Bovine spongiform encephalopathy: the effect of oral exposure dose on attack rate and incubation period in cattle

Chronic Wasting Disease (CWD) in Cervids and the Consequences of a Mutable Protein Conformation

Chronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose, etc.). It spreads readily from CWD-contaminated environments and among wild cervids. As of 2022, North American CWD has been found in 29 states, four Canadian provinces and South Korea. The Scandinavian form of CWD originated independently. Prions propagate their pathology by inducing a natively expressed prion protein (PrPC) to adopt the prion conformation (PrPSc). PrPC and PrPSc differ solely in their conformation. Like other prion diseases, transmissible CWD prions can arise spontaneously. The CWD prions can respond to selection pressures resulting in the emergence of new strain phenotypes. Annually, 11.5 million Americans hunt and harvest nearly 6 million deer, indicating that CWD is a potential threat to an important American food source. No tested CWD strain has been shown to be zoonotic. However, this may not be true for emerging strains. Should a zoonotic CWD strain emerge, it could adversely impact the hunting economy and game meat consumers.

The Microscopic Detection of Animal Proteins in Animal Feed Regarding Bovine Spongiform Encephalopathy

Due to the actuality of spongiform encephalopathies and their proven spreading by means of animal feed containing meat and bone meal, the description and measurement of osteocytic lacunae contributes to more easily distinguish bone fragments in meat and bone meal. Transmissible spongiform encephalopathies (TSEs) have attracted a lot of attention, especially after 1986, when the first case of BSE (bovine spongiform encephalopathy) was detected. Since the outbreak of spongiform encephalopathy (BSE), the use of animal protein including bone meal as an ingredient in animal feed has been controlled by several regulations including Regulation (EC) 999/2001, Regulation (EC) 1774/2002, and Regulation (EC) 1234/2003. The classical microscopic method is the only official method for detecting animal protein in animal feed in the European Union (Commission Regulation (EC) 152/2009). By applying the microscopic method to the animal feed samples, we performed detection in order to determine the presence of animal proteins that originate from mammals and fish. The microscopic analysis of all 421 samples, of which 115 were raw materials for the production of animal feed, 230 were concentrates for ruminant nutrition and 76 were concentrates for non-ruminant nutrition (32 concentrates for laying hens and 44 concentrates for pigs), did not provide positive results, that is, no remains of animal tissues of mammalian origin were found in any specimen. Whereas in 10 out of 32 (31.25%) concentrates intended for non-ruminant nutrition (laying hens), pieces of fish tissue were found. In these samples, we usually detected the presence of fish bones, gills and scales.

Prion Infectivity and PrPBSE in the Peripheral and Central Nervous System of Cattle 8 Months Post Oral BSE Challenge

After oral exposure of cattle with classical bovine spongiform encephalopathy (C-BSE), the infectious agent ascends from the gut to the central nervous system (CNS) primarily via the autonomic nervous system. However, the timeline of this progression has thus far remained widely undetermined. Previous studies were focused on later time points after oral exposure of animals that were already 4 to 6 months old when challenged. In contrast, in this present study, we have orally inoculated 4 to 6 weeks old unweaned calves with high doses of BSE to identify any possible BSE infectivity and/or PrP^BSE in peripheral nervous tissues during the first eight months post-inoculation (mpi). For the detection of BSE infectivity, we used a bovine PrP transgenic mouse bioassay, while PrP^BSE depositions were analyzed by immunohistochemistry (IHC) and by protein misfolding cyclic amplification (PMCA). We were able to show that as early as 8 mpi the thoracic spinal cord as well as the parasympathetic nodal ganglion of these animals contained PrP^BSE and BSE infectivity. This shows that the centripetal prion spread starts early after challenge at least in this age group, which represents an essential piece of information for the risk assessments for food, feed, and pharmaceutical products produced from young calves.

