British Medical Bulletin

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (23) Request Permissions Disclaimer Google Scholar Articles by Hill, A. F Articles by Collinge, J. Search for Related Content PubMed PubMed Citation Articles by Hill, A. F Articles by Collinge, J. Related Collections Immunology Infectious Diseases Neurology Social Bookmarking          What's this?

British Medical Bulletin 66:161-170 (2003)
© 2003 The British Council

Subclinical prion infection in humans and animals

Andrew F Hill and John Collinge

MRC Prion Unit, Department of Neurodegenerative Disease, Institute of Neurology, London, UK

	Abstract

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
Transmission of prion diseases between mammalian species is limited by a so-called ‘species’ or ‘transmission’ barrier. Recognition of prion transmission usually relies on the appearance of clinical symptoms in inoculated animals and the interval between inoculation and appearance of clinical disease is designated incubation period. At some point during this clinically silent period, neuropathological and biochemical changes as well as accumulation of prions in the brain can be detected and this stage can be called preclinical prion disease. Recently, several lines of evidence have suggested that subclinical forms of prion disease exist, in which high levels of infectivity and PrP^Sc are found in animals that do not develop clinically apparent disease during a normal life-span. Such asymptomatic prion ‘carrier’ states challenge our current understanding of pathogenesis as well as of the molecular basis of barriers to transmission. Subclinical as well as preclinical/clinical prion disease may be relevant when analysing the risk to public health of potential sources of prion exposure.

	Introduction

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
Prion diseases of humans and animals are transmissible disorders associated with distinct neuropathological lesions and the presence in the central nervous system (CNS) of an abnormal isoform, PrP^Sc, of the host-encoded prion protein, PrP^C. PrP^Sc differs from PrP^C in being partially protease-resistant, rich in ß-sheet structure, and occurring in an aggregated form. Prion diseases are unique in that they may be at the same time inherited and transmissible. Much evidence exists to suggest that the principal or sole component of the prion is PrP^Sc, although the exact structure of the infectious particle has yet to be determined. Prions are unique infectious agents in that they do not appear to contain any nucleic acid and are relatively resistant to many physical and chemical treatments that readily inactivate conventional infectious agents. These observations led to the protein-only hypothesis of prion replication, which proposes that prions are propagated by autocatalytic conversion of PrP^C to PrP^Sc, leading to accumulation of infectivity, PrP^Sc and to the development of clinical disease¹. One of the key observations supporting the role of PrP^C in the pathogenesis of prion diseases is the demonstration that mice lacking PrP^C are resistant to infection with prions².

	Prion strains and transmission barriers

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
The existence of multiple isolates, or strains, of prions was difficult to accommodate within the protein-only hypothesis, that in its current form excludes the presence of a conventional informational molecule other than PrP. Different prion strains produce distinct phenotypes, as defined by the pattern of neuropathological lesions (the ‘lesion profile’) and length of the incubation period when inoculated into susceptible animals. Several strains of naturally occurring sheep scrapie have been isolated by biological cloning in mice³. The finding that different prion strains can be propagated in in-bred mice expressing prion protein with the same primary sequence and that at least some strains can be re-isolated unchanged after passage through an intermediate host expressing PrP^C with a different primary sequence shows that strain variation is not necessarily dependent on the primary sequence of the host prion protein gene⁴. This highlights the remarkable biology underlying these diseases, whose causative agent, a protein entity apparently devoid of a nucleic acid genome, somehow encodes heritable properties.

Several lines of evidence now suggest that prion strain information is encoded by PrP^Sc itself. For example, the hyper (HY) and drowsy (DY) strains of transmissible mink encephalopathy (TME), which can be passaged in hamsters and distinguished by their disease phenotype, differ in the physicochemical properties of PrP^Sc deposited in the brains of the affected animals. Limited proteolytic cleavage of PrP^Sc from infected hamsters, followed by Western blot analysis, reveals distinct strain-specific banding patterns⁵. The different mobilities are due to cleavage at different amino-proximal sites, whose exposure to proteinase may reflect distinct conformations of PrP^Sc characteristic for each strain^6,7. Differences in the banding pattern of protease-treated PrP^Sc are also observed in cases of CJD in humans that present with distinct phenotypes^7,8; crucially, these biochemical differences can be transmitted to PrP in transgenic mice expressing human PrP^7,9,10.

