{"id":324,"date":"2016-11-04T03:33:06","date_gmt":"2016-11-04T03:33:06","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/microbiology\/?post_type=chapter&#038;p=324"},"modified":"2018-07-11T18:51:11","modified_gmt":"2018-07-11T18:51:11","slug":"viroids-virusoids-and-prions","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/chapter\/viroids-virusoids-and-prions\/","title":{"raw":"Viroids, Virusoids, and Prions","rendered":"Viroids, Virusoids, and Prions"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Objectives<\/h3>\r\n<ul>\r\n \t<li>Describe viroids and their unique characteristics<\/li>\r\n \t<li>Describe virusoids and their unique characteristics<\/li>\r\n \t<li>Describe prions and their unique characteristics<\/li>\r\n<\/ul>\r\n<\/div>\r\nResearch attempts to discover the causative agents of previously uninvestigated diseases have led to the discovery of nonliving disease agents quite different from viruses. These include particles consisting only of RNA or only of protein that, nonetheless, are able to self-propagate at the expense of a host\u2014a key similarity to viruses that allows them to cause disease conditions. To date, these discoveries include viroids, virusoids, and the proteinaceous prions.\r\n<h2>Viroids<\/h2>\r\n[caption id=\"\" align=\"alignright\" width=\"401\"]<img class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154631\/OSC_Microbio_06_04_PSTV.jpg\" alt=\"Photo of potatoes with odd, lumpy growths.\" width=\"401\" height=\"290\" \/> Figure 1. These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread when infected knives are used to cut healthy potatoes, which are then planted. (credit: Pamela Roberts, University of Florida Institute of Food and Agricultural Sciences, USDA ARS)[\/caption]\r\n\r\nIn 1971, Theodor <strong>Diener<\/strong>, a pathologist working at the Agriculture Research Service, discovered an acellular particle that he named a viroid, meaning \"virus-like.\" <strong>Viroids<\/strong> consist only of a short strand of circular RNA capable of self-replication. The first viroid discovered was found to cause <strong>potato tuber spindle disease<\/strong>, which causes slower sprouting and various deformities in potato plants (see Figure 1). Like viruses, potato spindle tuber viroids (PSTVs) take control of the host machinery to replicate their RNA genome. Unlike viruses, viroids do not have a protein coat to protect their genetic information.\r\n\r\nViroids can result in devastating losses of commercially important agricultural food crops grown in fields and orchards. Since the discovery of PSTV, other viroids have been discovered that cause diseases in plants. <strong>Tomato planta macho viroid (TPMVd)<\/strong> infects tomato plants, which causes loss of chlorophyll, disfigured and brittle leaves, and very small tomatoes, resulting in loss of productivity in this field crop. <strong>Avocado sunblotch viroid (ASBVd)<\/strong> results in lower yields and poorer-quality fruit. ASBVd is the smallest viroid discovered thus far that infects plants. <strong>Peach latent mosaic viroid (PLMVd)<\/strong> can cause necrosis of flower buds and branches, and wounding of ripened fruit, which leads to fungal and bacterial growth in the fruit. PLMVd can also cause similar pathological changes in plums, nectarines, apricots, and cherries, resulting in decreased productivity in these orchards, as well. Viroids, in general, can be dispersed mechanically during crop maintenance or harvesting, vegetative reproduction, and possibly via seeds and insects, resulting in a severe drop in food availability and devastating economic consequences.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>What is the genome of a viroid made of?<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Virusoids<\/h2>\r\nA second type of pathogenic RNA that can infect commercially important agricultural crops are the <strong>virusoids<\/strong>, which are subviral particles best described as non\u2013self-replicating ssRNAs. RNA replication of <strong>virusoids<\/strong> is similar to that of viroids but, unlike viroids, virusoids require that the cell also be infected with a specific \"helper\" virus. There are currently only five described types of virusoids and their associated <strong>helper viruses<\/strong>. The helper viruses are all from the family of <strong>Sobemoviruses<\/strong>. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid. Once the helper virus enters the host cell, the virusoids are released and can be found free in plant cell cytoplasm, where they possess ribozyme activity. The helper virus undergoes typical viral replication independent of the activity of the virusoid. The virusoid genomes are small, only 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA.\r\n\r\nVirusoids belong to a larger group of infectious agents called <strong>satellite RNAs<\/strong>, which are similar pathogenic RNAs found in animals. Unlike the plant virusoids, satellite RNAs may encode for proteins; however, like plant virusoids, satellite RNAs must coinfect with a helper virus to replicate. One satellite RNA that infects humans and that has been described by some scientists as a virusoid is the <strong>hepatitis delta virus (HDV)<\/strong>, which, by some reports, is also called hepatitis delta virusoid. Much larger than a plant virusoid, HDV has a circular, ssRNA genome of 1,700 nucleotides and can direct the biosynthesis of HDV-associated proteins. The HDV helper virus is the <strong>hepatitis B virus (HBV)<\/strong>. Coinfection with HBV and HDV results in more severe pathological changes in the liver during infection, which is how HDV was first discovered.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>What is the main difference between a viroid and a virusoid?<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h2>Prions<\/h2>\r\nAt one time, scientists believed that any infectious particle must contain DNA or RNA. Then, in 1982, Stanley <strong>Prusiner<\/strong>, a medical doctor studying scrapie (a fatal, degenerative disease in sheep) discovered that the disease was caused by proteinaceous infectious particles, or <strong>prions<\/strong>. Because proteins are acellular and do not contain DNA or RNA, Prusiner\u2019s findings were originally met with resistance and skepticism; however, his research was eventually validated, and he received the Nobel Prize in Physiology or Medicine in 1997.\r\n\r\nA prion is a misfolded rogue form of a normal protein (PrPc) found in the cell. This rogue prion protein (PrPsc), which may be caused by a genetic mutation or occur spontaneously, can be infectious, stimulating other endogenous normal proteins to become misfolded, forming plaques (see Figure 2). Today, prions are known to cause various forms of <strong>transmissible spongiform encephalopathy (TSE)<\/strong> in human and animals.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154634\/OSC_Microbio_06_04_prion.jpg\" alt=\"Figure A shows the process of how normal prion protein is converted to disease causing forms. Endogenous PrPc interact with PrpSC. This converts PRPC into PRPSC. The PRPSC accumulate. Figure B is a micrograph that shows holes in brain tissue.\" width=\"1300\" height=\"468\" \/> Figure 2. Endogenous normal prion protein (PrPc) is converted into the disease-causing form (PrPsc) when it encounters this variant form of the protein. PrPsc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may originate from misfolded prions consumed in food that eventually find their way into brain tissue. (credit b: modification of work by USDA)[\/caption]\r\n\r\nTSE is a rare degenerative disorder that affects the brain and nervous system. The accumulation of rogue proteins causes the brain tissue to become sponge-like, killing brain cells and forming holes in the tissue, leading to brain damage, loss of motor coordination, and dementia (see Figure 3). Infected individuals are mentally impaired and become unable to move or speak. There is no cure, and the disease progresses rapidly, eventually leading to death within a few months or years.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"1300\"]<img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154637\/OSC_Microbio_06_04_CJDbrain.jpg\" alt=\"The CJD brain has larger spaces as seen by more black regions in the image of the whole brain. The micrograph shows holes in the brain tissue.