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U.S. Food &
Drug Administration Center for Food Safety & Applied Nutrition Foodborne Pathogenic Microorganisms and Natural Toxins Handbook |
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Prions and Transmissible Spongiform
Encephalopathies
| 1. Name of the Agent: | The prion (proteinaceous infectious agent). Prions are proteins thought to originate as regular components of neurological tissues in animals: they are not cellular organisms or viruses. In their normal noninfectious state, these proteins may be involved in cell-to-cell communication. When these proteins become abnormally shaped i.e., prions, they are able to transform molecules of the normally shaped protein with which they come into contact to the abnormal prion configuration. This process is repeated numerous times until the number of abnormally shaped molecules causes overt illness. When consumed in the diet, prions are thought to be absorbed into the body where, again, they begin the process of changing their normal protein counterparts into prions. The specific prions of interest in disease and their normally configured proteins are those found in mammals; however, similar proteins occur in other organisms, from chickens to yeasts. While the "prion theory" of Transmissible Spongiform Encephalopathies (TSEs) is widely accepted and explains the occurrence of TSEs, there are other theories of the cause of these illnesses. |
| 2. Nature of Acute Disease: | Prions are associated with a group of diseases called Transmissible Spongiform Encephalopathies (TSEs). In humans, the illness suspected of being foodborne is variant Creutzfeldt-Jakob disease (vCJD). The human disease vCJD and the cattle disease, bovine spongiform encephalopathy (BSE), also known as "mad cow" disease, appear to be caused by the same agent. Other similar but not identical TSE diseases exist in animals, but there is no known transmission of these to humans. Included among these is chronic wasting disease (CWD) of deer and elk, and the oldest known of these diseases - scrapie - which occurs in sheep and goats . No early acute clinical indications for TSEs have been described. After an extended incubation period of years, these diseases result in irreversible neurodegeneration that becomes the cause of death. |
| 3. Nature of Disease: | The neurodegenerative phase of vCJD in humans typically involves the formation of "daisy-shaped" areas of damage in the central nervous system. There is also, in common with other TSEs, vacuolization (formation of holes) that gives brain tissue a spongy appearance. This damage is, likely, the result of accumulations of the abnormally shaped protein i.e., the prion, in the brain. The build-up causes cell death. |
| 4. Diagnosis of Human Illness: | The most reliable means for diagnosing any TSE is the microscopic examination of brain tissue - a post-mortem procedure. Preliminary diagnoses of vCJD are based on patient history, clinical symptoms, electroencephalograms, and magnetic resonance imaging of the brain. |
| 5. Associated Foods: | The major concern for consumers is the potential contamination of meat products by central nervous system tissue (brain and spinal cord) during routine slaughter. This indirect intake of high-risk tissues may have been the source of human illnesses in the United Kingdom and elsewhere. Bovine meat (if free of central nervous system tissue) and milk have shown no infectivity in test animals. Gelatin, derived from the hides and bones of cattle, appears to be very low risk , especially when produced from materials originating in countries free of BSE. Based upon many studies, scientists have concluded that forms of CJD other than vCJD are not associated with food consumption. |
| 6. Relative Frequency of Disease: | There are no reported human cases of vCJD or bovine cases of BSE in the United States.. In the United Kingdom, there have been 94 human cases of suspected or confirmed vCJD from 1993, when the illness was first recognized, through February 2001. Since 1986, more than 180,000 cases of BSE have occurred there in cattle, particularly dairy herds. The feeding of rendered TSE-infected animal by-products to cattle is believed to have caused the epidemic of BSE. Practices such as this have now been prohibited, resulting in a dramatic decline in the number of cases. |
| 7. Course of Disease and Complications: | Cases of vCJD usually present with psychiatric problems, such as depression. As the disease progresses, neurologic signs appear -- unpleasant sensations in the limbs and/or face. There are problems with walking and muscle coordination. Sometimes, late in the course of the disease, victims become forgetful and then experience severe problems with processing information and speaking. Uncontrolled muscle twitching and death follow. |