Experiments performed since the early 1960s have demonstrated that transmission of prion disease from one species to another is considerably less efficient than within species, and the term ‘species barrier’ was coined by Pattison in 1965¹¹. The effect of a species barrier is to lengthen the mean incubation period, increase the range of incubation periods and to reduce the fraction (in some examples to zero) of inoculated animals succumbing to clinical disease. When, following transmission across a species barrier, brain homogenate from the sick recipient is passaged through the same species, the incubation period shortens and becomes much more consistent. This marked difference in transmission parameters between primary and second passage is diagnostic for the existence of a species barrier and the extent of the fall provides a guide to the size of the barrier.

Several parameters are known to influence the transmissibility of prions both across and within species. These include polymorphisms in the prion protein gene that give rise to differences in PrP^C primary structure between donor and host, prion strain type, the route of inoculation (e.g. peripheral versus intracerebral) and the dose. The effect of a very substantial species barrier is that few, if any, animals succumb to disease on primary passage, and if they do so, then only after incubation periods approaching the natural life-span of the animal.

Abrogation of species barriers has been achieved using transgenic mice. Inoculation of transgenic mice that overexpress hamster PrP^C (and continue to express normal levels of mouse PrP^C) with either mouse or hamster prions results in clinical disease and the accumulation of PrP^Sc derived from mouse or hamster PrP^C, respectively¹². These findings led to the suggestion that the PrP primary structure is the major determinant of species barriers. Further experiments with transgenic mice expressing a variety of mammalian PrP genes have been useful in assessing barriers to prion disease. Transgenic mice devoid of mouse PrP but expressing human PrP lack a barrier to infection with human prions from sporadic CJD (sCJD), but not variant CJD (vCJD), cases^7,13. Such mice have been used to assess risk factors associated with human exposure to BSE and to support the view that vCJD and BSE are caused by exposure to the same prion strain^10,13.

Although a strong barrier to the development of clinical disease has frequently been observed when prions are transferred from one mammalian species to another, there are also instances where this is much less the case. An important example is the transmission, both natural and experimental, of cattle BSE prions to a wide variety of other species, where in some cases only a relatively low barrier to clinical disease is observed. This was observed in the transmission of cattle BSE into wild-type mice, which succumb to clinical disease with a low, but distinct, barrier to the development of clinical disease^10,14. On the other hand, a barrier to transmission, as determined by appearance of clinical disease, may also be due to differences in the prion strain rather than to differences in the PrP^C of donor and recipient. For example, transgenic mice devoid of mouse PrP but expressing human PrP show no species barrier to human sCJD prions but a pronounced barrier to human vCJD prions, even though the human donors (of the PRNP 129MM genotype) expressed prion proteins of identical primary structure¹⁰. Hence, it has been proposed that barriers to prion transmissibility be referred to as ‘transmission barriers’, to reflect the complex relationship between species, PrP primary structure and prion strain type, and the contribution these parameters make to the disease process¹⁵.

	Subclinical prion infection in animal models

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
It has long been argued that the incubation period of a prion disease can exceed the natural life-span of an animal¹⁶. After inoculation of mice with mouse-adapted scrapie prions, prion titres typically rise first in spleen and later in brain, and histopathological changes develop in the CNS during a long incubation period devoid of clinical symptoms. Under certain circumstances, the incubation period approaches the natural life-span of the animal and, if it were to exceed it, the brain and other tissues could harbour significant levels of infectious prions even though clinical signs of infection never appear. In reality, one can of course not determine whether or not clinical symptoms would have appeared had the animals lived longer, underlining the difficulty in distinguishing between preclinical and subclinical disease. Thus, clinically, asymptomatic animals may have significant infectious titres in brain and other tissues. However, there may also be subclinical, as distinct from such preclinical, forms of prion infection, where animals become asymptomatic carriers of infectivity and would not develop clinical disease. Such carrier states are well recognised in other infectious diseases. For example, chronic hepatitis C may in some cases persist for a life-time, as evidenced by constant replication of the virus and histopathological changes in the liver, without causing clinical symptoms and thus represents a subclinical disease state, while AIDS presents with a long preclinical phase that almost invariably results in severe clinical disease. In prion diseases, where incubation periods are extremely prolonged, distinction between subclinical and preclinical states is difficult. It certainly can be argued that animals dying after a typical life-span without clinical signs of prion disease but harbouring high levels of infectivity represent the late preclinical stage of ‘transmissions’ where the ‘incubation period’ exceeds the normal life-span. The distinction between the terms subclinical and preclinical is essentially a semantic one in this context. We defined the term subclinical prion infection operationally to refer to animals in which prion replication is occurring, but which have not developed clinical signs of prion disease during a normal life-span¹⁷.