\" width=\"1300\" height=\"629\" \/> Figure 3. Creutzfeldt-Jakob disease (CJD) is a fatal disease that causes degeneration of neural tissue. (a) These brain scans compare a normal brain to one with CJD. (b) Compared to a normal brain, the brain tissue of a CJD patient is full of sponge-like lesions, which result from abnormal formations of prion protein. (credit a (right): modification of work by Dr. Laughlin Dawes; credit b (top): modification of work by Suzanne Wakim; credit b (bottom): modification of work by Centers for Disease Control and Prevention)[\/caption]\r\n\r\nTSEs in humans include <strong>kuru<\/strong>, <strong>fatal familial insomnia<\/strong>, <strong>Gerstmann-Straussler-Scheinker disease<\/strong>, and <strong>Creutzfeldt-Jakob disease<\/strong> (see Figure 3). TSEs in animals include <strong>mad cow disease<\/strong>, <strong>scrapie<\/strong> (in sheep and goats), and <strong>chronic wasting disease<\/strong> (in elk and deer). TSEs can be transmitted between animals and from animals to humans by eating contaminated meat or animal feed. Transmission between humans can occur through heredity (as is often the case with GSS and CJD) or by contact with contaminated tissue, as might occur during a blood transfusion or organ transplant. There is no evidence for transmission via casual contact with an infected person. Table 1\u00a0lists TSEs that affect humans and their modes of transmission.\r\n<table id=\"fs-id1168327872775\" class=\"span-all\" summary=\"A two column, eight row table is shown. The rows in the first column contain \">\r\n<thead>\r\n<tr>\r\n<th colspan=\"2\">Table 1. Transmissible Spongiform Encephalopathies (TSEs) in Humans<\/th>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<th style=\"width: 25%\"><strong>Disease<\/strong><\/th>\r\n<th><strong>Mechanism(s) of Transmission<\/strong>[footnote]National Institute of Neurological Disorders and Stroke. \"Creutzfeldt-Jakob Disease Fact Sheet.\" http:\/\/www.ninds.nih.gov\/disorders\/cjd\/detail_cjd.htm (accessed December 31, 2015).[\/footnote]<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr valign=\"top\">\r\n<td>Sporadic CJD (sCJD)<\/td>\r\n<td>Not known; possibly by alteration of normal prior protein (PrP) to rogue form due to somatic mutation<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Variant CJD (vCJD)<\/td>\r\n<td>Eating contaminated cattle products and by secondary bloodborne transmission<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Familial CJD (fCJD)<\/td>\r\n<td>Mutation in germline PrP gene<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Iatrogenic CJD (iCJD)<\/td>\r\n<td>Contaminated neurosurgical instruments, corneal graft, gonadotrophic hormone, and, secondarily, by blood transfusion<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Kuru<\/td>\r\n<td>Eating infected meat through ritualistic cannibalism<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Gerstmann-Straussler-Scheinker disease (GSS)<\/td>\r\n<td>Mutation in germline PrP gene<\/td>\r\n<\/tr>\r\n<tr valign=\"top\">\r\n<td>Fatal familial insomnia (FFI)<\/td>\r\n<td>Mutation in germline PrP gene<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\nPrions are extremely difficult to destroy because they are resistant to heat, chemicals, and radiation. Even standard sterilization procedures do not ensure the destruction of these particles. Currently, there is no treatment or cure for TSE disease, and contaminated meats or infected animals must be handled according to federal guidelines to prevent transmission.\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ul>\r\n \t<li>Does a prion have a genome?<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Key Concepts and Summary<\/h3>\r\n<ul>\r\n \t<li>Other acellular agents such as <strong>viroids<\/strong>, <strong>virusoids<\/strong>, and <strong>prions<\/strong> also cause diseases. Viroids consist of small, naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to establish an infection. Prions are proteinaceous infectious particles that cause <strong>transmissible spongiform encephalopathies<\/strong>.<\/li>\r\n \t<li>Prions are extremely resistant to chemicals, heat, and radiation.<\/li>\r\n \t<li>There are no treatments for prion infection.<\/li>\r\n<\/ul>\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Multiple Choice<\/h3>\r\nWhich of these infectious agents do not have nucleic acid?