| 8. Target Populations: | All cases of vCJD to date have occurred in individuals of a single human genotype that is methionine homozygous at codon 139. About 40% of the total human population belongs to this methionine-methionine homozygous state. The susceptibility of other genotypes is not yet known. |
| 9. Food Analysis & Reconditioning: | No practical detection or inactivation methods exist, at present. The abnormally shaped prions are resistant to most heat and chemical treatments. Decontamination methods are so drastic that food subjected to these processes generally becomes inedible. Consequently, the key to food protection is obtaining meat from animals not infected with BSE and protecting against contamination of food with brain and spinal cord tissue. |
| 10. Selected Outbreaks: | Significant numbers of vCJD cases have occurred only in the United Kingdom; isolated cases have been reported in France, and Ireland. There have been no reported cases of vCJD in the United States. |
| 11. Education and Background Resources: | Literature references can be found at the links below. |
| CDC's Emerging Infectious Diseases | The epidemic of bovine spongiform encephalopathy in the United Kingdom, that began in 1986 and during its course affected nearly 200,000 cattle, is waning. It leaves in its wake a human outbreak of variant Creutzfeldt-Jakob disease, most probably resulting from the consumption of beef products contaminated by central nervous system tissue. Although averaging only 10-15 cases a year since its first appearance in 1994, the future magnitude and geographic distribution of this illness cannot yet be predicted. The possibility that large numbers of apparently healthy persons might be incubating the disease raises concerns about iatrogenic transmissions through instrumentation (surgery and medical diagnostic procedures) and blood and organ donations. Government agencies in many countries continue to implement new measures to minimize this risk. |
| APHIS USDA Background document on BSE | BSE has had a substantial impact on the livestock industry in the United Kingdom. The disease also has been confirmed in native-born cattle in Belgium, Denmark, France, Germany, Italy, Ireland, Liechtenstein, Luxembourg, the Netherlands, Northern Ireland, Portugal, Spain, and Switzerland. The Animal and Plant Health Inspection Service (APHIS) of the U.S. Department of Agriculture (USDA) is enforcing import restrictions and conducting surveillance for BSE to prevent this serious disease from becoming established in the United States. |
| Centers for Disease Control and Prevention Background document on CJD | Since 1996, evidence has been increasing
for a causal relationship between ongoing outbreaks in
Europe of a disease in cattle, called bovine spongiform
encephalopathy (BSE, or "mad cow disease"), and
a disease in humans, called variant Creutzfeldt-Jakob
disease (vCJD). Both disorders are inevitably fatal brain
diseases with unusually long incubation periods measured
in years, and are caused by an unconventional
transmissible agent. BSE has not been detected in United States cattle, despite active surveillance efforts by USDA that began in May 1990. The U.S. Department of Agriculture monitors BSE in the United Sates; the CDC monitors the trends and current incidence of human TSEs in the United States. |
| Food and Drug Administration | Rendered feed ingredients contaminated
with an infectious agent are believed to have been the
source of BSE infection in cattle in the United Kingdom.
Some of the feed given to cattle included remnants of the
slaughtering process, such as the brain and spinal cord,
which harbor the agent that is believed to cause BSE.
Although the material is cooked during the rendering
process, the BSE agent can survive cooking. To prevent the establishment and amplification of BSE through feed in the United States, FDA implemented a final rule that, in most cases, prohibits the feeding of mammalian protein to ruminant animals. This rule, Title 21 Part 589.2000 of the Code of Federal Regulations, became effective on August 4, 1997. |
| Food and Drug Administration |
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| Loci index for genome Homo sapiens | Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence. |
| 12. Molecular Structural Data: | |
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PrP Protein in Humans |
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PrP Protein in Cattle |
mow@cfsan.fda.gov
January 2001 with periodic updates
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