Subclinical prion infection has been described in rodent models that exhibit a transmission barrier between hamster prions (263K or Sc237 strain) and wild-type mice. As shown by two independent studies, wild-type mice inoculated intracerebrally with high doses of 263K hamster prions do not develop clinical disease. However, 1–2 years after inoculation, the brains contain high levels of prions that, when inoculated into further mice and hamsters, cause clinical prion disease (summarised in Table 1). The PrP^Sc found in the brains of the inoculated mice was derived from mouse and not hamster PrP^C, suggesting replication of mouse PrP^Sc and not due to the presence of the residual inoculum^17,18. The prion content of the brains was significantly higher than the amount originally inoculated, showing that there had been prion replication, and approached levels found in animals at terminal stages of clinical prion disease. The fact that these mice were able to tolerate such high levels of infectious prions without showing clinical symptoms is remarkable and underlines how little we currently know about the neurotoxic pathway(s) involved in prion diseases. The findings also stress that prion infection may, unless revealed by bio-assay or by the presence of PrP^Sc, go undetected in apparently healthy animals.

View this table: [in this window] [in a new window]   Table 1 Examples of subclinical prion infection in rodent same-species and across-species models

 
The occurrence of subclinical prion infections is not restricted to instances involving species barriers. In studies investigating the role of the lymphoreticular system in the pathogenesis of prion disease, B-cell-deficient mice appeared resistant to peripheral prion infection as judged by their failure to develop clinical disease¹⁹. They were, however, susceptible to prion infection when inoculated intracerebrally, exhibiting incubation periods similar to those seen in wild-type control animals. Although peripherally challenged immunodeficient mice showed no clinical signs of scrapie, marked accumulation of PrP^Sc in their brains was observed. The subclinically infected animals harboured significant prion levels in their brain that in some cases exceeded those in terminally sick, wild-type, controls²⁰. Subclinical prion infection has also been described in Tga20 mice (transgenic mice overexpressing mouse PrP) injected with low doses of RML or ME-7 prions²¹. The response of these animals oscillated between a healthy appearance and mild scrapie symptoms. However, animals that developed an ataxic syndrome always progressed to terminal stages of disease. Surprisingly, the subclinically infected animals that were sacrificed at over 200 days after inoculation contained similar levels of infectivity as terminally sick animals. This again shows that prion titres may reach maximum levels without eliciting clinical disease. Therefore, in judging susceptibility to infection of an animal exposed to prions, it is not sufficient to monitor for clinical signs, but it is necessary to assay for PrP^Sc and/or prion infectivity.

	Subclinical prion infection and risks to public health

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
The occurrence of clinically silent prion infection, be it sub- or preclinical, has several implications for public health, most notably regarding the iatrogenic transmission from apparently healthy individuals.

Iatrogenic CJD in humans can have incubation periods longer than 30 years²². Such cases have resulted from the use of pituitary-derived hormone treatments, dura mater grafts, corneal grafts and contaminated neurosurgical instruments²². Although such cases are relatively rare, they underline the need for identifying potential sources of infection and the establishment of effective control procedures. It is possible, although not formally demonstrated, that some cases of iatrogenic CJD may have been caused by infection from silent carriers.

Of critical importance to humans is the emergence of vCJD, which has been epidemiologically and experimentally linked to dietary exposure to cattle BSE prions^10,14. In sCJD, PrP^Sc is undetectable using current methods in tissues peripheral to the CNS^23,24, and inoculation into primates of these tissues only occasionally results in clinical disease²⁵. However, the tissue distribution of PrP^Sc is starkly different in cases of vCJD, where lymphoreticular tissues such as spleen, lymph nodes and tonsils as well as specific regions of the eye show PrP^Sc positivity^23,24. This feature of vCJD has been exploited diagnostically, with tonsil biopsy testing for the presence of PrP^Sc proving to be a sensitive and specific ante-mortem test for vCJD^23,26. With the large number of individuals exposed to BSE prions and the uncertainty of the number of clinical cases of vCJD that will occur in the future, clinically silent vCJD prion infection must remain a consideration when applying risk assessments to the iatrogenic transmission of this disease. Importantly, the demonstration of infectivity and PrP^Sc in tonsil tissue provides a strong case for the use of disposable instruments in tonsillectomy procedures, one of the most common surgical procedures.