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>viroids<\/li>\r\n \t<li>viruses<\/li>\r\n \t<li>bacteria<\/li>\r\n \t<li>prions<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"109829\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"109829\"]Answer d. Prions\u00a0do not have nucleic acid.[\/hidden-answer]\r\n\r\nWhich of the following is true of prions?\r\n<ol style=\"list-style-type: lower-alpha\">\r\n \t<li>They can be inactivated by boiling at 100 \u00b0C.<\/li>\r\n \t<li>They contain a capsid.<\/li>\r\n \t<li>They are a rogue form of protein, PrP.<\/li>\r\n \t<li>They can be reliably inactivated by an autoclave.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"15979\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"15979\"]Answer c. Prions are a rogue form of protein, PrP.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Fill in the Blank<\/h3>\r\nBoth viroids and virusoids have a(n) _________ genome, but virusoids require a(n) _________ to reproduce.\r\n\r\n[reveal-answer q=\"345431\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"345431\"]Both viroids and virusoids have an <strong>RNA<\/strong> genome, but virusoids require a <strong>helper virus<\/strong> to reproduce.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Think about It<\/h3>\r\n<ol>\r\n \t<li>Describe the disease symptoms observed in animals infected with prions.<\/li>\r\n \t<li>Does a prion replicate? Explain.<\/li>\r\n<\/ol>\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Objectives<\/h3>\n<ul>\n<li>Describe viroids and their unique characteristics<\/li>\n<li>Describe virusoids and their unique characteristics<\/li>\n<li>Describe prions and their unique characteristics<\/li>\n<\/ul>\n<\/div>\n<p>Research attempts to discover the causative agents of previously uninvestigated diseases have led to the discovery of nonliving disease agents quite different from viruses. These include particles consisting only of RNA or only of protein that, nonetheless, are able to self-propagate at the expense of a host\u2014a key similarity to viruses that allows them to cause disease conditions. To date, these discoveries include viroids, virusoids, and the proteinaceous prions.<\/p>\n<h2>Viroids<\/h2>\n<div style=\"width: 411px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154631\/OSC_Microbio_06_04_PSTV.jpg\" alt=\"Photo of potatoes with odd, lumpy growths.\" width=\"401\" height=\"290\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 1. These potatoes have been infected by the potato spindle tuber viroid (PSTV), which is typically spread when infected knives are used to cut healthy potatoes, which are then planted. (credit: Pamela Roberts, University of Florida Institute of Food and Agricultural Sciences, USDA ARS)<\/p>\n<\/div>\n<p>In 1971, Theodor <strong>Diener<\/strong>, a pathologist working at the Agriculture Research Service, discovered an acellular particle that he named a viroid, meaning &#8220;virus-like.&#8221; <strong>Viroids<\/strong> consist only of a short strand of circular RNA capable of self-replication. The first viroid discovered was found to cause <strong>potato tuber spindle disease<\/strong>, which causes slower sprouting and various deformities in potato plants (see Figure 1). Like viruses, potato spindle tuber viroids (PSTVs) take control of the host machinery to replicate their RNA genome. Unlike viruses, viroids do not have a protein coat to protect their genetic information.<\/p>\n<p>Viroids can result in devastating losses of commercially important agricultural food crops grown in fields and orchards. Since the discovery of PSTV, other viroids have been discovered that cause diseases in plants. <strong>Tomato planta macho viroid (TPMVd)<\/strong> infects tomato plants, which causes loss of chlorophyll, disfigured and brittle leaves, and very small tomatoes, resulting in loss of productivity in this field crop. <strong>Avocado sunblotch viroid (ASBVd)<\/strong> results in lower yields and poorer-quality fruit. ASBVd is the smallest viroid discovered thus far that infects plants. <strong>Peach latent mosaic viroid (PLMVd)<\/strong> can cause necrosis of flower buds and branches, and wounding of ripened fruit, which leads to fungal and bacterial growth in the fruit. PLMVd can also cause similar pathological changes in plums, nectarines, apricots, and cherries, resulting in decreased productivity in these orchards, as well. Viroids, in general, can be dispersed mechanically during crop maintenance or harvesting, vegetative reproduction, and possibly via seeds and insects, resulting in a severe drop in food availability and devastating economic consequences.