Preliminary observations on the pathogenesis of experimental bovine spongiform encephalopathy (BSE): an update

BSE risk posed by ruminant collagen and gelatine derived from bones

The European Commission requested an estimation of the BSE risk (C-, L- and H-BSE) from gelatine and collagen derived from ovine, caprine or bovine bones, and produced in accordance with Regulation (EC) No 853/2004, or Regulation (EC) No 1069/2009 and its implementing Regulation (EU) No 142/2011. A quantitative risk assessment was developed to estimate the BSE infectivity, measured in cattle oral infectious dose 50 (CoID50), in a small size batch of gelatine including one BSE-infected bovine or ovine animal at the clinical stage. The model was built on a scenario where all ruminant bones could be used for the production of gelatine and high-infectivity tissues remained attached to the skull (brain) and vertebral column (spinal cord). The risk and exposure pathways defined for humans and animals, respectively, were identified. Exposure routes other than oral via food and feed were considered and discussed but not assessed quantitatively. Other aspects were also considered as integrating evidence, like the epidemiological situation of the disease, the species barrier, the susceptibility of species to BSE and the assumption of an exponential dose-response relationship to determine the probability of BSE infection in ruminants. Exposure to infectivity in humans cannot be directly translated to risk of disease because the transmission barrier has not yet been quantified, although it is considered to be substantial, i.e. much greater amounts of infectivity would be needed to successfully infect a human and greater in the oral than in the parenteral route of exposure. The probability that no new case of BSE in the cattle or small ruminant population would be generated through oral exposure to gelatine made of ruminant bones is 99%-100% (almost certain) This conclusion is based on the current state of knowledge, the epidemiological situation of the disease and the current practices, and is also valid for collagen.

Conventional and State-of-the-Art Detection Methods of Bovine Spongiform Encephalopathy (BSE)

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to a group of diseases known as transmissible spongiform encephalopathies (TSEs). It is believed that the infectious agent responsible for prion diseases is abnormally folded prion protein (PrP^Sc), which derives from a normal cellular protein (PrP^C), which is a cell surface glycoprotein predominantly expressed in neurons. There are three different types of BSE, the classical BSE (C-type) strain and two atypical strains (H-type and L-type). BSE is primarily a disease of cattle; however, sheep and goats also can be infected with BSE strains and develop a disease clinically and pathogenically indistinguishable from scrapie. Therefore, TSE cases in cattle and small ruminants require discriminatory testing to determine whether the TSE is BSE or scrapie and to discriminate classical BSE from the atypical H- or L-type strains. Many methods have been developed for the detection of BSE and have been reported in numerous studies. Detection of BSE is mainly based on the identification of characteristic lesions or detection of the PrP^Sc in the brain, often by use of their partial proteinase K resistance properties. The objective of this paper was to summarize the currently available methods, highlight their diagnostic performance, and emphasize the advantages and drawbacks of the application of individual tests.

Large animal models for chronic wasting disease

Chronic wasting disease (CWD) is a fatal neurodegenerative prion disease of cervid species including deer, elk, moose and reindeer. The disease has shown both geographic and species expansion since its discovery in the late 1960's and is now recognized in captive and free-ranging cervid populations in North America, Asia and Europe. The facile transmission of CWD is unique among prion diseases and has resulted in growing concern for cervid populations and human public health. The development of native cervid host models with longitudinal monitoring has revealed new insights about CWD pathogenesis and transmission dynamics. More than 20 years of experimental studies conducted in these models, using biologically relevant routes of infection, have led to better understanding of many aspect of CWD infections. This review addresses some of these insights, including: (i) the temporal intra-host trafficking of CWD prions in tissues and bodily fluids, (ii) the presence of infectivity shed in bodily excretions that may help explain the facile transmission of CWD, (iii) mother-to-offspring CWD transmission, (iv) the influence of some Prnp polymorphisms on CWD susceptibility, and (vi) continued development of vaccine strategies to mitigate CWD.

Discrimination of Classical and Atypical BSE by a Distinct Immunohistochemical PrPSc Profile

Bovine spongiform encephalopathy (BSE) belongs to the group of transmissible spongiform encephalopathies and is associated with the accumulation of a pathological isoform of the host-encoded glycoprotein, designated prion protein (PrP^Sc). Classical BSE (C-type) and two atypical BSE forms (L- and H-type) are known, and can be discriminated by biochemical characteristics. The goal of our study was to identify type-specific PrP^Sc profiles by using Immunohistochemistry. In our study, brain samples from 21 cattle, intracerebrally inoculated with C-, H-, and L-type BSE, were used. In addition, the corresponding samples from three orally C-type BSE infected animals were also included. From all animals, a lesion and PrP^Sc-profiles of six brain regions were determined. The lesion profile and the neuroanatomical distribution of PrP^Sc was highly consistent between the groups, but the immunohistochemical analysis revealed a distinct PrP^Sc profile for the different BSE-types, which included both the topographic and cellular pattern of PrP^Sc. This qualitative and quantitative analysis of PrP^Sc affected structures sheds new light into the pathogenesis of the different BSE types. Furthermore, immunohistochemical characterization is supported as an additional diagnostic tool in BSE surveillance programs, especially when only formalin-fixed tissue samples are available.

Subclinical infection occurs frequently following low dose exposure to prions by blood transfusion

Infectious prion diseases have very long incubation periods, and the role that subclinical infections play in transmission, persistence and re-emergence of these diseases is unclear. In this study, we used a well-established model of vCJD (sheep experimentally infected with bovine spongiform encephalopathy, BSE) to determine the prevalence of subclinical infection following exposure by blood transfusion from infected donors. Many recipient sheep survived for years post-transfusion with no clinical signs and no disease-associated PrP (PrP^Sc) found in post mortem tissue samples by conventional tests. Using a sensitive protein misfolding cyclic amplification assay (PMCA), we found that the majority of these sheep had detectable PrP^Sc in lymph node samples, at levels approximately 10⁵-10⁶ times lower than in equivalent samples from clinically positive sheep. Further testing revealed the presence of PrP^Sc in other tissues, including brain, but not in blood samples. The results demonstrate that subclinical infection is a frequent outcome of low dose prion infection by a clinically relevant route for humans (blood transfusion). The long term persistence of low levels of infection has important implications for prion disease control and the risks of re-emergent infections in both humans and animals.

Animal prion diseases: A review of intraspecies transmission

Animal prion diseases are a group of neurodegenerative, transmissible, and fatal disorders that affect several animal species. The causative agent, prion, is a misfolded isoform of normal cellular prion protein, which is found in cells with higher concentration in the central nervous system. This review explored the sources of infection and different natural transmission routes of animal prion diseases in susceptible populations. Chronic wasting disease in cervids and scrapie in small ruminants are prion diseases capable of maintaining themselves in susceptible populations through horizontal and vertical transmission. The other prion animal diseases can only be transmitted through food contaminated with prions. Bovine spongiform encephalopathy (BSE) is the only animal prion disease considered zoonotic. However, due to its inability to transmit within a population, it could be controlled. The emergence of atypical cases of scrapie and BSE, even the recent report of prion disease in camels, demonstrates the importance of understanding the transmission routes of prion diseases to take measures to control them and to assess the risks to human and animal health.

