Executive Summary

HISTORICAL AND MEDICAL CONTEXT

Two New Prion Diseases Arise in the 1980s

The 1985 outbreak of mad cow disease in the United Kingdom generated global awareness of a previously obscure set of neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs) (Table 2-1). Unlike all other known infectious diseases, the infectious agent of TSEs appears to be associated with an abnormally folded protein known as a prion (Prusiner, 1982).

There is no cure, prophylaxis, or fail-safe antemortem diagnostic test for TSEs, often called prion diseases. Infected hosts incubate a TSE for months to decades, and their health declines rapidly after the onset of clinical symptoms, ending in death within a period of months.

Mad cow disease, or bovine spongiform encephalopathy (BSE), became an epidemic that affected hundreds of thousands of animals in the United Kingdom and that severely harmed the country's cattle farmers and beef industry. Cases of BSE have also been reported in continental Europe, Israel, Japan, and elsewhere.

Human consumption of BSE-infected beef products gave rise to a fatal, human neurodegenerative disease called variant Creutzfeldt-Jakob disease (vCJD), which was identified in 1996 (Will et al., 1996). There were 129 definite or probable vCJD cases in the United Kingdom as of December 2, 2002 (Department of Health, United Kingdom, 2002), and a handful of cases in other countries. Estimates of the total number of people who will contract vCJD as a result of the BSE epidemic vary from hundreds to tens of thousands depending on



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Executive Summary HISTORICAL AND MEDICAL CONTEXT Two New Prion Diseases Arise in the 1980s The 1985 outbreak of mad cow disease in the United Kingdom generated global awareness of a previously obscure set of neurodegenerative diseases called transmissible spongiform encephalopathies (TSEs) (Table 2-1). Unlike all other known infectious diseases, the infectious agent of TSEs appears to be associated with an abnormally folded protein known as a prion (Prusiner, 1982). There is no cure, prophylaxis, or fail-safe antemortem diagnostic test for TSEs, often called prion diseases. Infected hosts incubate a TSE for months to decades, and their health declines rapidly after the onset of clinical symptoms, ending in death within a period of months. Mad cow disease, or bovine spongiform encephalopathy (BSE), became an epidemic that affected hundreds of thousands of animals in the United Kingdom and that severely harmed the country's cattle farmers and beef industry. Cases of BSE have also been reported in continental Europe, Israel, Japan, and elsewhere. Human consumption of BSE-infected beef products gave rise to a fatal, human neurodegenerative disease called variant Creutzfeldt-Jakob disease (vCJD), which was identified in 1996 (Will et al., 1996). There were 129 definite or probable vCJD cases in the United Kingdom as of December 2, 2002 (Department of Health, United Kingdom, 2002), and a handful of cases in other countries. Estimates of the total number of people who will contract vCJD as a result of the BSE epidemic vary from hundreds to tens of thousands depending on

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assumptions regarding the incubation period, individual susceptibility, and the level of exposure. The incubation period for another human prion disease, kuru, is 4 to 40 years (Huillard d'Aignaux et al., 2002). The origin of vCJD in prion-infected cattle raises the concern that chronic wasting disease, a prion disease spreading among North American deer and elk (Williams and Miller, 2002), could cause disease in people who consume venison from the affected regions. The European Commission has poured millions of euros into research to develop better diagnostics for TSEs, especially BSE, with modest success. Some commercial diagnostic tests for postmortem BSE detection have been developed and are used throughout the United Kingdom and Europe. The tests cannot detect prions present at low levels, however. The lack of highly sensitive, accurate, and rapid tests has led to controls such as categorical importation bans and massive culling of herds to ensure the safety of beef products. To date the U.S. Food and Drug Administration (FDA) has not received a request from any of the European companies that manufacture BSE-screening tests to approve them for human use in the United States, nor has any company based in the United States submitted any TSE screening test to FDA for approval for human use (Personal communication, D.M.Asher, FDA, July 18, 2002). However, the U.S. Department of Agriculture's Center for Veterinary Biologics has approved the use of one test produced by Bio-Rad Laboratories for the detection of chronic wasting disease (CWD) in mule deer. Congress Creates the National Prion Research Project The economic and health consequences of BSE and vCJD in Europe and the risk that U.S. military forces stationed abroad and their dependents could contract a TSE through infected beef or contaminated blood products led the U.S. Congress to pass a law establishing the National Prion Research Project (NPRP) in 2002 (Senate Committee on Appropriations, 2001). NPRP will fund research on TSEs, with special emphasis on developing an antemortem diagnostic test. Congress mandated that the U.S. Department of Defense (DOD) administer the new project, and the department delegated it to the Army's Medical Research and Material Command (MRMC). MRMC administers grants through a two-tiered process of external scientific peer review, followed by programmatic review by a multidisciplinary group of DOD and civilian experts called an integration panel.

