibodies against PrPSc. After amplifying and then concentrating any PrPSc, the samples are labelled with a fluorescent dye using an antibody for specificity and then finally loaded into a micro-capillary tube. This tube is placed in a specially constructed apparatus so that it is totally surrounded by optical fibres to capture all light emitted once the dye is excited using a laser. The technique allowed detection of PrPSc after many fewer cycles of conversion than others have achieved, substantially reducing the possibility of artefacts, as well as speeding up the assay. The researchers also tested their method on blood samples from apparently healthy sheep that went on to develop scrapie. The animals' brains were analysed once any symptoms became apparent. The researchers could therefore compare results from brain tissue and blood taken once the animals exhibited symptoms of the diseases, with blood obtained earlier in the animals' lives, and from uninfected animals. The results showed very clearly that PrPSc could be detected in the blood of animals long before the symptoms appeared.[33][34]
There are currently no known ways to cure or prevent prion disease.[35] Certain medications can slow down the progression of the disease.[36] But ultimately,supportive care is currently the only option for infected individuals.
Transmissible spongiform encephalopathies (TSE) are very rare but can reach epidemic proportions.[clarification needed] It is very hard to map the spread of the disease due to the difficulty of identifying individual strains of the prions. This means that, if animals at one farm begin to show the disease after an outbreak on a nearby farm, it is very difficult to determine whether it is the same strain affecting both herds—suggesting transmission—or if the second outbreak came from a completely different source.[citation needed]
Classic Creutzfeldt-Jakob disease (CJD) was discovered in 1920. It occurs sporadically over the world but is very rare. It affects about one person per million each year. Typically, the cause is unknown for these cases. It has been found to be passed on genetically in some cases. 250 patients contracted the disease through iatrogenictransmission (from use of contaminated surgical equipment).[37] This was before equipment sterilization was required in 1976, and there have been no other iatrogenic cases since then. In order to prevent the spread of infection, the World Health Organization created a guide to tell health care workers what to do when CJD appears and how to dispose of contaminated equipment.[38] The Centers for Disease Control and Prevention (CDC) have been keeping surveillance on CJD cases, particularly by looking at death certificate information.[39]
Chronic wasting disease (CWD) is a prion disease found in North America in deer and elk. The first case was identified as a fatal wasting syndrome in the 1960s. It was then recognized as a transmissible spongiform encephalopathy in 1978. Surveillance studies showed that CWD was endemic among free-ranging deer and elk in northeastern Colorado, southeastern Wyoming and western Nebraska. It was also discovered that CWD may have been present in a proportion of free-ranging animals decades before the initial recognition. In the United States, the discovery of CWD raised concerns about the transmission of this prion disease to humans. It was suspected that many cases of CJD were transmitted by CWD, however the evidence was minimal.[24]
In the 1980s and 1990s, bovine spongiform encephalopathy (BSE or "mad cow disease") spread in cattle at an epidemic rate. The total estimated number of cattle infected was approximately 750,000 between 1980 and 1996. This occurred because the cattle were fed processed remains of other cattle. Then human consumption of these infected cattle caused an outbreak of the human form CJD. There was a dramatic decline in BSE when feeding bans were put in place. On May 20, 2003, the first case of BSE was confirmed in North America. The source could not be clearly identified, but researchers suspect it came from imported BSE-infected cow meat. In the United States, the USDA created safeguards to minimize the risk of BSE exposure to humans.[24]
Variant Creutzfeldt-Jakob disease (vCJD) was discovered in 1996 in England. There is strong evidence to suggest that vCJD was caused by the same prion as bovine spongiform encephalopathy.[40] "Since 1996 and as of August 2013, a total of 229 cases of variant CJD cases have been identified from 11 countries: 177 from the United Kingdom, 27 from France, 4 from Ireland, 4 from the United States, 5 from Spain, 3 in the Netherlands, 2 each from Portugal, Italy and Canada, and 1 each from Japan, Taiwan and Saudi Arabia."[41]
In the 5th centuryBCE,Hippocrates described a disease like TSE in cattle and sheep, which he believed also occurred in humans.[42]Publius Flavius Vegetius Renatus records cases of a disease with similar characteristics in the 4th and 5th centuries AD.[citation needed] In 1755, an outbreak ofscrapie was discussed in the British House of Commons and may have been present in Britain for some time before that.[43] Although there were unsupported claims in 1759 that the disease was contagious, in general it was thought to be due to inbreeding and countermeasures appeared to be successful. Early-20th-century experiments failed to show transmission ofscrapie between animals, until extraordinary measures were taken such as the intra-ocular injection of infected nervous tissue. No direct link betweenscrapie and human disease was suspected then or has been found since. TSE was first described in humans byAlfons Maria Jakob in 1921.[44]Daniel Carleton Gajdusek's discovery that Kuru was transmitted by cannibalism accompanied by the finding of scrapie-like lesions in the brains of Kuru victims strongly suggested an infectious basis to TSE.[45] A paradigm shift to a non-nucleic infectious entity was required when the results were validated with an explanation of how aprion protein might transmit spongiform encephalopathy.[46] Not until 1988 was the neuropathology of spongiform encephalopathy properly described in cows.[47] The alarming amplification ofBSE in the British cattle herd heightened fear of transmission to humans and reinforced the belief in the infectious nature of TSE. This was confirmed with the identification of a Kuru-like disease, called new variantCreutzfeldt–Jakob disease, in humans exposed toBSE.[48] Although the infectious disease model of TSE has been questioned in favour of aprion transplantation model that explains why cannibalism favours transmission,[42] the search for a viral agent was, as of 2007, being continued in some laboratories.[49][50]
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