Large-scale survey of prion protein genetic variability in scrapie disease-free goats from the United States

Scrapie is a slowly progressive neurodegenerative disease of small ruminants caused by an accumulation of an abnormal isoform of prion protein in the central nervous system. Polymorphisms of the prion protein gene (PRNP) strongly modulate scrapie resistance and incubation period in goats. The aim of this study was to identify PRNP genetic variability in goats across the United States. Blood from a total of 6,029 apparent scrapie disease-free goats from 654 operations and 19 breeds were analyzed. Sequencing of PRNP revealed 26 genotypes with different rates based on eight codons. The GG127, RR154, and QQ222 genotypes were predominant and showed a remarkably high rate across all goats. The QK222 and NS146 genotypes, known to be protective against scrapie, were found in 0.6% [with 95% CI = (0.3, 1.2)] and 22.0% [95% CI = (19.1, 25.2)] of goats, respectively. The QK222 genotype was found in 23.1% of Oberhasli goats tested, with 95%CI = (3.9, 68.7)] and 22.0% of Toggenburg goats tested with 95%CI = (9.7, 42.5)], while NS146 was found in 65.5% of Savannah goats tested, with 95%CI = (30.8, 89.9), 36.7% of Boer goats tested, with 95%CI = (33.1, 40.4), 36.3% of Nubian goats tested, with 95%CI = (27.0, 46.7)], and 35.6% of LaMancha goats tested, with 95%CI = (22.8, 50.8%). The MM142 and IM142 genotypes were found more frequently in goats on dairy operations, while the HR143, NS146, and ND146 genotypes were found more frequently in goats on meat operations. Goats in the east region had a higher percentage of goats with RH154, RQ211, and QK222 genotypes than goats in the west region. The results of this study showed high genetic variability of PRNP among the U.S. goat population, with differences by location and breed, and may serve as a rationale for development of goat breeding programs at the national level to mitigate the risk of scrapie.

Absence of classical and atypical (H- and L-) BSE infectivity in the blood of bovines in the clinical end stage of disease as confirmed by intraspecies blood transfusion

While the presence of bovine spongiform encephalopathy (BSE) infectivity in the blood of clinically affected sheep has been proven by intraspecies blood-transfusion experiments, this question has remained open in the case of BSE-affected cattle. Although the absence of infectivity can be anticipated from the restriction of the agent to neuronal tissues in this species, evidence for this was still lacking. This particularly concerns the production and use of medicinal products and other applications containing bovine blood or preparations thereof. We therefore performed a blood-transfusion experiment from cattle in the clinical end stage of disease after experimental challenge with either classical (C-BSE) or atypical (H- and l-) BSE into calves at 4–6 months of age. The animals were kept in a free-ranging group for 10 years. Starting from 24 months post-transfusion, a thorough clinical examination was performed every 6 weeks in order to detect early symptoms of a BSE infection. Throughout the experiment, the clinical picture of all animals gave no indication of a BSE infection. Upon necropsy, the brainstem samples were analysed by BSE rapid test as well as by the highly sensitive Protein Misfolding Cyclic Amplification (PMCA), all with negative results. These results add resilient data to confirm the absence of BSE infectivity in the donor blood collected from C-, H- and l-BSE-affected cattle even in the final clinical phase of the disease. This finding has important implications for the risk assessment of bovine blood and blood products in the production of medicinal products and other preparations.

Potential BSE risk posed by the use of ruminant collagen and gelatine in feed for non-ruminant farmed animals

EFSA was requested to estimate the cattle bovine spongiform encephalopathy (BSE) risk (C-, L- and H-BSE) posed by ruminant collagen and gelatine produced from raw material fit for human consumption, or from material classified as Category 3 animal by-products (ABP), to be used in feed intended for non-ruminant animals, including aquaculture animals. Three risk pathways (RP) were identified by which cattle could be exposed to ruminant feed cross-contaminated with ruminant collagen or gelatine: 1) recycled former foodstuffs produced in accordance with Regulation (EC) No 853/2004 (RP1), 2) technological or nutritional additives or 3) compound feed, produced either in accordance with Regulation (EC) No 853/2004 (RP2a) or Regulation (EU) No 142/2011 (RP2b). A probabilistic model was developed to estimate the BSE infectivity load measured in cattle oral ID 50 (CoID 50)/kg, in the gelatine produced from the bones and hide of one infected animal older than 30 months with clinical BSE (worst-case scenario). The amount of BSE infectivity (50th percentile estimate) in a member state (MS) with negligible risk status was 7.6 × 10-2 CoID 50/kg, and 3.1 × 10-4 CoID 50/kg in a MS with controlled risk status. The assessment considered the potential contamination pathways and the model results (including uncertainties) regarding the current epidemiological situation in the EU and current statutory controls. Given the estimated amount of BSE infectivity to which cattle would be exposed in a single year, and even if all the estimated undetected BSE cases in the EU were used for the production of collagen or gelatine (either using raw materials fit for human consumption or Category 3 ABP raw materials), it was concluded that the probability that no new case of BSE in the cattle population would be generated through any of the three RP is 99-100% (almost certain).

Deciphering the BSE-type specific cell and tissue tropisms of atypical (H and L) and classical BSE

After the discovery of two atypical bovine spongiform encephalopathy (BSE) forms in France and Italy designated H- and L-BSE, the question arose whether these new forms differed from classical BSE (C-BSE) in their pathogenesis. Samples collected from cattle in the clinical stage of BSE during an intracranial challenge study with L- and H-BSE were analysed using biochemical and histological methods as well as in a transgenic mouse bioassay. Our results generally confirmed what had been described for C-BSE to be true also for both atypical BSE forms, namely the restriction of the pathological prion protein (PrP^Sc) and BSE infectivity to the nervous system. However, analysis of samples collected under identical conditions from both atypical H- and L-BSE forms allowed us a more precise assessment of the grade of involvement of different tissues during the clinical end stage of disease as compared to C-BSE. One important feature is the involvement of the peripheral nervous and musculoskeletal tissues in both L-BSE and H-BSE affected cattle. We were, however, able to show that in H-BSE cases, the PrP^Sc depositions in the central and peripheral nervous system are dominated by a glial pattern, whereas a neuronal deposition pattern dominates in L-BSE cases, indicating differences in the cellular and topical tropism of both atypical BSE forms. As a consequence of this cell tropism, H-BSE seems to spread more rapidly from the CNS into the periphery via the glial cell system such as Schwann cells, as opposed to L-BSE which is mostly propagated via neuronal cells.