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To complement this rigorous research management process, MRMC requested that the Institute of Medicine (IOM) assist it (Department of Army Contract DAMD17-02-C-0094, May 2002). IOM was asked to produce a report that would advise the integration panel on the most pressing areas where TSE research is needed. This report would help guide the integration panel to recommend the highest-priority research for funding. STUDY PROCESS AND INFORMATION SOURCES In June 2002 IOM formed an 11-member committee supplemented by six consultants who are internationally recognized experts in prion research. The committee members were selected for their expertise in infectious disease, prion molecular biology, microbiology, neurology, epidemiology, blood banking, veterinary medicine, and food safety. The consultants provided essential technical insight. The Committee on Transmissible Spongiform Encephalopathies: Assessment of Relevant Science and its panel of consultants convened three times between July and October 2002. The committee was charged with evaluating the state of prion science, especially as it relates to research needed in diagnostics. The members of the committee were asked to look at novel technologies that might advance diagnostics; evaluate the reagents and assays used in prion research and recommend improvements; evaluate the adequacy of the TSE research infrastructure in the United States with respect to the number of investigators, physical facilities, and training needs; suggest opportunities for collaboration with foreign investigators; evaluate the threat of TSEs to U.S. military forces with respect to their food supply, with respect to their blood supply, or in any other way; and provide advice on public health policies or surveillance programs that require new research or that might affect the military. Finally, they were asked to recommend additional research to reduce or prevent TSEs (Box 1-1, p. 18). The committee evaluated information from the sponsor, peer-reviewed journal articles provided by committee staff, and presentations by invited guests with expertise relevant to TSE diagnostics (Appendix). As part of the contract, MRMC asked the committee to produce an interim report advising the integration panel on the most promising avenues of research for developing antemortem TSE diagnostic tests. That report, presented here, is intended to help the integration panel

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prioritize submitted research proposals that pass peer review. Table ES-1, on pages 13 and 14, summarizes the committee's recommendations. PRIONS The protein playing a critical role in prion disease is called prion protein or PrP. It is encoded by the PRNP gene on chromosome 20 in humans. Like all proteins, PrP has a characteristic conformation, but under certain conditions it folds into an abnormal shape that causes fatal neurodegeneration after a long incubation period. In this report, the terms "prion" and "PrPSc" refer to the protease-resistant protein associated with prion disease.1 TSE DIAGNOSTICS Obstacles to Developing Antemortem Diagnostics for TSEs Conventional methods used to diagnose most infectious diseases, such as malaria, tuberculosis, hepatitis, and human immunodeficiency virus, fail to detect prion diseases for numerous reasons. A prion is a host protein with an altered conformation such that the immune system does not recognize it as foreign and does not produce antibodies against it. Since it lacks DNA and RNA, it cannot be identified by molecular methods such as polymerase chain reaction and other nucleic acid-based tests, nor can prions be identified by customary methods such as direct visualization under a light microscope, cultivation in the laboratory, or detection of specific antibodies or antigens by standard immunology methods. Prions are insoluble, distributed unevenly in body tissues, and found in a limited set of tissues by currently available tests. PrPSc is neurotropic, so ultimately it affects cells of nervous system tissues. Where and how PrPSc progresses through the body before its final assault on the nervous system are largely unclear, complicating the ability to locate and detect it. 1   At times an additional term, PrPres, is used synonymously with PrPSc. PrPres is abnormally folded prion protein that is highly resistant to digestion by the enzyme proteinase K (PK) and that is strongly associated with prion disease. However, unlike PrPres, PrPSc demonstrates a gradient of resistance to PK. PrPSc is associated with infectious potential and with prion disease even in circumstances where it may be sensitive to PK digestion.