Concerns in this regard were highlighted by the demonstration that transgenic mice expressing human PrP encoding methionine at codon 129 (the PRNP genotype of all cases of vCJD recorded to date) demonstrated a very high level of subclinical infection on challenge with BSE or vCJD prions, raising the possibility that both primary BSE prion infection of humans and iatrogenic exposure to vCJD prions could lead to more subclinical infections than clinically apparent cases⁹.

The issue of clinically silent prion infection in cattle and other species exposed to BSE prions is also of obvious importance to public health. PrP^Sc can, unsurprisingly, be demonstrated in cattle with no obvious symptoms of BSE, using a Western blot assay²⁷. Another study, which examined specific regions of bovine brain with a tissue-slice-based immunoassay, also gave PrP^Sc-positive results in a screen of apparently healthy cattle²⁸. The results from experimental studies of subclinical prion disease, demonstrating that animals can harbour high levels of infectious prions in their brains without showing any clinical signs of disease, emphasises the need to test for prion infection as opposed to merely clinically-apparent prion disease in slaughtered cattle. In humans, because of the tissue distribution of PrP^Sc outside the CNS in vCJD, prevalence screening for the presence of PrP^Sc in tonsil tissue taken from routine tonsillectomies may provide information as to the number of individuals who may be incubating this disease¹⁵.

	Biological implications of subclinical forms of prion disease

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
Interesting biological implications are raised by the recognition of subclinical forms of prion infection. In view of the fact that high levels of prions and PrP^Sc accumulate in the brains of clinically healthy animals, the question of whether PrP^Sc itself is neurotoxic or not needs to be addressed. Several lines of evidence suggest that PrP^Sc itself may not be highly neurotoxic. Büeler et al. reported that by 20 weeks after inoculation with RML scrapie prions, mice carrying only one functional PrP allele accumulated levels of infectivity and PrP^Sc as high as wild-type animals, but remained healthy until almost a year after inoculation, while wild-type animals died by about 26 weeks²⁹. In another experiment, PrP over-expressing brain tissue was grafted into PrP knockout mice which were then inoculated with prions. While the grafted, PrP-overexpressing tissue developed hallmark signs of prion disease, such as spongiform change and accumulation of PrP^Sc, the surrounding tissue (in which PrP was not expressed) suffered no deleterious effects³⁰. Besides confirming the absolute requirement for PrP expression to propagate these diseases, these experiments failed to demonstrate any neurotoxicity to surrounding areas of the graft. Also, in the human prion disease fatal familial insomnia there are severe clinical symptoms although the levels of PrP^Sc are low or undetectable^31,32; some of these cases can be transmitted to susceptible animals, confirming the presence of infectious prions³³. Transmission of BSE to wild-type mice resulting in characteristic prion disease in the absence of detectable PrP^Sc has also been reported³⁴.

What these observations suggest is that PrP^Sc, as defined by its physicochemical properties of insolubility and partial resistance to protease treatment, while being an accurate marker of prion infection, may not be the neurotoxic molecule responsible for the pathogenesis of these diseases. It is possible that the neurotoxic species may be a different form of PrP, possibly an intermediate formed in the conversion of PrP^C to PrP^Sc, which remains to be defined at the molecular level^17,35. Perhaps PrP^Sc is a relatively inert end-product, with the rate of neurodegeneration governed by the rate at which the neurotoxic intermediate is formed¹⁷.

	Conclusions

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
The demonstration of PrP^Sc and infectivity in mice thought to be resistant to prions from a different species as well as clinically silent prion infection after same-species passage questions our current understanding of prion ‘species barriers’ and mechanisms of pathogenicity¹⁵. Assessment of ‘species barriers’ have relied on the failure of inoculated animals to develop clinical symptoms. From the studies discussed above, it now appears prudent to include molecular and neuropathological assessment of clinically healthy inoculated animals to exclude the possibility of subclinical prion infection. The possibility of iatrogenic transmission of vCJD is of obvious concern in public health issues and, given the results obtained from the experimental studies discussed above, warrants appropriate infection control guidelines to be considered when using surgical instruments and ‘at-risk’ tissues from apparently healthy humans for medical procedures. Further research into developing early, non-invasive diagnostic tests and defining molecular pathways involved in the long, clinically silent period of these diseases will be of great importance. Resulting insights could lead to early therapeutic strategies for these diseases and perhaps help solve the most enigmatic problem in the prion field at present – defining the neurotoxic and infectious entity at the molecular level.