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>What is the genome of a viroid made of?<\/li>\n<\/ul>\n<\/div>\n<h2>Virusoids<\/h2>\n<p>A second type of pathogenic RNA that can infect commercially important agricultural crops are the <strong>virusoids<\/strong>, which are subviral particles best described as non\u2013self-replicating ssRNAs. RNA replication of <strong>virusoids<\/strong> is similar to that of viroids but, unlike viroids, virusoids require that the cell also be infected with a specific &#8220;helper&#8221; virus. There are currently only five described types of virusoids and their associated <strong>helper viruses<\/strong>. The helper viruses are all from the family of <strong>Sobemoviruses<\/strong>. An example of a helper virus is the subterranean clover mottle virus, which has an associated virusoid packaged inside the viral capsid. Once the helper virus enters the host cell, the virusoids are released and can be found free in plant cell cytoplasm, where they possess ribozyme activity. The helper virus undergoes typical viral replication independent of the activity of the virusoid. The virusoid genomes are small, only 220 to 388 nucleotides long. A virusoid genome does not code for any proteins, but instead serves only to replicate virusoid RNA.<\/p>\n<p>Virusoids belong to a larger group of infectious agents called <strong>satellite RNAs<\/strong>, which are similar pathogenic RNAs found in animals. Unlike the plant virusoids, satellite RNAs may encode for proteins; however, like plant virusoids, satellite RNAs must coinfect with a helper virus to replicate. One satellite RNA that infects humans and that has been described by some scientists as a virusoid is the <strong>hepatitis delta virus (HDV)<\/strong>, which, by some reports, is also called hepatitis delta virusoid. Much larger than a plant virusoid, HDV has a circular, ssRNA genome of 1,700 nucleotides and can direct the biosynthesis of HDV-associated proteins. The HDV helper virus is the <strong>hepatitis B virus (HBV)<\/strong>. Coinfection with HBV and HDV results in more severe pathological changes in the liver during infection, which is how HDV was first discovered.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>What is the main difference between a viroid and a virusoid?<\/li>\n<\/ul>\n<\/div>\n<h2>Prions<\/h2>\n<p>At one time, scientists believed that any infectious particle must contain DNA or RNA. Then, in 1982, Stanley <strong>Prusiner<\/strong>, a medical doctor studying scrapie (a fatal, degenerative disease in sheep) discovered that the disease was caused by proteinaceous infectious particles, or <strong>prions<\/strong>. Because proteins are acellular and do not contain DNA or RNA, Prusiner\u2019s findings were originally met with resistance and skepticism; however, his research was eventually validated, and he received the Nobel Prize in Physiology or Medicine in 1997.<\/p>\n<p>A prion is a misfolded rogue form of a normal protein (PrPc) found in the cell. This rogue prion protein (PrPsc), which may be caused by a genetic mutation or occur spontaneously, can be infectious, stimulating other endogenous normal proteins to become misfolded, forming plaques (see Figure 2). Today, prions are known to cause various forms of <strong>transmissible spongiform encephalopathy (TSE)<\/strong> in human and animals.<\/p>\n<div style=\"width: 1310px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154634\/OSC_Microbio_06_04_prion.jpg\" alt=\"Figure A shows the process of how normal prion protein is converted to disease causing forms. Endogenous PrPc interact with PrpSC. This converts PRPC into PRPSC. The PRPSC accumulate. Figure B is a micrograph that shows holes in brain tissue.