Detection of Pathognomonic Biomarker PrPSc and the Contribution of Cell Free-Amplification Techniques to the Diagnosis of Prion Diseases

Transmissible spongiform encephalopathies or prion diseases are rapidly progressive neurodegenerative diseases, the clinical manifestation of which can resemble other promptly evolving neurological maladies. Therefore, the unequivocal ante-mortem diagnosis is highly challenging and was only possible by histopathological and immunohistochemical analysis of the brain at necropsy. Although surrogate biomarkers of neurological damage have become invaluable to complement clinical data and provide more accurate diagnostics at early stages, other neurodegenerative diseases show similar alterations hindering the differential diagnosis. To solve that, the detection of the pathognomonic biomarker of disease, PrP^Sc, the aberrantly folded isoform of the prion protein, could be used. However, the amounts in easily accessible tissues or body fluids at pre-clinical or early clinical stages are extremely low for the standard detection methods. The solution comes from the recent development of in vitro prion propagation techniques, such as Protein Misfolding Cyclic Amplification (PMCA) and Real Time-Quaking Induced Conversion (RT-QuIC), which have been already applied to detect minute amounts of PrP^Sc in different matrixes and make early diagnosis of prion diseases feasible in a near future. Herein, the most relevant tissues and body fluids in which PrP^Sc has been detected in animals and humans are being reviewed, especially those in which cell-free prion propagation systems have been used with diagnostic purposes.

Transmissible spongiform encephalopathy in goats: is PrP rapid test sensitivity affected by genotype?

Transmissible spongiform encephalopathy (TSE) surveillance in goats relies on tests initially approved for cattle, subsequently assessed for sheep, and approval extrapolated for use in "small ruminants." The current EU-approved immunodetection tests employ antibodies against various epitopes of the prion protein PrPSc, which is encoded by the host PRNP gene. The caprine PRNP gene is polymorphic, mostly at codons different from the ovine PRNP. The EU goat population is much more heterogeneous than the sheep population, with more PRNP-related polymorphisms, and with marked breed-related differences. The ability of the current tests to detect disease-specific PrPSc generated against these different genetic backgrounds is currently assumed, rather than proven. We examined whether common polymorphisms within the goat PRNP gene might have any adverse effect on the relative performance of EU-approved rapid tests. The sample panel comprised goats from the UK, Cyprus, France, and Italy, with either experimental or naturally acquired scrapie at both the preclinical and/or unknown and clinical stages of disease. Test sensitivity was significantly lower and more variable when compared using samples from animals that were preclinical or of unknown status. However, all of the rapid tests included in our study were able to correctly identify all samples from animals in the clinical stages of disease, apart from samples from animals polymorphic for serine or aspartic acid at codon 146, in which the performance of the Bio-Rad tests was profoundly affected. Our data show that some polymorphisms may adversely affect one test and not another, as well as underline the dangers of extrapolating from other species.

Bovine Spongiform Encephalopathy - A Review from the Perspective of Food Safety

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative disease that belongs to transmissible spongiform encephalopathy (TSE). Since the first case was identified in the UK in 1986, BSE spread to other countries including Japan. Its incidence peaked in 1992 in the UK and from 2001 to 2006 in many other countries, but a feed ban aimed at eliminating the recycling of the BSE agent and other control measures aimed at preventing food and feed contamination with the agent were highly effective at reducing the spread of BSE. In 2004, two types of atypical BSE, H-type BSE (H-BSE) and L-type BSE (L-BSE), which differ from classical BSE (C-BSE), were found in France and Italy. Atypical BSE, which is assumed to occur spontaneously, has also been detected among cattle in other countries including Japan. The BSE agent including atypical BSE agent is a unique food-safety hazard with different chemical and biological properties from the microbial pathogens and toxic chemicals that contaminate food. In this review, we summarize the reported findings on the tissue distribution of BSE prions in infected cattle and other aspects of BSE, as well as the control measures against the disease employed in Japan. Topics that require further studies are discussed based on the summarized findings from the perspective of food safety.

Animal prion diseases: the risks to human health

Transmissible spongiform encephalopathies (TSEs) or prion diseases of animals notably include scrapie in small ruminants, chronic wasting disease (CWD) in cervids and classical bovine spongiform encephalopathy (C-BSE). As the transmission barrier phenomenon naturally limits the propagation of prions from one species to another, and the lack of epidemiological evidence for an association with human prion diseases, the zoonotic potential of these diseases was for a long time considered negligible. However, in 1996, C-BSE was recognized as the cause of a new human prion disease, variant Creutzfeldt-Jakob disease (vCJD), which triggered an unprecedented public health crisis in Europe. Large-scale epidemio-surveillance programs for scrapie and C-BSE that were implemented in the EU after the BSE crisis revealed that the distribution and prevalence of prion diseases in the ruminant population had previously been underestimated. They also led to the recognition of new forms of TSEs (named atypical) in cattle and small ruminants and to the recent identification of CWD in Europe. At this stage, the characterization of the strain diversity and zoonotic abilities associated with animal prion diseases remains largely incomplete. However, transmission experiments in nonhuman primates and transgenic mice expressing human PrP clearly indicate that classical scrapie, and certain forms of atypical BSE (L-BSE) or CWD may have the potential to infect humans. The remaining uncertainties about the origins and relationships between animal prion diseases emphasize the importance of the measures implemented to limit human exposure to these potentially zoonotic agents, and of continued surveillance for both animal and human prion diseases.

Low levels of classical BSE infectivity in rendered fat tissue

BSE infectivity in mesentery fat is most likely associated with embedded nervous tissue. To prove this mesentery containing celiac ganglion was taken from oral BSE infected cattle in different stages of the disease and from one control animal. Fat was rendered according to standard tallow production methods and the prion infectivity therein analysed in transgenic mouse bioassay. Rendered fat of the clinical animal revealed low infectivity levels, whereas preclinical and control animals remained negative. This study, although not representative, provides a proof of principle, indicating the potential contamination of melted mesenteric fat by embedded nervous structures during standard tallow production.

Detection of PrPres in peripheral tissue in pigs with clinical disease induced by intracerebral challenge with sheep-passaged bovine spongiform encephalopathy agent

Bovine spongiform encephalopathy (BSE) can be efficiently transmitted to pigs via intracerebral inoculation. A clear link has been established between the consumption of products of bovine origin contaminated with the BSE agent and the development of variant Creutzfeldt-Jakob disease in humans. Small ruminants can also naturally develop BSE, and sheep-adapted BSE (Sh-BSE) propagates more efficiently than cattle BSE in pigs and in mouse models expressing porcine prion protein. In addition, Sh-BSE shows greater efficiency of transmission to human models than original cow BSE. While infectivity and/or abnormal PrP accumulation have been reported in the central nervous system in BSE-infected pigs, the ability of the agent to replicate in peripheral tissues has not been fully investigated. We previously characterized the presence of prions in a panel of tissues collected at the clinical stage of disease from pigs experimentally infected with Sh-BSE. Western blot revealed low levels of PrP^res accumulation in lymphoid tissues, nerves, and skeletal muscles from 4 of the 5 animals analysed. Using protein misfolding cyclic amplification (PMCA), which we found to be 6 log fold more sensitive than direct WB for the detection of pig BSE, we confirmed the presence of the Sh-BSE agent in lymphoid organs, nerves, ileum, and striated muscles from all 5 inoculated pigs. Surprisingly, PrP^res positivity was also detected in white blood cells from one pig using this method. The presence of infectivity in lymphoid tissues, striated muscles, and peripheral nerves was confirmed by bioassay in bovine PrP transgenic mice. These results demonstrate the ability of BSE-derived agents to replicate efficiently in various peripheral tissues in pigs. Although no prion transmission has been reported in pigs following oral BSE challenge, our data support the continuation of the Feed Ban measure implemented to prevent entry of the BSE agent into the feed chain.

The use of PrP transgenic Drosophila to replace and reduce vertebrate hosts in the bioassay of mammalian prion infectivity

Prion diseases are fatal neurodegenerative conditions of humans and vertebrate species. The transmissible prion agent is a novel infectious particle composed principally of PrP Sc, an abnormal isomer of the normal host protein PrP C. The only reliable method to detect mammalian prion infectivity is by bioassay, invariably in a vertebrate host. The current prion bioassays typically involve intracerebral or peripheral inoculation of test material into the experimental host and subsequent euthanasia when clinical signs of terminal prion disease become evident. It may be months or years before the onset of clinical disease becomes evident and a pre-determined clinical end-point is reached. Consequently, bioassay of prion infectivity in vertebrate species is cumbersome, time consuming, expensive, and increasingly open to ethical debate because these animals are subjected to terminal neurodegenerative disease. Prions are a significant risk to public health through the potential for zoonotic transmission of animal prion diseases. Attention has focussed on the measurement of prion infectivity in different tissues and blood from prion-infected individuals in order to determine the distribution of infectious prions in diseased hosts. New animal models are required in order to replace or reduce, where possible, the dependency on the use of vertebrate species, including the 'gold standard' mouse prion bioassay, to assess prion infectivity levels. Here we highlight the development of a  Drosophila-based prion bioassay, a highly sensitive and rapid invertebrate animal system that can efficiently detect mammalian prions. This novel invertebrate model system will be of considerable interest to biologists who perform prion bioassays as it will promote reduction and replacement in the number of sentient animals currently used for this purpose. This article is a composite of previous methods that provides an overview of the methodology of the model and discusses the experimental data to promote its viability for use instead of more sentient hosts.

Creutzfeldt-Jakob disease and blood transfusion safety

Transmissible spongiform encephalopathies (TSEs) are untreatable, fatal neurologic diseases affecting mammals. Human disease forms include sporadic, familial and acquired Creutzfeldt-Jakob disease (CJD). While sporadic CJD (sCJD) has been recognized for near on 100 years, variant CJD (vCJD) was first reported in 1996 and is the result of food-borne transmission of the prion of bovine spongiform encephalopathy (BSE, 'mad cow disease'). Currently, 230 vCJD cases have been reported in 12 countries, the majority in the UK (178) and France (27). Animal studies demonstrated highly efficient transmission of natural scrapie and experimental BSE by blood transfusion and fuelled concern that sCJD was potentially transfusion transmissible. No such case has been recorded and case-control evaluations and lookback studies indicate that, if transfusion transmission occurs at all, it is very rare. In contrast, four cases of apparent transfusion transmission of vCJD infectivity have been identified in the UK. Risk minimization strategies in response to the threat of vCJD include leucodepletion, geographically based donor deferrals and deferral of transfusion recipients. A sensitive and specific, high-throughput screening test would provide a potential path to mitigation but despite substantial effort no such test has yet appeared. The initial outbreak of vCJD appears to be over, but concern remains about subsequent waves of disease among those already infected. There is considerable uncertainty about the size of the infected population, and there will be at least a perception of some continuing risk to blood safety. Accordingly, at least some precautionary measures will remain in place and continued surveillance is necessary.

The zoonotic potential of animal prion diseases

Bovine spongiform encephalopathy (BSE) is the only animal prion disease that has been demonstrated to be zoonotic, causing variant Creutzfeldt-Jakob disease (vCJD) in humans. The link between BSE and vCJD was established by careful surveillance, epidemiologic investigations, and experimental studies using in vivo and in vitro models of cross-species transmission. Similar approaches have been used to assess the zoonotic potential of other animal prion diseases, including atypical forms identified through active surveillance. There is no epidemiologic evidence that classical or atypical scrapie, atypical forms of BSE, or chronic wasting disease (CWD) is associated with human prion disease, but the limitations of the epidemiologic data should be taken into account when interpreting these results. Transmission experiments in nonhuman primates and human PrP transgenic mice suggest that classic scrapie, L-type atypical BSE (L-BSE), and CWD may have zoonotic potential, which for L-BSE appears to be equal to or greater than that of classic BSE. The results of in vitro conversion assays to analyze the human transmission barrier correlate well with the in vivo data. However, it is still difficult to predict the likelihood that an animal prion disease will transmit to humans under conditions of field exposure from the results of in vivo or in vitro experiments. This emphasizes the importance of continuing systematic surveillance for both human and animal prion diseases in identifying zoonotic transmission of diseases other than classic BSE.

Experimental transmission to a calf of an isolate of Spanish classical scrapie

Multiple theories exist regarding the origin of bovine spongiform encephalopathy (BSE). An early and prominent theory proposed that BSE was the result of the adaptation of sheep scrapie to cattle. The reports to date indicate that the distribution of the pathological prion protein (PrP^Sc) in experimental bovine scrapie is largely restricted to the central nervous system (CNS). Here, we describe pathological findings in a calf intracerebrally inoculated with a Spanish classical scrapie isolate. While clinical disease was observed 30 months after inoculation and PrP^Sc was detected in the CNS, the corresponding phenotype differed from that of BSE. Immunohistochemistry and PMCA also revealed the presence of PrP^Sc in the peripheral nerves, lymphoid tissues, skeletal muscle and gastrointestinal tract, suggesting centrifugal spread of the scrapie agent from the brain. To the best of our knowledge, this is the first report describing the detection of PrP^Sc in tissues other than the CNS after experimental transmission of scrapie to cattle.

Biosafety of Prions

Prions are the infectious agents that cause devastating and untreatable disorders known as Transmissible Spongiform Encephalopathies (TSEs). The pathologic events and the infectious nature of these transmissible agents are not completely understood yet. Due to the difficulties in inactivating prions, working with them requires specific recommendations and precautions. Moreover, with the advent of innovative technologies, such as the Protein Misfolding Cyclic Amplification (PMCA) and the Real Time Quaking-Induced Conversion (RT-QuIC), prions could be amplified in vitro and the infectious features of the amplified products need to be carefully assessed.

Rapid Testing for Creutzfeldt-Jakob Disease in Donors of Cornea

BACKGROUND

Creutzfeldt-Jakob disease (CJD) has been accidentally transmitted by contaminated corneal transplants. Eye donors are not ordinarily tested for CJD, in part because an easy test is not available. We propose a relatively simple postmortem procedure to collect brain samples without performing full autopsy and show that a test currently marketed for veterinary diagnosis would offer an effective screening test.

METHODS

We selected 6 brains from confirmed cases of human sporadic CJD and sampled each in triplicate (18 specimens), 28 control brains of individuals with non-CJD neurodegenerative diseases and 10 normal brains. We also applied a procedure involving retro-orbital puncture after enucleation and biopsied the frontal lobes and optic nerves of a macaque experimentally infected with variant CJD. All samples were tested with the IDEXX HerdChek BSE-Scrapie Ag Kit to detect the abnormal prion protein, PrP.

RESULTS

The test discriminated between control and CJD-infected brains. All 18 infected brain samples diluted to 0.1%, except one, showed signals above cutoff, and a number of samples were reactive at even higher dilutions. These results suggest the test could detect the low concentrations of PrP probably present in brains of donors at early stages of CJD. Our collection procedure obtained sufficient macaque brain and optic nerve tissues to detect PrP.

CONCLUSIONS

We showed that a commercial test combined with rapid sample collection might offer a practical solution to screen brains of cornea donors for evidence of CJD. Such a test might enhance safety of corneal transplants and some other tissue-derived products.

Control Points To Reduce Movement of Central Nervous System Tissue during Beef Slaughter

Consumption of central nervous system tissue (CNST) from cattle with bovine spongiform encephalopathy (BSE) is thought to cause the human neurological disease, variant Creutzfeldt-Jacob disease. To identify points of cross-contamination of beef carcasses with CNST, 55 young beef cattle were slaughtered and processed through a federally inspected multispecies abattoir. The objectives of this study were to evaluate CNST spread following the placement of a plug in the penetration site of the skull after captive bolt stunning, to evaluate cross-contamination of carcasses before and after splitting, to compare the effects of hot water pasteurization (84°C for 10 s) versus cold water wash (10°C for 30 s) for reducing CNST on the carcass, and to examine other possible sources of cross-contamination in the abattoir. Results indicated that the use of a plastic plug reduced CNST contamination near the bolt penetration site. This study also confirmed that carcass splitting resulted in an increase in CNST contamination at various areas of the carcass. Hot water pasteurization appeared to be an effective means of removing CNST contamination from carcasses in most of the areas sampled.

Intra- and Interspecies Transmission of Atypical BSE - What Can We Learn from It?

After the detection of the first cases of atypical bovine spongiform encephalopathy (BSE) more than ten years ago, the etiology, pathogenesis and agent distribution of these novel BSE forms in cattle were completely unknown. Many studies have been performed in the meantime to elucidate the pathogenic mechanisms of these diseases. A wealth of data has been accumulated regarding the distribution of the abnormal isoform of the prion protein, PrP^Sc, in tissues of affected cattle, confirming the general restriction of the PrP^Sc and agent distribution to the central and peripheral nervous system, albeit at slightly higher levels as compared to classical BSE. However, due to lack of data, the assumptions regarding the spontaneous etiology of both atypical BSE forms (H-BSE and L-BSE) and also the origin of the classical BSE epidemic are still mainly speculative. By performing subpassage experiments of both the atypical BSE forms in a variety of conventional and transgenic mice and Syrian Gold hamsters, we aimed to improve our understanding of the strain stability of these BSE forms. It turned out that under these experimental conditions, both the atypical BSE forms may alter their phenotypes and become indistinguishable from classical BSE. Information about the classical and atypical BSE strain characteristics help to improve our understanding of the correlation between all three BSE forms.

Prions and the Oral Cavity

Prion diseases have recently emerged as a significant challenge to health-care workers, including those involved in dentistry. Abnormal prion proteins are resistant to complete inactivation by conventional sterilization techniques. In the last decade, a new form of prion disease emerged in the UK, termed "variant CJD", thought to be acquired by consumption of bovine spongiform encephalopathy-contaminated food products. At present, CJD is an invariably fatal disease with no immediate prospect of treatment or vaccination. Of concern with the variant form of CJD, unlike the more classic forms of the disease, is the appearance of significant levels of infectivity outside the central nervous system. This raises concerns for the potential transmission of prion proteins via surgical procedures from individuals in the asymptomatic stage of the disease. This article reviews the existing knowledge base on the nature of prions, their distribution in oral tissues, and the implications for dental treatment.

Validation of a portable pneumatic captive bolt device as a one-step method of euthanasia for use in depopulation of feedlot cattle

OBJECTIVE

To validate the effectiveness of a penetrating captive bolt device with a built-in low-pressure air channel pithing mechanism (PCBD) as a 1-step method for euthanasia of cattle.

DESIGN

Clinical trial.

ANIMALS

66 feedlot steers and heifers (weight, 227 to 500 kg [500 to 1,100 lb]) that were not expected to survive or finish the feeding period with their cohorts.

PROCEDURES

Cattle were transported to a university facility and euthanized with the PCBD. For each calf, clinical variables were monitored and recorded immediately before and for at least 10 minutes after application of the PCBD. Following euthanasia, the head of each calf was removed and trauma to the brain and skull was assessed and scored.

RESULTS

Death was successfully achieved with the PCBD without application of an ancillary technique in all 66 cattle; however, 4 (6%) cattle required a second or third shot from the PCBD because of technical errors in its placement. All shots from the PCBD that entered the cranial vault successfully rendered cattle unconscious without a return to sensibility.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that the PCBD was an effective 1-step method of euthanasia for use in mass depopulation of feedlot cattle.

Minimum Effective Dose of Cattle and Sheep BSE for Oral Sheep Infection

The minimum dose required to cause infection of Romney and Suffolk sheep of the ARQ/ARQ or ARQ/ARR prion protein gene genotypes following oral inoculation with Romney or Suffolk a sheep Bovine spongiform encephalopathy (BSE)-derived or cattle BSE-derived agent was investigated using doses ranging from 0.0005g to 5g. ARQ/ARQ sheep which were methionine (M) / threonine (T) heterozygous or T/T homozygous at codon 112 of the Prnp gene, dosed ARQ/ARR sheep and undosed controls did not show any evidence of infection. Within groups of susceptible sheep, the minimum effective oral dose of BSE was found to be 0.05g, with higher attack rates following inoculation with the 5g dose. Surprisingly, this study found no effect of dose on survival time suggesting a possible lack of homogeneity within the inoculum. All clinical BSE cases showed PrP^d accumulation in brain; however, following cattle BSE inoculation, LRS involvement within Romney recipients was found to be significantly lower than within the Suffolk sheep inoculated group which is in agreement with previous reports.

Transmission of sheep-bovine spongiform encephalopathy to pigs

Experimental transmission of the bovine spongiform encephalopathy (BSE) agent has been successfully reported in pigs inoculated via three simultaneous distinct routes (intracerebral, intraperitoneal and intravenous). Sheep derived BSE (Sh-BSE) is transmitted more efficiently than the original cattle-BSE isolate in a transgenic mouse model expressing porcine prion protein. However, the neuropathology and distribution of Sh-BSE in pigs as natural hosts, and susceptibility to this agent, is unknown. In the present study, seven pigs were intracerebrally inoculated with Sh-BSE prions. One pig was euthanized for analysis in the preclinical disease stage. The remaining six pigs developed neurological signs and histopathology revealed severe spongiform changes accompanied by astrogliosis and microgliosis throughout the central nervous system. Intracellular and neuropil-associated pathological prion protein (PrP^Sc) deposition was consistently observed in different brain sections and corroborated by Western blot. PrP^Sc was detected by immunohistochemistry and enzyme immunoassay in the following tissues in at least one animal: lymphoid tissues, peripheral nerves, gastrointestinal tract, skeletal muscle, adrenal gland and pancreas. PrP^Sc deposition was revealed by immunohistochemistry alone in the retina, optic nerve and kidney. These results demonstrate the efficient transmission of Sh-BSE in pigs and show for the first time that in this species propagation of bovine PrP^Sc in a wide range of peripheral tissues is possible. These results provide important insight into the distribution and detection of prions in non-ruminant animals.

Mathematical Models for Estimating the Risks of Bovine Spongiform Encephalopathy (BSE)

When the bovine spongiform encephalopathy (BSE) epidemic first emerged in the United Kingdom in the mid 1980s, the etiology of animal prion diseases was largely unknown. Risk management efforts to control the disease were also subject to uncertainties regarding the extent of BSE infections and future course of the epidemic. As understanding of BSE increased, mathematical models were developed to estimate risk of BSE infection and to predict reductions in risk in response to BSE control measures. Risk models of BSE-transmission dynamics determined disease persistence in cattle herds and relative infectivity of cattle prior to onset of clinical disease. These BSE models helped in understanding key epidemiological features of BSE transmission and dynamics, such as incubation period distribution and age-dependent infection susceptibility to infection with the BSE agent. This review summarizes different mathematical models and methods that have been used to estimate risk of BSE, and discusses how such risk projection models have informed risk assessment and management of BSE. This review also provides some general insights on how mathematical models of the type discussed here may be used to estimate risks of emerging zoonotic diseases when biological data on transmission of the etiological agent are limited.

Accelerated, spleen-based titration of variant Creutzfeldt-Jakob disease infectivity in transgenic mice expressing human prion protein with sensitivity comparable to that of survival time bioassay

The dietary exposure of the human population to the prions responsible for the bovine spongiform encephalopathy (BSE) epizooty has led to the emergence of variant Creutzfeldt-Jakob disease (vCJD). This fatal, untreatable neurodegenerative disorder is a growing public health concern because the prevalence of the infection seems much greater than the disease incidence and because secondary transmission of vCJD by blood transfusion or use of blood products has occurred. A current limitation in variant CJD risk assessment is the lack of quantitative information on the infectivity of contaminated tissues. To address this limitation, we tested the potential of a transgenic mouse line overexpressing human prion protein (PrP), which was previously reported to propagate vCJD prions. Endpoint titration of vCJD infectivity in different tissues was evaluated by two different methods: (i) the "classical" bioassay, based on the appearance of clinical symptoms and the detection of pathological prion protein in tissues of the inoculated mouse, and (ii) a shortened bioassay based on the detection of the protein in the mouse spleen at defined time points. The two methods proved equally sensitive in quantifying infectivity, even after very-low-dose inoculation of infected material, but the time schedule was shortened from ~2.5 years to ~1 year with the spleen bioassay. Compared to the "gold-standard" RIII model routinely used for endpoint titration of vCJD/BSE prions, either method improved the sensitivity by >2 orders of magnitude and allowed reevaluating the infectious titer of spleen from a vCJD individual at disease end stage to >1,000-fold-higher values.

IMPORTANCE

Here, we provide key reevaluation of the infectious titer of variant CJD brain and spleen tissues. The highly sensitive, accelerated spleen-based assay should thus constitute a key advance for variant CJD epidemiological and risk assessment purposes and should greatly facilitate future titration studies, including, for example, those aimed at validating decontamination procedures. The overlooked notion that the lymphoid tissue exhibits a higher capacity than the brain to replicate prions even after low-dose infection raises new questions about the molecular and/or cellular determinant(s) involved, a key issue regarding potent silent carriers of variant CJD in the lymphoid tissue.

Advances in screening test development for transmissible spongiform encephalopathies

The blood of patients with transmissible spongiform encephalopathy or prion disease can no longer be considered free of infectivity. There have been two recent reports of highly probable transfusion-associated iatrogenic variant Creutzfeldt-Jakob disease infections, and there is supporting experimental evidence of scrapie transmission by the transfusion of blood from sheep with naturally occurring disease. In the absence of a preclinical diagnostic test for transmissible spongiform encephalopathy, the main precautionary measures undertaken by blood agencies employ donor exclusion criteria, ensuring that the number of any further iatrogenic cases will be small. The development of a sensitive, specific and reliable diagnostic test is urgently needed for early identification of infected individuals in order to ensure the safety of blood supplies. During the past 5 years, significant progress has been made in improving the sensitivity and specificity of tests using brain and lymphoreticular tissues to identify Creutzfeldt-Jakob disease-infected individuals. However, the quest for a blood test is still in its infancy and requires extensive further research.

Enhanced virulence of sheep-passaged bovine spongiform encephalopathy agent is revealed by decreased polymorphism barriers in prion protein conversion studies

Bovine spongiform encephalopathy (BSE) can be efficiently transmitted to small ruminants (sheep and goats) with certain prion protein (PrP) genotypes. Polymorphisms in PrP of both the host and donor influence the transmission efficiency of transmissible spongiform encephalopathies (TSEs) in general. These polymorphisms in PrP also modulate the PrP conversion underlying TSE agent replication. Here we demonstrate that single-round protein misfolding cyclic amplification (PMCA) can be used to assess species and polymorphism barriers at the molecular level. We assessed those within and between the ovine and bovine species in vitro using a variety of natural scrapie and experimentally generated cross-species BSE agents. These BSE agents include ovBSE-ARQ isolates (BSE derived from sheep having the ARQ/ARQ PrP genotype), and two unique BSE-derived variants: BSE passaged in VRQ/VRQ sheep and a cow BSE agent isolate generated by back-transmission of ovBSE-ARQ into its original host. PMCA allowed us to quantitatively determine PrP conversion profiles that correlated with known in vivo transmissibility and susceptibility in the two ruminant species in which strain-specific molecular signatures, like its molecular weight after protease digestion, were maintained. Furthermore, both BSE agent isolates from ARQ and VRQ sheep demonstrated a surprising transmission profile in which efficient transmissions to both sheep and bovine variants was combined. Finally, all data support the notion that ARQ-derived sheep BSE points to a significant increase in virulence compared to all other tested scrapie- and BSE-derived variants reflected by the increased conversion efficiencies of previously inefficient convertible PrP variants (including the so-called "resistant" sheep ARR variant).

IMPORTANCE

Prion diseases such as scrapie in sheep and goats, BSE in cattle, and Creutzfeldt-Jakob disease (CJD) in humans are fatal neurodegenerative diseases caused by prions. BSE is known to be transmissible to a variety of hosts, including sheep and humans. Based on the typical BSE agent strain signatures and epidemiological data, the occurrence of a novel variant of CJD in humans was linked to BSE occurrence in the United Kingdom. Measures, including genetic selection of sheep toward less susceptible PrP genotypes, have been implemented to lower the risk of BSE transmission into sheep, since the disease could potentially spread into a natural reservoir. In this study, we demonstrated using molecular PrP conversion studies that when BSE is first transmitted through sheep, the host range is modified significantly and the PrP converting potency increased, allowing the ovine BSE to transmit more efficiently than cow BSE into supposedly less susceptible hosts.

Complementary studies detecting classical bovine spongiform encephalopathy infectivity in jejunum, ileum and ileocaecal junction in incubating cattle

Recently we have described the distribution of bovine spongiform encephalopathy (BSE) infectivity and/or PrPSc in Peyer's patches (PP) of the small intestine of orally BSE infected cattle. In this follow-up study additional jejunal and ileal PP's and ileocaecal-junction tissue samples from 1, 4, and 24 months post infection (mpi) were examined by mouse (Tgbov XV) bioassay. Infectivity was demonstrated in ileal PP's 4 mpi and the distribution/extent of infectivity at 24 mpi was comparable to those seen at earlier time points, revealing no indication for a decline/clearance. These data are relevant for the definition of Specified Risk Materials in the context of the TSE legislation worldwide.

Rapid PrPScDetection in Lymphoid Tissue and Application to Scrapie Surveillance of Fallen Stock in Japan: Variable PrPScAccumulation in Palatal Tonsil in Natural Scrapie

Rapid western blot (WB) procedure for an abnormal isoform of prion protein (PrP(Sc) ) detection in lymphoid tissues was established and has been applied to the surveillance of fallen stock. In this program, brain and palatal tonsil were examined by WB and three cases of sheep scrapie were detected. While one clinically scrapie-infected sheep harbored PrP(Sc) in the brain and palatal tonsil, the two sheep in the pre-clinical stage harbored PrP(Sc) in the brain, but not in the palatal tonsil. This study shows that PrP(Sc) accumulation in palatal tonsil is variable in natural scrapie, even among genetically susceptible sheep.

Modeling of bovine spongiform encephalopathy in a two-species feedback loop

Bovine spongiform encephalopathy, otherwise known as mad cow disease, can spread when an individual cow consumes feed containing the infected tissues of another individual, forming a one-species feedback loop. Such feedback is the primary means of transmission for BSE during epidemic conditions. Following outbreaks in the European Union and elsewhere, many governments enacted legislation designed to limit the spread of such diseases via elimination or reduction of one-species feedback loops in agricultural systems. However, two-species feedback loops-those in which infectious material from one-species is consumed by a secondary species whose tissue is then consumed by the first species-were not universally prohibited and have not been studied before. Here we present a basic ecological disease model which examines the rôle feedback loops may play in the spread of BSE and related diseases. Our model shows that there are critical thresholds between the infection's expansion and decrease related to the lifespan of the hosts, the growth rate of the prions, and the amount of prions circulating between hosts. The ecological disease dynamics can be intrinsically oscillatory, having outbreaks as well as refractory periods which can make it appear that the disease is under control while it is still increasing. We show that non-susceptible species that have been intentionally inserted into a feedback loop to stop the spread of disease do not, strictly by themselves, guarantee its control, though they may give that appearance by increasing the refractory period of an epidemic's oscillations. We suggest ways in which age-related dynamics and cross-species coupling should be considered in continuing evaluations aimed at maintaining a safe food supply.

Sensitivity and specificity of a commercial BSE kit for the detection of ovine scrapie

To examine the sensitivity of a commercially available bovine spongiform encephalopathy (BSE) kit (NippIBL) for the detection of ovine scrapie, 50 scrapie-positive ovine samples from the UK, and 54 scrapie-negative ovine samples from Japan were obtain and tested using this kit. The sensitivity and specificity of NippIBL for ovine samples were 96% and 100%, respectively. The detection limit of the abnormal isoform of prion protein (PrP(Sc) ) of NippIBL was examined using diluted scrapie-positive samples. The sensitivity of NippIBL to ovine scrapie was 3-10 times superior to that of another commercial BSE diagnosis kit. Thus, the NippIBL kit proved more effective for the detection of ovine scrapie.

Performance analysis of rapid diagnostic tests on atypical bovine spongiform encephalopathy

The preferred method to determine the prevalence of bovine spongiform encephalopathy (BSE) in a country is to use immunology-based rapid-tests. Though these tests are validated to detect C-type BSE disease-associated prion (PrP(sc)), test-specific properties may influence their ability to detect H- and/or L-type BSE PrP(sc), where both are atypical from C-type PrP(sc). Molecular characterization shows atypical BSE PrP(sc) to have a different sensitivity to proteinase activity and different affinities for certain prion-specific antibodies. It is important to understand how atypical BSE PrP(sc) may affect the performance of rapid-tests, which are typically dependent on the use of specific proteases and antibodies. The current study used experimentally generated C-, H-, and L-type BSE PrP(sc) to evaluate 3 tests used in various national BSE surveillance programs: an immunochromatographic assay, a standard sandwich enzyme-linked immunosorbent assay (stndELISA), and a PrP(sc)-conformation-specific ELISA (confELISA). Although BSE PrP(sc) type had some effects on rapid-test performance, analytical sensitivity for atypical BSE PrP(sc) on all 3 platforms was not significantly compromised. When testing for atypical BSE PrP(sc), the 3 tests were able to meet the same requirements that the European Food Safety Authority set when evaluating the tests for C-type BSE PrP(sc).

Spread of classic BSE prions from the gut via the peripheral nervous system to the brain

An experimental oral bovine spongiform encephalopathy (BSE) challenge study was performed to elucidate the route of infectious prions from the gut to the central nervous system in preclinical and clinical infected animals. Tissue samples collected from the gut and the central and autonomic nervous system from animals sacrificed between 16 and 44 months post infection (mpi) were examined for the presence of the pathological prion protein (PrP(Sc)) by IHC. Moreover, parts of these samples were also bioassayed using bovine cellular prion protein (PrP(C)) overexpressing transgenic mice (Tgbov XV) that lack the species barrier for bovine prions. A distinct accumulation of PrP(Sc) was observed in the distal ileum, confined to follicles and/or the enteric nervous system, in almost all animals. BSE prions were found in the sympathetic nervous system starting at 16 mpi, and in the parasympathetic nervous system from 20 mpi. A clear dissociation between prion infectivity and detectable PrP(Sc) deposition became obvious. The earliest presence of infectivity in the brain stem was detected at 24 mpi, whereas PrP(Sc) accumulation was first detected after 28 mpi. In summary, our results decipher the centripetal spread of BSE prions along the autonomic nervous system to the central nervous system, starting already halfway in the incubation time.

Disease-associated prion protein in neural and lymphoid tissues of mink (Mustela vison) inoculated with transmissible mink encephalopathy

Transmissible spongiform encephalopathies (TSEs) are diagnosed by immunodetection of disease-associated prion protein (PrP(d)). The distribution of PrP(d) within the body varies with the time-course of infection and between species, during interspecies transmission, as well as with prion strain. Mink are susceptible to a form of TSE known as transmissible mink encephalopathy (TME), presumed to arise due to consumption of feed contaminated with a single prion strain of ruminant origin. After extended passage of TME isolates in hamsters, two strains emerge, HY and DY, each of which is associated with unique structural isoforms of PrP(TME) and of which only the HY strain is associated with accumulation of PrP(TME) in lymphoid tissues. Information on the structural nature and lymphoid accumulation of PrP(TME) in mink is limited. In this study, 13 mink were challenged by intracerebral inoculation using late passage TME inoculum, after which brain and lymphoid tissues were collected at preclinical and clinical time points. The distribution and molecular nature of PrP(TME) was investigated by techniques including blotting of paraffin wax-embedded tissue and epitope mapping by western blotting. PrP(TME) was detected readily in the brain and retropharyngeal lymph node during preclinical infection, with delayed progression of accumulation within other lymphoid tissues. For comparison, three mink were inoculated by the oral route and examined during clinical disease. Accumulation of PrP(TME) in these mink was greater and more widespread, including follicles of rectoanal mucosa-associated lymphoid tissue. Western blot analyses revealed that PrP(TME) accumulating in the brain of mink is structurally most similar to that accumulating in the brain of hamsters infected with the DY strain. Collectively, the results of extended passage in mink are consistent with the presence of only a single strain of TME, the DY strain, capable of inducing accumulation of PrP(TME) in the lymphoid tissues of mink but not in hamsters. Thus, mink are a relevant animal model for further study of this unique strain, which ultimately may have been introduced through consumption of a TSE of ruminant origin.

Experimental interspecies transmission studies of the transmissible spongiform encephalopathies to cattle: comparison to bovine spongiform encephalopathy in cattle

Prion diseases or transmissible spongiform encephalopathies (TSEs) of animals include scrapie of sheep and goats; transmissible mink encephalopathy (TME); chronic wasting disease (CWD) of deer, elk and moose; and bovine spongiform encephalopathy (BSE) of cattle. The emergence of BSE and its spread to human beings in the form of variant Creutzfeldt-Jakob disease (vCJD) resulted in interest in susceptibility of cattle to CWD, TME and scrapie. Experimental cross-species transmission of TSE agents provides valuable information for potential host ranges of known TSEs. Some interspecies transmission studies have been conducted by inoculating disease-causing prions intracerebrally (IC) rather than orally; the latter is generally effective in intraspecies transmission studies and is considered a natural route by which animals acquire TSEs. The "species barrier" concept for TSEs resulted from unsuccessful interspecies oral transmission attempts. Oral inoculation of prions mimics the natural disease pathogenesis route whereas IC inoculation is rather artificial; however, it is very efficient since it requires smaller dosage of inoculum, and typically results in higher attack rates and reduces incubation time compared to oral transmission. A species resistant to a TSE by IC inoculation would have negligible potential for successful oral transmission. To date, results indicate that cattle are susceptible to IC inoculation of scrapie, TME, and CWD but it is only when inoculated with TME do they develop spongiform lesions or clinical disease similar to BSE. Importantly, cattle are resistant to oral transmission of scrapie or CWD; susceptibility of cattle to oral transmission of TME is not yet determined.

Animal prion diseases

Prion diseases occur in many animal species, most notably in ruminants. While scrapie in sheep has been recognised for three centuries and goat scrapie has been recognised for decades, BSE in cattle is a relatively novel disease which was first diagnosed in the UK in the mid 1980s. BSE was most likely caused through dietary exposure to animal feed contaminated with prions and disease was subsequently transmitted to people. The BSE epidemic is almost at an end, but the recent identification of so called atypical forms of BSE and scrapie pose many questions about the possible spectrum of prion diseases in animals and their transmissibility to other species, including humans.The pathogenesis of animal prion diseases has been studied both in natural infections and in experimental animal models. Detection of infectivity is greatly helped by suitable rodent models, in particular transgenic mice. Clinically infected animals show characteristic neuropathology in the brain and spinal cord which is accompanied by the accumulation of a conformationally altered, protease-resistant host protein. The post-mortem diagnosis is based on the detection of this protein, PrP(Sc), but despite recent impressive developments a routine ante-mortem diagnostic test has proved elusive.There is no treatment for prion diseases in animals, but disease outbreaks are controlled through a mixture of movement restrictions on holdings, culling of affected animals and herds and, for classical scrapie in sheep, selective breeding for genetic resistance. Prions are very stable and can remain in the environment for prolonged periods. This poses serious practical questions with regard to the decontamination of infected premises. The control of BSE specifically through restrictions in animal feeding practises has been successful, but the changing spectrum of these diseases plus the economic pressures to relax feed bans and reduce levels of surveillance will require constant vigilance to safeguard animal and public health.

Risk of prion disease transmission through bovine-derived bone substitutes: a systematic review

BACKGROUND

Despite the causal association between variant Creutzfeldt - Jakob disease and bovine spongiform encephalopathy (BSE), bovine origin graft materials are widely used during dental surgical procedures. The aim of this study was to assess the risk of BSE transmission through anorganic bovine bone substitutes.

METHODS

Electronic database of MEDLINE was searched to identify relevant studies regarding our focused questions, presence of BSE prion infectivity in raw bovine bone, BSE prion inactivation by bone substitute manufacturing process, protein contents in anorganic bovine bone substitutes, and validity of current BSE diagnostic methods. Search terms yielded 1,704 titles. After title/abstract screening and duplicates removal, 36 full-text articles were screened for inclusion.

RESULTS

A total of 16 studies were included in the final analysis. No eligible studies were identified regarding the efficacy of BSE prion inactivation by the treatments used for anorganic bovine bone manufacturing. BSE infectivity and PrP(Sc) , pathological prion, were detected in bovine bone marrow and serum samples. Proteins were detected in Tutoplast® (bovine), Bio-Oss®, and tibia samples treated at the similar condition for Bio-Oss deproteinization. Inconsistent results of different BSE diagnostic tests were not unusual findings (Iwata et al. 2006; Arnold et al. 2007; Murayama et al. 2010), and a study by Balkema-Buschmann and colleagues showed an apparent discrepancy between BSE infectivity and detection of PrP(27-30), the current surrogate marker for prion disease infectivity.

CONCLUSION

This review indicates that bovine-derived graft biomaterials may carry a risk of prion transmission to patients.

Prion diseases: risks, characteristics, and infection control considerations in dentistry

Prion diseases are a group of fatal neurodegenerative diseases that are rapidly progressive and fatal, with no definite cure. There are no reported cases of prion disease transmission arising from dental procedures. Nevertheless, there is a theoretical but real risk of transmission of prion disease from dental instruments. A review was made of studies up to 2008 to provide an update of the characteristics, risk of transmission, and the infection-control implications of prions in the field of dentistry. As the prions are resistant to conventional sterilization methods, highly-specific, cross-infection control measures are required when managing patients infected with these.