	Acknowledgements

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 
We are grateful to Giovanna Mallucci and Charles Weissmann for their critical review of the manuscript. AFH is supported by grants from the National Health and Medical Research Council of Australia.

	Footnotes

 
Correspondence to: Dr Andrew F Hill, Department of Biochemistry & Molecular Biology and Department of Pathology, University of Melbourne, Parkville, Victoria 3010, Australia

	References

Top Abstract Introduction Prion strains and transmission... Subclinical prion infection in... Subclinical prion infection and... Biological implications of... Conclusions Acknowledgements References

 

1. Prusiner SB. Novel proteinaceous infectious particles cause scrapie. Science 1982; 216: 136–44[Abstract/Free Full Text]
2. Büeler H, Aguzzi A, Sailer A et al. Mice devoid of PrP are resistant to scrapie. Cell 1993; 73: 1339–47[CrossRef][Web of Science][Medline]
3. Bruce ME, Fraser H, McBride PA, Scott JR, Dickinson AG. The basis of strain variation in scrapie. In: Prusiner SB, Collinge J, Powell J, Anderton B. (eds) Prion Diseases in Human and Animals. London: Ellis Horwood, 1992: 497–508
4. Bruce M, Chree A, McConnell I, Foster J, Pearson G, Fraser H. Transmission of bovine spongiform encephalopathy and scrapie to mice: strain variation and the species barrier. Philos Trans R Soc Lond B Biol Sci 1994; 343: 405–11[Web of Science][Medline]
5. Bessen RA, Marsh RF. Biochemical and physical properties of the prion protein from two strains of the transmissible mink encephalopathy agent. J Virol 1992; 66: 2096–101[Abstract/Free Full Text]
6. Bessen RA, Marsh RF. Distinct PrP properties suggest the molecular basis of strain variation in transmissible mink encephalopathy. J Virol 1994; 68: 7859–68[Abstract/Free Full Text]
7. Collinge J, Sidle KCL, Meads J, Ironside J, Hill AF. Molecular analysis of prion strain variation and the aetiology of ‘new variant’ CJD. Nature 1996; 383: 685–90[CrossRef][Medline]
8. Parchi P, Giese A, Capellari S et al. Classification of sporadic Creutzfeldt-Jakob disease based on molecular and phenotypic analysis of 300 subjects. Ann Neurol 1999; 46: 224–33[CrossRef][Web of Science][Medline]
9. Asante EA, Linehan JM, Desbruslais M et al. BSE prions propagate as either variant CJD-like or sporadic CJD-like prion strains in transgenic mice expressing human prion protein. EMBO J 2002; 21: 6358–66[CrossRef][Web of Science][Medline]
10. Hill AF, Desbruslais M, Joiner S, Sidle KCL, Gowland I, Collinge J. The same prion strain causes vCJD and BSE. Nature 1997; 389: 448–50[CrossRef][Medline]
11. Pattison IH. Experiments with scrapie with special reference to the nature of the agent and the pathology of the disease. In: Gajdusek CJ, Gibbs CJ, Alpers MP (eds) Slow, Latent and Temperate Virus Infections, NINDB Monograph 2. Washington, DC: US Government Printing, 1965; 249–57
12. Scott M, Foster D, Mirenda C et al. Transgenic mice expressing hamster prion protein produce species-specific scrapie infectivity and amyloid plaques. Cell 1989; 59: 847–57[CrossRef][Web of Science][Medline]
13. Collinge J, Palmer MS, Sidle KCL et al. Unaltered susceptibility to BSE in transgenic mice expressing human prion protein. Nature 1995; 378: 779–83[CrossRef][Medline]
14. Bruce ME, Will RG, Ironside JW et al. Transmissions to mice indicate that ‘new variant’ CJD is caused by the BSE agent. Nature 1997; 389: 498–501[CrossRef][Medline]
15. Collinge J. Variant Creutzfeldt-Jakob disease. Lancet 1999; 354: 317–23[CrossRef][Web of Science][Medline]
16. Dickinson AG, Fraser H, Outram GW. Scrapie incubation time can exceed natural lifespan. Nature 1975; 256: 732–3[CrossRef][Medline]
17. Hill AF, Joiner S, Linehan J, Desbruslais M, Lantos PL, Collinge J. Species barrier independent prion replication in apparently resistant species. Proc Natl Acad Sci USA 2000; 97: 10248–53[Abstract/Free Full Text]