\" width=\"1300\" height=\"468\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. Endogenous normal prion protein (PrPc) is converted into the disease-causing form (PrPsc) when it encounters this variant form of the protein. PrPsc may arise spontaneously in brain tissue, especially if a mutant form of the protein is present, or it may originate from misfolded prions consumed in food that eventually find their way into brain tissue. (credit b: modification of work by USDA)<\/p>\n<\/div>\n<p>TSE is a rare degenerative disorder that affects the brain and nervous system. The accumulation of rogue proteins causes the brain tissue to become sponge-like, killing brain cells and forming holes in the tissue, leading to brain damage, loss of motor coordination, and dementia (see Figure 3). Infected individuals are mentally impaired and become unable to move or speak. There is no cure, and the disease progresses rapidly, eventually leading to death within a few months or years.<\/p>\n<div style=\"width: 1310px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1094\/2016\/11\/03154637\/OSC_Microbio_06_04_CJDbrain.jpg\" alt=\"The CJD brain has larger spaces as seen by more black regions in the image of the whole brain. The micrograph shows holes in the brain tissue.\" width=\"1300\" height=\"629\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 3. Creutzfeldt-Jakob disease (CJD) is a fatal disease that causes degeneration of neural tissue. (a) These brain scans compare a normal brain to one with CJD. (b) Compared to a normal brain, the brain tissue of a CJD patient is full of sponge-like lesions, which result from abnormal formations of prion protein. (credit a (right): modification of work by Dr. Laughlin Dawes; credit b (top): modification of work by Suzanne Wakim; credit b (bottom): modification of work by Centers for Disease Control and Prevention)<\/p>\n<\/div>\n<p>TSEs in humans include <strong>kuru<\/strong>, <strong>fatal familial insomnia<\/strong>, <strong>Gerstmann-Straussler-Scheinker disease<\/strong>, and <strong>Creutzfeldt-Jakob disease<\/strong> (see Figure 3). TSEs in animals include <strong>mad cow disease<\/strong>, <strong>scrapie<\/strong> (in sheep and goats), and <strong>chronic wasting disease<\/strong> (in elk and deer). TSEs can be transmitted between animals and from animals to humans by eating contaminated meat or animal feed. Transmission between humans can occur through heredity (as is often the case with GSS and CJD) or by contact with contaminated tissue, as might occur during a blood transfusion or organ transplant. There is no evidence for transmission via casual contact with an infected person. Table 1\u00a0lists TSEs that affect humans and their modes of transmission.<\/p>\n<table id=\"fs-id1168327872775\" class=\"span-all\" summary=\"A two column, eight row table is shown. The rows in the first column contain\">\n<thead>\n<tr>\n<th colspan=\"2\">Table 1. Transmissible Spongiform Encephalopathies (TSEs) in Humans<\/th>\n<\/tr>\n<tr valign=\"top\">\n<th style=\"width: 25%\"><strong>Disease<\/strong><\/th>\n<th><strong>Mechanism(s) of Transmission<\/strong><a class=\"footnote\" title=\"National Institute of Neurological Disorders and Stroke. &quot;Creutzfeldt-Jakob Disease Fact Sheet.&quot; http:\/\/www.ninds.nih.gov\/disorders\/cjd\/detail_cjd.htm (accessed December 31, 2015).\" id=\"return-footnote-324-1\" href=\"#footnote-324-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a><\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr valign=\"top\">\n<td>Sporadic CJD (sCJD)<\/td>\n<td>Not known; possibly by alteration of normal prior protein (PrP) to rogue form due to somatic mutation<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Variant CJD (vCJD)<\/td>\n<td>Eating contaminated cattle products and by secondary bloodborne transmission<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Familial CJD (fCJD)<\/td>\n<td>Mutation in germline PrP gene<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Iatrogenic CJD (iCJD)<\/td>\n<td>Contaminated neurosurgical instruments, corneal graft, gonadotrophic hormone, and, secondarily, by blood transfusion<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Kuru<\/td>\n<td>Eating infected meat through ritualistic cannibalism<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Gerstmann-Straussler-Scheinker disease (GSS)<\/td>\n<td>Mutation in germline PrP gene<\/td>\n<\/tr>\n<tr valign=\"top\">\n<td>Fatal familial insomnia (FFI)<\/td>\n<td>Mutation in germline PrP gene<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p>Prions are extremely difficult to destroy because they are resistant to heat, chemicals, and radiation. Even standard sterilization procedures do not ensure the destruction of these particles. Currently, there is no treatment or cure for TSE disease, and contaminated meats or infected animals must be handled according to federal guidelines to prevent transmission.