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The similarities between prions and the normal host cellular protein PrPC pose a fundamental problem. Since it is normal to find PrPC in healthy individuals, detection tests must differentiate between the normal and abnormal prion protein molecules. The strategy so far has been to mix the test material with the proteinase K enzyme, which digests normal prion protein but only a portion of the abnormally folded protein. Then, various techniques, described below, detect the residual PrPSc after digestion. This process inadvertently reduces the small amount of original PrPSc captured, making it less sensitive than experimental methods that do not rely on proteinase K digestion. The fact that only small amounts of abnormal prion protein may be available for detection in accessible living tissues such as blood, urine, and cerebrospinal fluid challenges diagnosticians to develop a sufficiently sensitive test. The tests must not only differentiate between normal and abnormal prion proteins, but also, for some purposes, discriminate between one or more strains of PrPSc-a challenge resulting from basic deficiencies in the understanding of prion strain diversity and the nature of strain variation. This then introduces the ultimate objective of a prion detection test: find a single infectious unit while avoiding a falsely positive test result. Presently Available TSE Diagnostics The diagnostic assays available today are generally used only after the death of an animal or person. These assays test brain tissue, where the greatest concentrations of prions can be found. Standard histopathological and immunohistochemical techniques are used to view the tissue microscopically to see characteristic vacuoles, plaques, or other abnormal features and staining associated with prion diseases. The standard confirmatory test is the Western blot. Attempts to develop accurate, rapid, and highly sensitive antemortem tests to date have largely failed, especially for detecting prions early in the course of infection. Also, most tests still involve proteinase K digestion. The specificities and sensitivities of tests that do not use proteinase K digestion must be demonstrated further. Newer tests have seemingly improved the limits of detection, but it will take improvements in sensitivity of several orders of magnitude to reliably detect an infectious unit of the prion particle. To date, all these newer detection methods are experimental and have not been independently verified and reported. New testing methods are critically needed. Researchers have attempted a variety of novel ways to improve the sensitivities of tests for TSEs (Table 3-4, p. 54).

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Novel Approaches Will Achieve NPRP's Goals in Diagnostics The committee concluded that it will take breakthroughs to achieve the levels of sensitivity and specificity needed to detect prions in live animal and human tissues. The integration panel should support proposals for research to develop assays capable of radically superceding the quality of existing tests, which are described in Chapter 3. Recommendation: Focus funding for new assays on the proposals most likely to achieve quantum leaps in the quality of prion detection tools rather than incremental improvements to existing tests. Any efforts to improve existing tests should aim to increase their sensitivities by several orders of magnitude (at least 103). The optimal test should detect less than 1 IU of PrPScper unit of ultimate product used (e.g., 1 liter of blood or 100 grams of beef). Major improvements must await the availability of more novel testing techniques or reagents. These reagents include new types of antibodies, peptides, nucleic acids, synthetic derivatives, and chimeric molecules that can be designed to specifically target the PrPSc molecule. Recommendation: Develop novel methods and reagents that detect or bind to prions, including new antibodies, peptides, nucleic acids, synthetic derivatives, and chimeric molecules. This may lead not only to better diagnostics, but also to therapeutic and prophylactic strategies. A strategy other than the direct detection of PrPSc is to detect surrogate markers of prion infection. Cells that have been injured by prion invasion perhaps produce other unique proteins or protein mixes that can be detected. The committee determined that the rapidly expanding field of proteomics may offer new tools for the development of highly sensitive prion detection tests that use such surrogate markers. This strategy is successfully being used for the detection of certain cancers (Petricoin et al., 2002a,b), and the committee suggests that it be applied to TSEs. Recommendation: Identify surrogate markers or signatures for the detection of prions or prion diseases. The committee also sees promise in strategies for amplification of the PrPSc material before further testing (Saborio et al., 2001).