18. Race R, Raines A, Raymond GJ, Caughey B, Chesebro B. Long-term subclinical carrier state precedes scrapie replication and adaptation in a resistant species: Analogies to bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease in humans. J Virol 2001; 75: 10106–12[Abstract/Free Full Text]
19. Klein M, Frigg R, Raeber A et al. PrP expression in B-lymphocytes is not required for prion neuroinvasion. Nat Med 1998; 4: 1429–33[CrossRef][Web of Science][Medline]
20. Frigg R, Klein MA, Hegyi I, Zinkernagel RM, Aguzzi A. Scrapie pathogenesis in subclinically infected B-cell-deficient mice. J Virol 1999; 73: 9584–8[Abstract/Free Full Text]
21. Thackray AM, Klein MA, Aguzzi A, Bujdoso R. Chronic subclinical prion disease induced by low-dose inoculum. J Virol 2002; 76: 2510–7[Abstract/Free Full Text]
22. Brown P, Preece M, Brandel JP et al. Iatrogenic Creutzfeldt-Jakob disease at the millennium. Neurology 2000; 55: 1075–81[Abstract/Free Full Text]
23. Hill AF, Butterworth RJ, Joiner S et al. Investigation of variant Creutzfeldt-Jakob disease and other human prion diseases with tonsil biopsy samples. Lancet 1999; 353: 183–9[CrossRef][Web of Science][Medline]
24. Wadsworth JDF, Joiner S, Hill AF et al. Tissue distribution of protease resistant prion protein in variant CJD using a highly sensitive immuno-blotting assay. Lancet 2001; 358: 171–80[CrossRef][Web of Science][Medline]
25. Brown P, Gibbs CJJ, Rodgers Johnson P et al. Human spongiform encephalopathy: the National Institutes of Health series of 300 cases of experimentally transmitted disease. Ann Neurol 1994; 35: 513–29[CrossRef][Web of Science][Medline]
26. Hill AF, Zeidler M, Ironside J, Collinge J. Diagnosis of new variant Creutzfeldt-Jakob disease by tonsil biopsy. Lancet 1997; 349: 99–100[CrossRef][Web of Science][Medline]
27. Schaller O, Fatzer R, Stack M et al. Validation of a Western immunoblotting procedure for bovine PrP^Sc detection and its use as a rapid surveillance method for the diagnosis of bovine spongiform encephalopathy (BSE). Acta Neuropathol 1999; 98: 437–43[CrossRef][Medline]
28. Schulz-Schaeffer WJ, Tschüke S, Kranefuss N et al. The paraffin-embedded tissue blot detects PrP^Sc early in the incubation time in prion diseases. Am J Pathol 2000; 156: 51–6[Abstract/Free Full Text]
29. Büeler H, Raeber A, Sailer A, Fischer M, Aguzzi A, Weissmann C. High prion and PrP^Sc levels but delayed onset of disease in scrapie-inoculated mice heterozygous for a disrupted PrP gene. Mol Med 1994; 1: 19–30[Medline]
30. Brandner S, Isenmann S, Raeber A et al. Normal host prion protein necessary for scrapie-induced neurotoxicity. Nature 1996; 379: 339–43[CrossRef][Medline]
31. Collinge J, Owen F, Poulter M et al. Prion dementia without characteristic pathology. Lancet 1990; 336: 7–9[CrossRef][Web of Science][Medline]
32. Medori R, Montagna P, Tritschler HJ et al. Fatal familial insomnia: a second kindred with mutation of prion protein gene at codon 178. Neurology 1992; 42: 669–70[Abstract/Free Full Text]
33. Collinge J, Palmer MS, Sidle KCL et al. Transmission of fatal familial insomnia to laboratory animals. Lancet 1995; 346: 569–70[CrossRef][Web of Science][Medline]
34. Lasmezas CI, Deslys J-P, Robain O, et al. Transmission of the BSE agent to mice in the absence of detectable abnormal prion protein. Science 1997; 275: 402–5[Abstract/Free Full Text]
35. Weissmann C. A ‘unified theory’ of prion propagation. Nature 1991; 352: 679–83[CrossRef][Medline]
36. Zlotnik I. Observations on the experimental transmission of scrapie of various origins to laboratory animals. In: Gajdusek CJ, Gibbs CJ, Alpers MP (eds) Slow latent and temperate virus infections. Washington DC:Us Government Printing, 1965:237–48
37. Kimberlin RH, Walker CA. Evidence that the transmission of one source of scrapie agent to hamsters involves separation of agent strains from a mixture. J Gen Virol 1978; 39: 487–496[Abstract/Free Full Text]
38. Race R, Chesebro B. Scrapie infectivity found in resistant species. Nature 1998; 392: 770[CrossRef][Medline]