<\/p>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ul>\n<li>Does a prion have a genome?<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Key Concepts and Summary<\/h3>\n<ul>\n<li>Other acellular agents such as <strong>viroids<\/strong>, <strong>virusoids<\/strong>, and <strong>prions<\/strong> also cause diseases. Viroids consist of small, naked ssRNAs that cause diseases in plants. Virusoids are ssRNAs that require other helper viruses to establish an infection. Prions are proteinaceous infectious particles that cause <strong>transmissible spongiform encephalopathies<\/strong>.<\/li>\n<li>Prions are extremely resistant to chemicals, heat, and radiation.<\/li>\n<li>There are no treatments for prion infection.<\/li>\n<\/ul>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Multiple Choice<\/h3>\n<p>Which of these infectious agents do not have nucleic acid?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>viroids<\/li>\n<li>viruses<\/li>\n<li>bacteria<\/li>\n<li>prions<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q109829\">Show Answer<\/span><\/p>\n<div id=\"q109829\" class=\"hidden-answer\" style=\"display: none\">Answer d. Prions\u00a0do not have nucleic acid.<\/div>\n<\/div>\n<p>Which of the following is true of prions?<\/p>\n<ol style=\"list-style-type: lower-alpha\">\n<li>They can be inactivated by boiling at 100 \u00b0C.<\/li>\n<li>They contain a capsid.<\/li>\n<li>They are a rogue form of protein, PrP.<\/li>\n<li>They can be reliably inactivated by an autoclave.<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q15979\">Show Answer<\/span><\/p>\n<div id=\"q15979\" class=\"hidden-answer\" style=\"display: none\">Answer c. Prions are a rogue form of protein, PrP.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Fill in the Blank<\/h3>\n<p>Both viroids and virusoids have a(n) _________ genome, but virusoids require a(n) _________ to reproduce.<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q345431\">Show Answer<\/span><\/p>\n<div id=\"q345431\" class=\"hidden-answer\" style=\"display: none\">Both viroids and virusoids have an <strong>RNA<\/strong> genome, but virusoids require a <strong>helper virus<\/strong> to reproduce.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Think about It<\/h3>\n<ol>\n<li>Describe the disease symptoms observed in animals infected with prions.<\/li>\n<li>Does a prion replicate? Explain.<\/li>\n<\/ol>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-324\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>OpenStax Microbiology. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\">http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-324-1\">National Institute of Neurological Disorders and Stroke. \"Creutzfeldt-Jakob Disease Fact Sheet.\" http:\/\/www.ninds.nih.gov\/disorders\/cjd\/detail_cjd.htm (accessed December 31, 2015). <a href=\"#return-footnote-324-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":5,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"OpenStax Microbiology\",\"author\":\"\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/e42bd376-624b-4c0f-972f-e0c57998e765@4.2\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-324","chapter","type-chapter","status-publish","hentry"],"part":286,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/324","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":7,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/324\/revisions"}],"predecessor-version":[{"id":2156,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/324\/revisions\/2156"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/parts\/286"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapters\/324\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/media?parent=324"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/pressbooks\/v2\/chapter-type?post=324"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/contributor?post=324"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/suny-mcc-microbiology\/wp-json\/wp\/v2\/license?post=324"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}