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Analogous to the polymerase chain reaction technique for amplifying small amounts of DNA, these strategies could significantly boost the power of prion diagnostics. Recommendation: Improve in vitro techniques that amplify small amounts of PrPSc to enhance the sensitivities of diagnostic tests. Cell Culture Assays In vitro culture systems have been used for prion detection with moderate success. Yet, the committee believes that these assays would hold great promise if a stable and robust cell culture assay were developed. Their speed and biological simplicity would make them very effective for testing for TSEs. The committee encourages NPRP to support the development of a cell culture assay. Recommendation: Improve techniques for propagating prions in cultured cells and develop new in vitro cell systems as a means to assay and study prions. Clinical Diagnostics Although clinical criteria for the characterization of prion diseases have been established, they are adjunctive at present. Neuroimaging offers promise as a future clinical diagnostic tool for prion diseases. The committee concluded that newer magnetic resonance imaging techniques, positron emission tomography scanning applications, and multiphoton microscopy should be developed for antemortem detection of TSEs. Multiphoton microscopy uses near-infrared light, which penetrates more deeply than visible or ultraviolet light and which permits imaging of microscopic structures within the cortex of the living animal at an extraordinarily high resolution with no apparent deleterious effects. It has been used to characterize the natural history of senile plaques and to evaluate antiplaque therapy in mouse models of Alzheimer's disease (Bacskai et al., 2001, 2002; Christie et al., 2001; Klunk et al., 2002). Similar studies could be performed with transgenic mouse models of prion disease to characterize the progression of PrPSc accumulation and localization by repeatedly imaging the same diseased brain region over time.

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Recommendation: Develop functional imaging for the presence of PrPScin brain tissue leading to an early diagnostic test similar to the imaging diagnostics being developed for Alzheimer's disease. BASIC RESEARCH IS ESSENTIAL TO DEVELOPING TSE DIAGNOSTICS The committee has determined that the main obstacle to developing sensitive, specific antemortem diagnostics for TSEs is the lack of knowledge about prions and their normal cellular isoform, PrPC (Chapter 4, pp. 71-78). Recommendation: Fund basic research to elucidate the structural features of prions, the molecular mechanisms of prion replication, the mechanisms of TSE pathogenesis, the epidemiology and natural history of TSEs, and the physiological function of PrPC. The committee believes that basic research in these five areas will supply the knowledge required to advance TSE diagnostics more quickly than applied research alone. Boxes 4-1 through 4-5 list the unanswered questions that the committee finds relevant to the development of diagnostic tools. Present models of prion conformation and tertiary structure are neither complete nor conclusive. Defining the structural differences between PrP isoforms could enable scientists to synthesize a PrPSc-specific antibody probe or aptamer. Defining the structures of PrPC and PrPSc at the sites where they interact during binding and conformational change could support the development of molecules that would block those interactions. It is believed that both the conversion of PrPC to PrPSc and the accumulation of prions depend on the help of one or more molecules (Caughey, 2001), which may be easier to detect than prions themselves. These unidentified ancillary or chaperoning factors could serve as surrogate markers for prion detection and as drug targets for TSE therapeutics and prophylaxes. Current mysteries about the pathogenesis of prion disease prevent better characterization of diagnostic targets and strategies. Explanations for those mysteries will result in tests with greater sensitivities and specificities. In addition, isolating the multiprotein complexes that

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contain prions might identify new cofactors important to the formation and stabilization of PrPSc and infectivity. Understanding the normal role of PrPC may also reveal associated molecules and pathways that are appropriate detection targets for TSE diagnostics. Investigators must clarify whether the basis for nerve cell dysfunction and death in prion disease is related to the toxicity of PrPSc, to the loss of function of PrPC as a result of its conversion to PrPSc and its aggregation during a prion infection, or to other factors. The committee concluded that focusing too much on applied, rather than basic, research accounts for the European Union's slow progress in TSE diagnostics. RESEARCH INFRASTRUCTURE The United States Must Improve Its Prion Research Infrastructure The committee determined that prion science would advance more rapidly in the United States if more investigators worked in this small research community and if more funds were consistently available. The U.S. prion research infrastructure, although of high quality, is small compared with the size of its European counterpart. Fewer than 20 principal investigators in the United States receive the full, annual allocation of funds from the National Institutes of Health (NIH) to conduct prion research. In fiscal year 2000, NIH spent only 0.16 percent ($23.86 million) of its $14.69 billion research budget on TSE research (Johnson, 2002; Kirschstein, 2001). Furthermore, 75 percent of the funds given out for TSE research go to only two laboratories (Personal communication, R.T.Johnson, special consultant to NIH on TSEs, 2002). Investigators and Facilities It has been difficult to attract new investigators to prion research for several reasons. They include the lack of available laboratory space and the high start-up costs associated with setting up a prion research laboratory. The laboratories have special containment requirements and rely on costly laboratory animals and dedicated equipment that cannot be shared with other researchers because of concerns about crosscontamination. In addition, it often takes years to reach the experimental end points because prion diseases have relatively long

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incubation periods. This makes it difficult to attract doctoral and postdoctoral fellows, whose academic programs last for a relatively short time. For these reasons, the committee determined that programs funding TSE research should attract and train more investigators and should expand the granting periods for investigators conducting bioassay research to 5 to 7 years. The committee also encourages NPRP to provide funds to increase the capacities of animal facilities and containment laboratories (biosafety levels 2 and 3) for research on prions and TSEs. Reagents Because prion biology is a relatively new science, many of the reagents and other materials used by investigators are not commercially available, so each prion laboratory has needed to produce its own reagents and materials. Consequently, standardization of these reagents between different laboratories or even at the same laboratory is lacking. As a result, the experimental conclusions reported by investigators can be difficult to replicate and easy to challenge. The committee views this as an area that would benefit from attention and funding. Specifically, the NPRP should support the establishment of a collection of reference materials and genetically engineered animals (including transgenic mice), as well as reference repositories, useful for the development of TSE diagnostics and for TSE research. All investigators involved in prion research should have access to this collection. The committee believes it would be reasonable to include the collection in existing, high-quality repositories for similar standardized reagents. The committee also recommends that the Food and Drug Administration (FDA) develop a standard set of panels of reagents that would be useful for validating the accuracies of new PrP detection tests. These panels would be used to confirm the performance characteristics of test kits before they are approved for public use, as well as to perform quality control on test kit lots before their release to the market. International Collaboration An additional strategy to improve TSE research capacity is to leverage research opportunities to work with European investigators. The committee believes that opportunities for U.S. investigators to conduct TSE research on site in a European laboratory or to work in a

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collaborative fashion with a European investigator on a joint research project are not only feasible but also highly desirable. THE RISK OF TSEs TO THE U.S. MILITARY The committee considered the risk of prion infection for members of the U.S. military and their families who are or have been deployed to Europe. The deliberation focused mainly on the possibilities of the consumption of beef products or the transfusion of blood products contaminated by prions. The food and blood supplies of deployed U.S. troops are closely controlled and generally originate at U.S. sources. However, some beef products sold in U.S. military commissaries and post exchanges (PXs) in Europe were procured from suppliers in the United Kingdom and continental Europe that later reported cases of BSE among their livestock. Some therapeutic blood is also obtained from a host nation's medical facilities in exceptional circumstances. The committee determined that the U.S. military was at an increased risk for acquiring vCJD as a result of its deployment to Europe. However, that risk was judged to be small and certainly less than that of the local population in the United Kingdom and other European countries reporting BSE. The committee recommends that existing passive surveillance systems be used to monitor the incidence of Creutzfeldt-Jakob disease (CJD) among members of the U.S. military and veterans receiving health care through the DOD and the U.S. Department of Veterans Affairs health systems. Research on TSEs in Blood The committee also concluded that more research is needed to determine the risks of acquiring vCJD and sporadic Creutzfeldt-Jakob disease (sCJD) from blood products. This risk was judged to be small but unknown. Since accurate antemortem tests for the detection of prions in humans are not available, conservative health policies have been established. Because experimental studies have demonstrated that the blood of animals can transmit prion disease by blood transfusion (Hunter et al., 2002), caution dictates that individuals who have been exposed to BSE-tainted beef products be prevented from donating blood or organs. This has created hardship for blood-servicing organizations in the United States and abroad as well as anxiety in people who have been deferred from donating blood. The committee recommends that the amount of prions in the blood of individuals with sCJD and vCJD be determined. Numerous

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experimental studies with animals indicate that that prions theoretically could be present in the blood of humans who have sCJD or vCJD, although there is no evidence that the blood of TSE-infected people contains prions. The committee also recommends that NPRP fund research to estimate the amount of PrPSc that corresponds to 1 ID50 (a dose that infects 50 percent of the population exposed to the agent) of sCJD and vCJD in human blood. Knowing both the titer of prions in human blood and the estimated size of 1 ID50 would enable the determination of whether a blood product could transmit the TSE agent to another person. Ultimately, it would be desirable to have a blood test whose PrPSc detection level was at or below the level needed to infect another person by use of a blood product. Individuals who are now deferred from donating blood simply because of their possible contact with BSE-tainted beef products might then be able to rejoin the donor pool. THE RISK OF BSE IN THE UNITED STATES Despite some assurance from a study commissioned by the U.S. Department of Agriculture that concluded that the United States is at very low risk of a BSE epidemic (Harvard Center for Risk Analysis, Harvard School of Public Health, Center for Computational Epidemiology, College of Veterinary Medicine, Tuskegee University, 2001), a recent General Accounting Office report cautioned that existing vulnerabilities could allow BSE to occur (GAO, 2002). This justifies careful attention and heightened efforts to control TSEs in the United States. CONCLUSION The recommendations in this report should equip the programmatic review panel of NPRP to prioritize the prion research proposals of scientific merit submitted in 2002. The final report of the IOM Committee on Transmissible Spongiform Encephalopathies: Assessment of Relevant Science will expand upon this report, addressing all of the committee's tasks in detail.

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TABLE ES-1 Committee Recommendations Recommendation Chapter Improving Diagnostics ➢ Focus funding for new assays on the proposals most likely to achieve quantum leaps in the quality of prion detection tools rather than incremental improvements to existing tests. Any efforts to improve existing tests should aim to increase their sensitivities by several orders of magnitude (at least 103). The optimal test should detect less than 1 IU of PrPSc per unit of ultimate product used (e.g., 1 liter of blood or 100 grams of beef). 4 ➢ Improve in vitro techniques that amplify small amounts of PrPSc to enhance the sensitivities of diagnostic tests. 4 ➢ Develop novel methods and reagents that detect or bind to prions, including new antibodies, peptides, nucleic acids, synthetic derivatives, and chimeric molecules. This may lead not only to better diagnostics, but also to therapeutic and prophylactic strategies. 4 ➢ Identify surrogate markers or signatures for the detection of prions or prion diseases. 4 ➢ Improve techniques for propagating prions in cultured cells and develop new in vitro cell systems as a means to assay and study prions. 4 ➢ Develop functional imaging for the presence of PrPSc in brain tissue leading to an early diagnostic test similar to the imaging diagnostics being developed for Alzheimer's disease. 4 Basic Research ➢ Fund basic research to elucidate the: • structural features of prions • molecular mechanisms of prion replication • mechanisms of TSE pathogenesis • epidemiology and natural history of TSEs • physiological function of PrPC 4

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Recommendation Chapter Prion Research Infrastructure ➢ Support programs that attract and train more investigators in prion disease research. In addition, for investigators conducting prion bioassay research, provide grants for 5- to 7-year periods. 5 ➢ Provide funds to increase the capacities of animal facilities and containment laboratories (biosafety levels 2 and 3) to conduct prion research. 5 ➢ Provide funding for collaborative research and training with European investigators and facilities that provide unique opportunities for prion research. 5 ➢ Establish a collection of reference materials and genetically engineered animals (including transgenic mice), as well as reference repositories, useful for the development of TSE diagnostics and for TSE research. All investigators involved in prion research must have access to this collection. It would be reasonable to include the collection in existing, high-quality repositories with similar standardized reagents. 5 ➢ The Food and Drug Administration should have panels of reference reagents available to evaluate the performance characteristics of tests to detect the prion protein and infectivity. 5 Risks to the U.S. Military ➢ Use existing passive surveillance systems to monitor the incidence of Creutzfeldt-Jakob disease and variant Creutzfeldt-Jakob disease among individuals receiving medical care from the U.S. Department of Defense and the U.S. Department of Veterans Affairs health systems. 5 ➢ Determine the amount of sCJD and vCJD prions in human blood and estimate the amount of PrPSc corresponding to one ID50 of sCJD and vCJD prions in human blood. 5

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