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				G. G. Kovacs and H. Budka Prion Diseases: From Protein to Cell Pathology Am. J. Pathol., March 1, 2008; 172(3): 555 - 565. [Abstract] [Full Text] [PDF]

				S. Webb, T. Lekishvili, C. Loeschner, S. Sellarajah, F. Prelli, T. Wisniewski, I. H. Gilbert, and D. R. Brown Mechanistic Insights into the Cure of Prion Disease by Novel Antiprion Compounds J. Virol., October 1, 2007; 81(19): 10729 - 10741. [Abstract] [Full Text] [PDF]

				P. Clarke, R. G Will, and A. C Ghani Is there the potential for an epidemic of variant Creutzfeldt-Jakob disease via blood transfusion in the UK? J R Soc Interface, August 22, 2007; 4(15): 675 - 684. [Abstract] [Full Text] [PDF]

				E. M. Norstrom, M. F. Ciaccio, B. Rassbach, R. Wollmann, and J. A. Mastrianni Cytosolic Prion Protein Toxicity Is Independent of Cellular Prion Protein Expression and Prion Propagation J. Virol., March 15, 2007; 81(6): 2831 - 2837. [Abstract] [Full Text] [PDF]

				I. Paspaltsis, K. Kotta, R. Lagoudaki, N. Grigoriadis, I. Poulios, and T. Sklaviadis Titanium dioxide photocatalytic inactivation of prions. J. Gen. Virol., October 1, 2006; 87(Pt 10): 3125 - 3130. [Abstract] [Full Text] [PDF]

				G. Tamguney, K. Giles, E. Bouzamondo-Bernstein, P. J. Bosque, M. W. Miller, J. Safar, S. J. DeArmond, and S. B. Prusiner Transmission of Elk and Deer Prions to Transgenic Mice J. Virol., September 15, 2006; 80(18): 9104 - 9114. [Abstract] [Full Text] [PDF]

				N. Piening, R. Nonno, M. Di Bari, S. Walter, O. Windl, U. Agrimi, H. A. Kretzschmar, and U. Bertsch Conversion Efficiency of Bank Vole Prion Protein in Vitro Is Determined by Residues 155 and 170, but Does Not Correlate with the High Susceptibility of Bank Voles to Sheep Scrapie in Vivo J. Biol. Chem., April 7, 2006; 281(14): 9373 - 9384. [Abstract] [Full Text] [PDF]

				C. Jacquemot, C. Cuche, D. Dormont, and F. Lazarini High Incidence of Scrapie Induced by Repeated Injections of Subinfectious Prion Doses J. Virol., July 15, 2005; 79(14): 8904 - 8908. [Abstract] [Full Text] [PDF]

				M. R. Scott, D. Peretz, H.-O. B. Nguyen, S. J. DeArmond, and S. B. Prusiner Transmission Barriers for Bovine, Ovine, and Human Prions in Transgenic Mice J. Virol., May 1, 2005; 79(9): 5259 - 5271. [Abstract] [Full Text] [PDF]

				P. Clarke and A. C Ghani Projections of the future course of the primary vCJD epidemic in the UK: inclusion of subclinical infection and the possibility of wider genetic susceptibility J R Soc Interface, March 22, 2005; 2(2): 19 - 31. [Abstract] [Full Text] [PDF]

				B. Lunenfeld Historical perspectives in gonadotrophin therapy Hum. Reprod. Update, November 1, 2004; 10(6): 453 - 467. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Search for citing articles in: ISI Web of Science (23) Request Permissions Disclaimer Google Scholar Articles by Hill, A. F Articles by Collinge, J. Search for Related Content PubMed PubMed Citation Articles by Hill, A. F Articles by Collinge, J. Related Collections Immunology Infectious Diseases Neurology Social Bookmarking          What's this?

Online ISSN 1471-8391 - Print ISSN 0007-1420

Copyright © 2010 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics