Limited detection of sternal bone marrow infectivity in the clinical phase of experimental bovine spongiform encephalopathy (BSE)

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.

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).

Osteoporotic Goat Spine Implantation Study Using a Synthetic, Resorbable Ca/P/S-Based Bone Substitute

One primary purpose of the present study is to clarify whether the highly porous, resorbable Ca/P/S-based bone substitute used in this study would still induce an osteoporotic bone when implanted into the osteoporotic vertebral defects of ovariectomized (OVX) goats, or the newly-grown bone would expectantly be rather healthy bone. The bone substitute material used for the study is a synthetic, 100% inorganic, highly porous and fast-resorbable Ca/P/S-based material (Ezechbone^® Granule CBS-400). The results show that the OVX procedure along with a low calcium diet and breeding away from light can successfully induce osteoporosis in the present female experimental goats. The histological examination reveals a newly-formed trabecular bone network within the surgically-created defect of the CBS-400-implanted (OVX_IP) goat. This new trabecular bone network in the OVX_IP goat appears much denser than the OVX goat and comparable to the healthy control goat. Histomorphometry show that, among all the experimental goats, the OVX_IP goat has the highest trabecular thickness and lowest trabecular bone packet prevalence. The differences in trabecular plate separation, trabecular number and trabecular bone tissue area ratio between the OVX_IP goat and the control goat are not significant, indicating that the trabecular bone architecture of the OVX_IP goat has substantially recovered to the normal level in about 6 months after implantation without signs of osteoporosis-related delay in the bone maturing process. The quick and nicely recovered trabecular architecture parameters observed in the OVX_IP goat indicate that the present Ca/P/S-based bone substitute material has a high potential to treat osteoporotic fractures.

DISCONTOOLS: Identifying gaps in controlling bovine spongiform encephalopathy

This article summarizes the 2016 update of the DISCONTOOLS project gap analysis on bovine spongiform encephalopathy (BSE), which was based on a combination of literature review and expert knowledge. Uncertainty still exists in relation to the pathogenesis, immunology and epidemiology of BSE, but provided that infected material is prohibited from entering the animal feed chain, cases should continue to decline. BSE does not appear to spread between cattle, but if new strains with this ability appear then control would be considerably more difficult. Atypical types of BSE (L-BSE and H-BSE) have been identified, which have different molecular patterns and pathology, and do not display the same clinical signs as classical BSE. Laboratory transmission experiments indicate that the L-BSE agent has zoonotic potential. There is no satisfactory conclusion regarding the origin of the BSE epidemic. C-BSE case numbers declined rapidly following strict controls banning ruminant protein in animal feed, but occasional cases still occur. It is unclear whether these more recent cases indicate inadequate implementation of the bans, or the possibility that C-BSE might occur spontaneously, as has been postulated for H- and L-BSE. All of this will have implications once existing bans and levels of surveillance are both relaxed. Immunochemical tests can only be applied post-mortem. There is no immunological basis for diagnosis in the live animal. All aspects of disease control are expensive, particularly surveillance, specified risk material removal and feed controls. There is pressure to relax feed controls, and concurrent pressure from other sources to reduce surveillance. While the cost benefit argument can be applied successfully to either of these approaches, it would be necessary to maintain the ban on intraspecies recycling and some baseline surveillance. However, the potential risk is not limited to intraspecies recycling; recycling with cross-species transmission may be an ideal way to select or/and modify properties of transmissible spongiform encephalopathies agents in the future.

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.

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.

The potential of mesenchymal stem cell in prion research

Scrapie and bovine spongiform encephalopathy are fatal neurodegenerative diseases caused by the accumulation of a misfolded protein (PrP(res)), the pathological form of the cellular prion protein (PrP(C)). For the last decades, prion research has greatly progressed, but many questions need to be solved about prion replication mechanisms, cell toxicity, differences in genetic susceptibility, species barrier or the nature of prion strains. These studies can be developed in murine models of transmissible spongiform encephalopathies, although development of cell models for prion replication and sample titration could reduce economic and timing costs and also serve for basic research and treatment testing. Some murine cell lines can replicate scrapie strains previously adapted in mice and very few show the toxic effects of prion accumulation. Brain cell primary cultures can be more accurate models but are difficult to develop in naturally susceptible species like humans or domestic ruminants. Stem cells can be differentiated into neuron-like cells and be infected by prions. However, the use of embryo stem cells causes ethical problems in humans. Mesenchymal stem cells (MSCs) can be isolated from many adult tissues, including bone marrow, adipose tissue or even peripheral blood. These cells differentiate into neuronal cells, express PrP(C) and can be infected by prions in vitro. In addition, in the last years, these cells are being used to develop therapies for many diseases, including neurodegenerative diseases. We review here the use of cell models in prion research with a special interest in the potential use of MSCs.

Detection of PrP(Sc) in peripheral tissues of clinically affected cattle after oral challenge with bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) is a fatal neurodegenerative prion disease that mainly affects cattle. Transmission of BSE to humans caused a variant form of Creutzfeldt-Jakob disease. Following infection, the protease-resistant, disease-associated isoform of prion protein (PrP(Sc)) accumulates in the central nervous system and in other tissues. Many countries have defined bovine tissues that may contain prions as specified risk materials, which must not enter the human or animal food chains and therefore must be discarded. Ultrasensitive techniques such as protein misfolding cyclic amplification (PMCA) have been developed to detect PrP(Sc) when present in minuscule amounts that are not readily detected by other diagnostic methods such as immunohistochemistry or Western blotting. This study was conducted to determine when and where PrP(Sc) can be found by PMCA in cattle orally challenged with BSE. A total of 48 different tissue samples from four cattle infected orally with BSE at various clinical stages of disease were examined using a standardized PMCA protocol. The protocol used brain homogenate from bovine PrP transgenic mice (Tgbov XV) as substrate and three consecutive rounds of PMCA. Using this protocol, PrP(Sc) was found in the brain, spinal cord, nerve ganglia, optic nerve and Peyer's patches. The presence of PrP(Sc) was confirmed in adrenal glands, as well as in mesenteric lymph nodes - a finding that was reported recently by another group. Interestingly, additional positive results were obtained for the first time in the oesophagus, abomasum, rumen and rectum of clinically affected cattle.

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.

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.

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.

Fukuoka-1 strain of transmissible spongiform encephalopathy agent infects murine bone marrow-derived cells with features of mesenchymal stem cells

BACKGROUND

The possible risk of iatrogenic transmissible spongiform encephalopathies (TSEs, prion diseases) from transplantation of marrow-derived mesenchymal stem cells (MSCs) is uncertain. While most cell lines resist infection, a few propagate TSE agents.

STUDY DESIGN AND METHODS

We generated MSC-like (MSC-L) cell cultures from bone marrow (BM) of mice inoculated with the human-derived Fukuoka-1 (Fu) strain of TSE agent. Cultured cells were characterized for various markers and cellular prion protein (PrP(C) ) by fluorescence-activated cell sorting and for PrP(C) and its pathologic TSE-associated form (PrP(TSE) ) by Western blotting (WB). Cell cultures were tested for their susceptibility to infection with Fu in vitro. The infectivity of one Fu-infected cell culture was assayed in mice.

RESULTS

BM cells from Fu-infected mice expressed neither PrP(C) nor PrP(TSE) after 3 days in culture as demonstrated by WB. Cells adherent to plastic and maintained under two different culture conditions became spontaneously immortalized and began to express PrP(C) at about the same time. One culture became transformed shortly after exposure to Fu in vitro and remained persistently infected, continuously generating PrP(TSE) through multiple passages; the infectivity of cultured cells was confirmed by intracerebral inoculation of lysates into mice. Both persistently TSE-infected and uninfected cells expressed a number of typical MSC markers.

CONCLUSION

BM-derived MSC-L cells of mice became persistently infected with the Fu agent under certain conditions in culture-conditions that differ substantially from those currently used to develop investigational human stem cell therapies.

Age-dependent windows for cohort culling in BSE herds

With the discovery of a BSE case in a herd, practice in Denmark prior to 2005 has been to cull any herd in its entirety, to avoid the risk of further cases. However, a growing dissatisfaction with this practice has led to a desire to be able to cull a minor fraction of a BSE herd only, while still removing the majority of the risk. One proposed method has been cohort culling. All animals in a certain age window around the age of the infected animal are culled, assuming that the BSE case and all other potential cases were infected at roughly the same time. This paper presents a method to characterize the animals in a BSE infected herd most at risk for developing BSE from the exposure that resulted in the index case. The results suggest that a +/-one-and-a-half year cohort cull would work well in Denmark for index BSE cases with an age of 6 years or less, but for older BSE cases the method is completely inadequate, making age-dependent windows a necessity. Formulas for calculating age-dependent windows are provided, and age-dependent windows for a herd size corresponding to a herd size equal to the 85% percentile of the Danish herds are provided in graphical form. A case study with a 9-year-old index case is presented.

Bovine spongiform encephalopathy

Bovine spongiform encephalopathy is an infectious disease of cattle that is transmitted through the consumption of meat-and-bone meal from infected cattle. The etiologic agent is an aberrant isoform of the native cellular prion protein that is a normal component of neurologic tissue. There currently are no approved tests that can detect BSE in live cattle.

Pathogenesis of experimental bovine spongiform encephalopathy (BSE): estimation of tissue infectivity according to incubation period

This paper reports the results of tissue infectivity assays of bovine spongiform encephalopathy (BSE) agent in orally exposed cattle at stages during the incubation period. Estimations of the titre of infectivity in central nervous system (CNS), certain peripheral nerve ganglia and distal ileum tissue were made according to time post exposure from the relationship between incubation period and dose for RIII mice and C57bl mice using data from titrations of brain material from cases of BSE. The rate of increase of infectivity in the bovine CNS was then estimated, taking into account these tissue infectivity titres, the variability of the brain titre of clinical field cases of BSE, and the probability density of the expected number of months before clinical onset of each infected bovine. The doubling time for CNS was shown to equal 1.2 months. The titre in the thoracic dorsal root ganglia (DRG) was, on average, approximately 1 log units less than CNS, and cervical DRG approximately 0.5 log less than thoracic DRG. The pattern of increase of infectivity in the distal ileum is that of an initial increase up to 14-18 months post exposure, followed by a decrease, which is likely to be highly variable between animals. These results will be informative for future risk assessments of BSE, especially in relation to reviewing current control measures.

Approaches to investigating transmission of spongiform encephalopathies in domestic animals using BSE as an example

Bovine spongiform encephalopathy was a novel spongiform encephalopathy, in an hitherto unaffected species, that had characteristics of a point source epidemic, with an agent that could have been incorporated into a wide variety of feedstuffs and iatrogenically administered to naïve populations, and there was early evidence that it was not restricted to bovines. It was vital to establish, albeit experimentally, which other species might be affected, and whether the epidemic could be maintained by natural transmission, if the source was removed. In contrast, scrapie has been endemic throughout Great Britain for centuries, is maintained naturally (even if we don't know exactly how) and has a known host range. The principles, process and integration of evidence from different types of studies, however, are similar for both of these transmissible spongiform encephalopathies (TSE) and can be applied to any emerging or suspected spongiform encephalopathy. This review discusses the experimental approaches used to determine TSE transmissibility and infectivity and how they relate to natural disease and control measures.

Estimating the temporal relationship between PrPSc detection and incubation period in experimental bovine spongiform encephalopathy of cattle

This study examines tissues from sequential-kill, time-course pathogenesis studies to refine estimates of the age at which disease-specific PrP (PrPSc) can first be detected in the central nervous system (CNS) and related peripheral nervous system ganglia of cattle incubating bovine spongiform encephalopathy (BSE). Such estimates are important for risk assessments of the age at which these tissues should be removed from cattle at slaughter to prevent human and animal exposure to BSE infection. Tissues were examined from cattle dosed orally with 100 or 1 g BSE-infected brain. Incubation period data for the doses were obtained from attack rate and the sequential-kill studies. A statistical model, fitted by maximum likelihood, accounted for the differences in the lognormal incubation period and the logistic probability of infection between different dose groups. Initial detection of PrPSc during incubation was invariably in the brainstem and the earliest was at 30 and 44 months post-exposure for the 100 g- and 1 g-dosed sequential-kill study groups, respectively. The point at which PrPSc in 50 % of the animals would be detected by immunohistochemistry applied to medulla–obex was estimated at 9.6 and 1.7 months before clinical onset for the 100 g- and 1 g-dosed cattle, respectively, with a low probability of detection in any of the tissues examined at more than 12 months before clinical onset. PrPSc was detected inconsistently in dorsal root ganglia, concurrent with or after detection in CNS, and not at all in certain sympathetic nervous system ganglia.

A model (BSurvE) for estimating the prevalence of bovine spongiform encephalopathy in a national herd

We developed the BSurvE spreadsheet model to estimate the true prevalence of bovine spongiform encephalopathy (BSE) in a national cattle population, and evaluate national BSE surveillance programs. BSurvE uses BSE surveillance data and demographic information about the national cattle population. The proportion of each cohort infected with BSE is found by equating the observed number of infected animals with the number expected, following a series of probability calculations and assuming a binomial distribution for the number of infected animals detected in each surveillance stream. BSurvE has been used in a series of international workshops, where analysis of national datasets demonstrated patterns of cohort infection that were consistent with infection-control activities within the country. The results also reflected the timing of known events that were high-risk for introduction of the infectious agent.

Prions in the peripheral nerves of bovine spongiform encephalopathy-affected cattle

With the use of increasingly sensitive methods for detection of the abnormal isoform of prion protein (PrPSc) and infectivity in prion diseases, it has recently been shown that parts of the peripheral nervous system (PNS) of bovine spongiform encephalopathy (BSE)-affected cattle may become infected. It has been reported that prions spread to the central nervous system (CNS) via the PNS in sheep scrapie, but the pathogenesis of BSE in cattle is less well understood. To determine whether parts of the PNS other than those implicated directly in the hypothetical pathogenetic spread of agent from the intestine to the CNS become involved before or after the CNS is affected, PrPSc distribution was investigated by a highly sensitive Western blotting technique in dorsal root ganglia, stellate ganglion, phrenic, radial and sciatic nerves, adrenal gland and CNS of cattle that were inoculated orally with BSE-affected brain and culled sequentially. In experimentally BSE-affected cattle, PrPSc was first detected in the CNS and dorsal root ganglia; subsequently, PrPSc accumulation was detected in the peripheral nerve trunks. PrPSc was also detected in the adrenal glands of cattle that showed clinical signs. No PrPSc was detected in the PNS of BSE-negative cattle. This study shows that, with respect to dorsal root ganglia, a paravertebral sympathetic ganglion and the somatic nerves examined, PrPSc is detected in the PNS during the disease course at the same time as, or after, it accumulates in the CNS.

No abnormal prion protein detected in the milk of cattle infected with the bovine spongiform encephalopathy agent

Milk specimens were collected from lactating cows that had previously been challenged with bovine spongiform encephalopathy (BSE)-infected brain at 4–6 months of age. One group of 10 animals received a single oral dose of 100 g, a second group received 1 g and the third was made up of unexposed controls. The cows were inseminated artificially, and calved at approximately 2 years of age and annually thereafter. Milking was done within the first week following calving and at 10-weekly intervals during the lactation period. Specimens were centrifuged to obtain a fraction enriched for somatic cells and these fractions were analysed for disease-associated, abnormal prion protein (PrPBSE) by using a modified commercial BSE ELISA and a different confirmatory assay. No abnormal prion protein has so far been identified in the cell fraction of milk from cattle incubating BSE by using these methods at their limits of detection.

An evaluation of United Kingdom environmental bovine spongiform encephalopathy risk assessment

As a member of the group of diseases known as transmissible spongiform encephalopathies (TSEs), bovine spongiform encephalopathy (BSE) has been causally associated with a new variant of Creutzfeldt-Jakob disease (vCJD) in humans. Given the many uncertainties on the transmission and persistence of TSE pathogens in the environment, quantitative assessment of risks to humans and animals continues to remain a public health issue. This paper reviews quantitative BSE risk assessments undertaken in the United Kingdom since 1997 to address the potential for human exposure and theoretical health risks through environmental pathways. The review focuses on how model assumptions and methodology may influence the results.

Bovine spongiform encephalopathy infectivity in greater kudu (Tragelaphus strepsiceros)

Of all the species exposed naturally to the bovine spongiform encephalopathy (BSE) agent, the greater kudu (Tragelaphus strepsiceros), a nondomesticated bovine from Africa, appears to be the most susceptible to the disease. We present the results of mouse bioassay studies to show that, contrary to findings in cattle with BSE in which the tissue distribution of infectivity is the most limited recorded for any of the transmissible spongiform encephalopathies (TSE), infectivity in greater kudu with BSE is distributed in as wide a range of tissues as occurs in any TSE. BSE agent was also detected in skin, conjunctiva, and salivary gland, tissues in which infectivity has not previously been reported in any naturally occurring TSE. The distribution of infectivity in greater kudu with BSE suggests possible routes for transmission of the disease and highlights the need for further research into the distribution of TSE infectious agents in other host species.

TSE risk assessments: a decision support tool

The paper presents a practical approach to assessing the risks of exposure to the infective agents from transmissible spongiform encephalopathies. Current data on the infectivity of bovine tissues have been reviewed and values for use in risk assessment studies proposed for the infectivity of central nervous system and other tissues, the development of infectivity through the incubation period and the cattle-human species barrier. A study to assess the risk of exposure to BSE infectivity in cattle disposed of during the 2001 foot and mouth epidemic in the UK by burning in pyres or burial is used to illustrate the application of TSE risk assessment and the way in which it can be used as an input to decision making.

Studies of the transmissibility of the agent of bovine spongiform encephalopathy to pigs

Studies to test the transmissibility of the bovine spongiform encephalopathy (BSE) agent to pigs began in 1989. Parenteral inoculation of the agent by three routes simultaneously (intracranially, intravenously and intraperitoneally) produced disease with an incubation period range of 69-150 weeks. Pre-clinical pathological changes were detected in two pigs killed electively at 105 and 106 weeks post-inoculation. Infectivity was detected by bioassay in inbred mice in the CNS of those pigs that developed spongiform encephalopathy. Infectivity was also found in the stomach, jejunum, distal ileum and pancreas of terminally affected pigs. These findings show that pigs are susceptible to BSE. In contrast, disease failed to occur in pigs retained for 7 years after exposure by feeding BSE-affected brain on three separate days, at 1-2 week intervals. The amounts fed each day were equivalent to the maximum daily intake of meat and bone meal in rations for pigs aged 8 weeks. No infectivity was found in tissues assayed from the pigs exposed orally. This included tissues of the alimentary tract. It is suggested that these pigs did not become infected. The relatively high oral exposure used in these experiments compared with feed-borne exposure in the field may explain the absence of an epidemic of spongiform encephalopathy in domestic pigs concurrent with the BSE epidemic in the UK.

Bovine spongiform encephalopathy (BSE) and its epidemiology

Since the recognition of BSE in 1986, over 180,000 cattle in the UK have developed the disease and 1-3 million are likely to have been infected with the BSE agent, most of which were slaughtered for human consumption before developing signs of the disease. The origin of the first case of BSE is unknown, but the epidemic was caused by the recycling of processed waste parts of cattle, some of which were infected with the BSE agent, to other cattle in feed. Control measures have resulted in the consistent decline of the epidemic in the UK since 1992. Cattle and feed exported from the UK have seeded smaller epidemics in other European countries, where control measures were applied later. If the control measures now in place to protect public and animal health are well enforced, the epidemic in cattle should be largely under control and any remaining risk to humans through the consumption of beef should be very small.

Prions, BSE and food

Biochemical and biophysical properties of prions including possible inactivation methods are reviewed. Possible molecular markers of transmissible spongiform encephalopathy (TSE) and mechanisms behind infectivity and correlation with clinical symptoms are discussed. The risk of Bovine Spongiform Encephalopathy (BSE) for humans i.e. variant Creutzfeldt-Jakob Disease (cCJD) is addressed in detail. The consequences of the emergence of the new cCJD and the lack of information on the infectivity of cCJD at the clinical stage of the disease in relation to the need to reconsider the biological concepts currently used in microbiology.

Tissue distribution of protease resistant prion protein in variant Creutzfeldt-Jakob disease using a highly sensitive immunoblotting assay

BACKGROUND

Variant Creutzfeldt-Jakob disease (vCJD) has a pathogenesis distinct from other forms of human prion disease: disease-related prion protein (PrP(Sc)) is readily detectable in lymphoreticular tissues. Quantitation of risk of secondary transmission, and targeting of risk reduction strategies, is limited by lack of knowledge about relative prion titres in these and other peripheral tissues, the unknown prevalence of preclinical vCJD, and a transmission barrier which limits the sensitivity of bioassay. We aimed to improve immunoblotting methods for high sensitivity detection of PrP(Sc) to investigate the distribution of PrP(Sc) in a range of vCJD tissues.

METHODS

We obtained tissues at necropsy from four patients with neuropathologically confirmed vCJD and from individuals without neurological disease. Tissues were analysed by sodium phosphotungstic acid precipitation of PrP(Sc) and western blotting using high sensitivity enhanced chemiluminescence.

FINDINGS

We could reliably detect PrP(Sc) in the equivalent of 50 nL 10% vCJD brain homogenate, with a maximum limit of detection equivalent to 5 nl. PrP(Sc) could be detected in tissue homogenates when present at concentrations 10(4)-10(5) fold lower than those reported in brain. Tonsil, spleen, and lymph node were uniformly positive for PrP(Sc) at concentrations in the range of 0.1-15% of those found in brain: the highest concentrations were consistently seen in tonsil. PrP(Sc) was readily detected in the retina and proximal optic nerve of vCJD eye at levels of 2.5 and 25%, respectively of those found in brain. Other peripheral tissues studied were negative for PrP(Sc) with the exception of low concentrations in rectum, adrenal gland, and thymus from a single patient with vCJD. vCJD appendix and blood (Buffy coat fraction) were negative for PrP(Sc) at this level of assay sensitivity.

INTERPRETATION

We have developed a highly sensitive immunoblot method for detection of PrP(Sc) in vCJD tissues that can be used to provide an upper limit on PrP(Sc) concentrations in peripheral tissues, including blood, to inform risk assessment models. Rectal and other gastrointestinal tissues should be further investigated to assess risk of iatrogenic transmission via biopsy instruments. Ophthalmic surgical instruments used in procedures involving optic nerve and the posterior segment of the eye, in particular the retina, might represent a potential risk for iatrogenic transmission of vCJD. Tonsil is the tissue of choice for diagnostic biopsy and for population screening of surgical tissues to assess prevalence of preclinical vCJD infection within the UK and other populations.

Inconsistent detection of PrP in extraneural tissues of cats with feline spongiform encephalopathy

Feline spongiform encephalopathy (FSE), a transmissible spongiform encephalopathy or prion disease of cats, first reported in Great Britain in 1990, is believed to result from the consumption of food contaminated by the agent of bovine spongiform encephalopathy (BSE). The accumulation of PrP in non-neural tissues of cats diagnosed as suffering from FSE was investigated by immunohistochemistry. In the majority of the cats no disease-specific PrP was detected in lymphoid tissues. Small amounts of PrP were detected in the spleen of only two of 13 samples examined, in Peyer's patches of one of the two cases for which suitable material was available, but in the myenteric plexus of all four cats in which sections of intestine were examined. In addition PrP immunostaining was found in the kidney of all the cats with FSE whose kidneys were examined.

A novel erythroid-specific marker of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE) are a group of invariably fatal neurodegenerative diseases and include scrapie in sheep, bovine spongiform encephalopathy (BSE) in cattle, chronic wasting disease in deer and elk, and Kuru disease, Creutzfeldt-Jakob disease (CJD) and variant CJD in humans. The pathological effects of disease occur predominantly in the CNS (central nervous system), where common hallmarks include vacuolation, gliosis, accumulation of a protease-resistant, abnormally folded isoform of the prion protein (PrPSc) and neuronal cell death. Lack of understanding of the molecular mechanisms underlying disease pathogenesis, particularly in non-CNS tissues, means that there are currently no effective strategies for early diagnosis or therapeutic intervention of TSEs. Here we report the first identification of a molecular marker that is easily detectable in readily accessible tissues. We demonstrate that a dramatic decrease in expression of a transcript specific to erythroid lineage cells is a common feature of TSEs. Our findings indicate a previously unrecognized role for involvement of the erythroid lineage in the etiology of TSE pathogenesis and should provide a new focus for research into diagnostic and therapeutic strategies.