Epidemiological verification of the mechanism of occurrence of atypical L-type bovine spongiform encephalopathy

Since 2004, a novel bovine spongiform encephalopathy (BSE), distinct from the conventional 'classical BSE' (C-BSE), has been reported as an atypical BSE. Atypical BSE is detected mostly in aged cattle, and it is suggested that atypical BSE may occur spontaneously. Relaxation of the relevant countermeasures such as feed ban, which prevents the use of bovine meat-and-bone meal as feed, has been discussed in recent years owing to the decrease in C-BSE cases. If atypical BSE occurs spontaneously without exposure to an agent called abnormal prion protein (PrP^Sc ), complete removal of these measures will be difficult. In this study, we verified the possibility that L-BSE, which is a subtype of atypical BSE, occurs spontaneously. We first hypothesized that L-BSE occurs only through the process of infection via oral exposure. If the hypothesis was true, the infection of L-BSE would be mostly limited to calves under 1 year of age due to their high susceptibility, and the feed ban would effectively reduce the number of infected calves by birth cohort. Thus, we created a mathematical model to estimate the number of infected calves by birth cohort and compared the effectiveness of the feed ban on C-BSE and L-BSE. The number of tested animals and detected cases in nine European countries were used for this analysis. Our results showed that the estimated number of infected calves in the birth cohort indicated that feed ban was less effective on L-BSE. This result supports the alternative hypothesis that at least a part of the L-BSE can occur without infection via oral exposure. Our results suggest that the complete abolition of countermeasures, such as feed ban, should be discussed carefully. As for the occurrence mechanism, although there remains uncertainty to reach conclusions, it is reasonable to assume that L-BSE can occur spontaneously at present.

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).

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.

Updated quantitative risk assessment (QRA) of the BSE risk posed by processed animal protein (PAP)

EFSA was requested: to assess the impact of a proposed quantitative real-time polymerase chain reaction (qPCR) 'technical zero' on the limit of detection of official controls for constituents of ruminant origin in feed, to review and update the 2011 QRA, and to estimate the cattle bovine spongiform encephalopathy (BSE) risk posed by the contamination of feed with BSE-infected bovine-derived processed animal protein (PAP), should pig PAP be re-authorised in poultry feed and vice versa, using both light microscopy and ruminant qPCR methods, and action limits of 100, 150, 200, 250 and 300 DNA copies. The current qPCR cannot discriminate between legitimately added bovine material and unauthorised contamination, or determine if any detected ruminant material is associated with BSE infectivity. The sensitivity of the surveillance for the detection of material of ruminant origin in feed is currently limited due to the heterogeneous distribution of the material, practicalities of sampling and test performance. A 'technical zero' will further reduce it. The updated model estimated a total BSE infectivity four times lower than that estimated in 2011, with less than one new case of BSE expected to arise each year. In the hypothetical scenario of a whole carcass of an infected cow entering the feed chain without any removal of specified risk material (SRM) or reduction of BSE infectivity via rendering, up to four new cases of BSE could be expected at the upper 95th percentile. A second model estimated that at least half of the feed containing material of ruminant origin will not be detected or removed from the feed chain, if an interpretation cut-off point of 100 DNA copies or more is applied. If the probability of a contaminated feed sample increased to 5%, with an interpretation cut-off point of 300 DNA copies, there would be a fourfold increase in the proportion of all produced feed that is contaminated but not detected.

Oral Transmission of L-Type Bovine Spongiform Encephalopathy Agent among Cattle

To determine oral transmissibility of the L-type bovine spongiform encephalopathy (BSE) prion, we orally inoculated 16 calves with brain homogenates of the agent. Only 1 animal, given a high dose, showed signs and died at 88 months. These results suggest low risk for oral transmission of the L-BSE agent among cattle.

Using an epidemiological framework and bovine spongiform encephalopathy investigation questionnaire to investigate suspect bovine spongiform encephalopathy cases: an example from a bovine spongiform encephalopathy case in Ireland in 2015

In several EU member states, bovine spongiform encephalopathy (BSE) cases have been identified in cattle born after the reinforced ban (BARB cases), for reasons that are not entirely clear. Epidemiological investigation of these cases has proved challenging. The European Food Safety Authority recently recommended the collection of a predefined set of epidemiological data from BSE suspects and confirmed BSE cases to aid future investigations. In this study, we present an epidemiological framework and BSE investigation questionnaire to aid the investigation of suspect BSE cases, and illustrate its application during the investigation of a BSE case in Ireland in 2015. It is recommended that the framework and questionnaire are used concurrently: the framework provides structure and focus, whereas the questionnaire (with 135 questions) aids data collection. The framework focuses on confirmation and discrimination, estimating the date and location of exposure, and determining the method/source of exposure. The BSE case in Ireland in 2015 was a BARB case born in 2010. It was identified with classical BSE at an authorised knackery as part of Ireland's targeted active surveillance programme for BSE. No definitive source of infection with the BSE agent could be attributed in this case.

Evidence for more cost-effective surveillance options for bovine spongiform encephalopathy (BSE) and scrapie in Great Britain

Transmissible spongiform encephalopathies (TSEs) are an important public health concern. Since the emergence of bovine spongiform encephalopathy (BSE) during the 1980s and its link with human Creutzfeldt-Jakob disease, active surveillance has been a key element of the European Union's TSE control strategy. Success of this strategy means that now, very few cases are detected compared with the number of animals tested. Refining surveillance strategies would enable resources to be redirected towards other public health priorities. Cost-effectiveness analysis was performed on several alternative strategies involving reducing the number of animals tested for BSE and scrapie in Great Britain and, for scrapie, varying the ratio of sheep sampled in the abattoir to fallen stock (which died on the farm). The most cost-effective strategy modelled for BSE involved reducing the proportion of fallen stock tested from 100% to 75%, producing a cost saving of ca GBP 700,000 per annum. If 50% of fallen stock were tested, a saving of ca GBP 1.4 million per annum could be achieved. However, these reductions are predicted to increase the period before surveillance can detect an outbreak. For scrapie, reducing the proportion of abattoir samples was the most cost-effective strategy modelled, with limited impact on surveillance effectiveness.

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.

Bovine spongiform encephalopathy (BSE) cases born after the total feed ban

Sixty bovine spongiform encephalopathy (BSE) cases of Classical or unknown type (BARB‐60 cases) were born after the date of entry into force of the EU total feed ban on 1 January 2001. The European Commission has requested EFSA to provide a scientific opinion on the most likely origin(s) of these BARB‐60 cases; whether feeding with material contaminated with the BSE agent can be excluded as the origin of any of these cases and, if so, whether there is enough scientific evidence to conclude that such cases had a spontaneous origin. The source of infection cannot be ascertained at the individual level for any BSE case, including these BARB‐60 cases, so uncertainty remains high about the origin of disease in each of these animals, but when compared with other biologically plausible sources of infection (maternal, environmental, genetic, iatrogenic), feed‐borne exposure is the most likely. This exposure was apparently excluded for only one of these BARB‐60 cases. However, there is considerable uncertainty associated with the data collected through the field investigation of these cases, due to a time span of several years between the potential exposure of the animal and the confirmation of disease, recall difficulty, and the general paucity of documented objective evidence available in the farms at the time of the investigation. Thus, feeding with material contaminated with the BSE agent cannot be excluded as the origin of any of the BARB‐60 cases, nor is it possible to definitively attribute feed as the cause of any of the BARB‐60 cases. A case of disease is classified as spontaneous by a process of elimination, excluding all other definable possibilities; with regard to the BARB‐60 cases, it is not possible to conclude that any of them had a spontaneous origin.

Modeling Variant Creutzfeldt-Jakob Disease and Its Pathogenesis in Non-human Primates

In the early 90s', Europe was shaken by the fear that the prions from "mad cow disease" (bovine spongiform encephalopathy) would transmit the disease to humans via beef products. In 1996, the first variant Creutzfeldt-Jakob (vCJD) patients were described, and the same year our Bovine Spongiform Encephalopathy (BSE) transmission studies to cynomolgus macaques demonstrated that the BSE prion was highly infectious for primates, inducing brain lesions identical to those observed in vCJD patients. These studies provided the first experimental evidence that vCJD was BSE in humans. Subsequent studies established the BSE/vCJD-infected cynomolgus macaque as a robust model to study the pathogenesis of vCJD. We showed rapid adaptation of BSE prions to primates upon subsequent passage, and their distribution in peripheral tissues and blood. Some key studies are summarized in the present paper.

Assessing the time taken for a surveillance system to detect a re-emergence of bovine spongiform encephalopathy in cattle

During the bovine spongiform encephalopathy (BSE) epidemic in July 2001 the European Commission established a surveillance scheme for the comprehensive sampling of all BSE clinical suspects, healthy slaughter (HS) animals >30months, and all emergency slaughter and fallen stock animals tested when >24months. With the exponential decline in classical BSE cases, this comprehensive surveillance system has been successively modified to become risk-based, targeting those exit streams and ages where cases from the original epidemic are most likely to be detected. Such reductions in testing are not without losses in the information subsequently collected, which could affect the sensitivity of the surveillance system to relatively small changes in the underlying prevalence of BSE across the European Union (EU). Here we report on a cohort-based approach to estimate the time taken for EU surveillance to observe a theoretical re-emergence of BSE in cattle. A number of surveillance schemes were compared. The baseline scheme considered detection being triggered by at least one case in the 'age window' 48-72 months in the fallen stock or emergency slaughter exit streams. Alternative schemes changed the start and end of this age window as well as considering testing for HS cattle. Under the baseline scheme, an estimated 15 years would lapse ([2.5th, 97.5th] percentiles=[10,24]) prior to detection, during which time 2867 infected animals ([2.5th, 97.5th]=[1722,6967]) would enter the slaughter population. These animals would be predominantly young animals (majority <24months) showing no clinical signs. This baseline scheme reduced the time to detection by 2 years, compared to a scheme where only clinical suspects were tested assuming BSE symptoms are recognised to the same degree by veterinary surgeons. Additional testing of younger animals did not improve detection as young infected animals were unlikely to test positive, but testing of older animals reduced the time to detection. Testing of HS animals >72months reduced the time to detection by one year compared to the baseline model, but would incur a high financial cost, e.g. testing HS animals >72months of age for 14 years would entail approximately 50.4 million additional tests. A limitation of the results is that there is no guarantee that current detection methods, optimised for detection of classical BSE, would identify a novel prion disease in cattle and it is currently difficult to envisage plausible routes by which a re-emergence of classical BSE could occur in Europe.

The transmissible spongiform encephalopathies of livestock

Prion diseases or transmissible spongiform encephalopathies (TSEs) are fatal protein-misfolding neurodegenerative diseases. TSEs have been described in several species, including bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, chronic wasting disease (CWD) in cervids, transmissible mink encephalopathy (TME) in mink, and Kuru and Creutzfeldt-Jakob disease (CJD) in humans. These diseases are associated with the accumulation of a protease-resistant, disease-associated isoform of the prion protein (called PrP(Sc)) in the central nervous system and other tissues, depending on the host species. Typically, TSEs are acquired through exposure to infectious material, but inherited and spontaneous TSEs also occur. All TSEs share pathologic features and infectious mechanisms but have distinct differences in transmission and epidemiology due to host factors and strain differences encoded within the structure of the misfolded prion protein. The possibility that BSE can be transmitted to humans as the cause of variant Creutzfeldt-Jakob disease has brought attention to this family of diseases. This review is focused on the TSEs of livestock: bovine spongiform encephalopathy in cattle and scrapie in sheep and goats.

A Bayesian back-calculation method to estimate the risk of bovine spongiform encephalopathy (BSE) in Canada during the period 1996-2011

Seventeen typical cases of bovine spongiform encephalopathy (BSE) were detected in Canada the period of 2003-2011. The clinical incidence of BSE was censored by early slaughter, death, or exportation of infected cattle due to the long incubation period of BSE disease. The aim of this study was to estimate the infection incidence of BSE in birth cohorts during 1996-2004 and project infection frequency through to 2007. An estimate of the number of asymptomatic infected cattle slaughtered for human consumption is also provided. The number of incident, asymptomatic cases was assumed to follow a Poisson process. A Bayesian back-calculation approach was used to project the risk of contracting BSE in those birth cohorts. Model parameters and inputs were taken from scientific literature and governmental data sources. The projected number of infected cattle in birth cohorts spanning the period 1996-2007 was 492, with median 95% credible interval 258-830. If the requirement to remove specified risk material (SRM) from cattle prior to entering the food chain was not in place, the predicted number of slaughtered infected in the human food chain from 1996-2010 was 298, with a 95% credible interval 156-500. The magnitude of the BSE epidemic in Canada for 1996-2007 birth cohorts was estimated to be approximately 28-fold higher than the number of clinical cases detected through to October 2011. Although some of those cattle were slaughtered for human consumption, the requirement of SRM removal may have prevented most of the infectious material from entering the food chain.

A stochastic model of the bovine spongiform encephalopathy epidemic in Canada

Bovine spongiform encephalopathy (BSE) appeared in the United Kingdom in the mid 1980s, and has been attributed to the use of meat and bone meal (MBM) in cattle feed contaminated with a scrapie-like agent. Import of infectious materials from a country where BSE has occurred is believed to be the major factor underlying the spread of the BSE epidemic to other countries. This study presents a new stochastic model developed to estimate risk of BSE from importation of cattle infected with the BSE agent. The model describes the propagation of the BSE agent through the Canadian cattle herd through rendering and feeding processes, following importation of cattle with infectious prions. This model was used estimate the annual number of newly infected animals each year over the period 1980-2019. Model predictions suggested that the number of BSE infections in Canada might have been approximately 40-fold greater than the actual number of clinically diagnosed cases. Under complete compliance with the 2007 ban on feeding MBM, this model further predicts that BSE is disappearing from the Canadian cattle system. A series of sensitivity analyses was also conducted to test the robustness of model predictions to alternative assumptions about factors affecting the evolution of the Canadian BSE epidemic.

NEURODEGENERATION. Alzheimer's and Parkinson's diseases: The prion concept in relation to assembled Aβ, tau, and α-synuclein

The pathological assembly of Aβ, tau, and α-synuclein is at the heart of Alzheimer's and Parkinson's diseases. Extracellular deposits of Aβ and intraneuronal tau inclusions define Alzheimer's disease, whereas intracellular inclusions of α-synuclein make up the Lewy pathology of Parkinson's disease. Most cases of disease are sporadic, but some are inherited in a dominant manner. Mutations frequently occur in the genes encoding Aβ, tau, and α-synuclein. Overexpression of these mutant proteins can give rise to disease-associated phenotypes. Protein assembly begins in specific regions of the brain during the process of Alzheimer's and Parkinson's diseases, from where it spreads to other areas.

Foodborne-Transmitted Prions From the Brain of Cows With Bovine Spongiform Encephalopathy Ascend in Afferent Neurons to the Simian Central Nervous System and Spread to Tonsils and Spleen at a Late Stage of the Incubation Period

BACKGROUND

Protease-resistant prion protein (PrP(res)) accumulation in lymphoreticular tissues indicates prion infection. To date, tonsillectomy and appendectomy samples have been used in population prevalence surveys to detect clinically silent carriers of variant Creutzfeldt-Jakob disease (vCJD). However, the temporal sequence of prion spread in the human body is still not known. We therefore traced the temporal-spatial pattern of PrP(res) accumulation in the body of a simian vCJD model.

METHODS

Cynomolgus monkeys were fed brain of (eleven) cows with bovine spongiform encephalopathy, and some were euthanized before and some after onset of neurological signs. PrP(res) was detected in tissues by a paraffin-embedded tissue blot technique and a semiquantitative Western immunoblot assay.

RESULTS

Bovine spongiform encephalopathy (BSE)-associated prions were preferentially transported from the gut to the central nervous system (CNS) along sensory nerve fibers and initially entered the simian CNS at lumbar spinal cord levels. In asymptomatic animals, we found BSE in 50% and 12% of gut- and tonsil-derived samples, respectively.

CONCLUSIONS

Unlike in rodents and ruminants, foodborne BSE-associated prions entered the simian CNS via afferent neurons. From sites of initial CNS invasion, prions spread centrifugally to tonsils and spleen at an advanced stage of the incubation period, thus explaining why tonsil specimens were not reliable for detection of simian disease carriers before onset of clinical signs.

Lesion of the olfactory epithelium accelerates prion neuroinvasion and disease onset when prion replication is restricted to neurons

Natural prion diseases of ruminants are moderately contagious and while the gastrointestinal tract is the primary site of prion agent entry, other mucosae may be entry sites in a subset of infections. In the current study we examined prion neuroinvasion and disease induction following disruption of the olfactory epithelium in the nasal mucosa since this site contains environmentally exposed olfactory sensory neurons that project directly into the central nervous system. Here we provide evidence for accelerated prion neuroinvasion and clinical onset from the olfactory mucosa after disruption and regeneration of the olfactory epithelium and when prion replication is restricted to neurons. In transgenic mice with neuron restricted replication of prions, there was a reduction in survival when the olfactory epithelium was disrupted prior to intranasal inoculation and there was >25% decrease in the prion incubation period. In a second model, the neurotropic DY strain of transmissible mink encephalopathy was not pathogenic in hamsters by the nasal route, but 50% of animals exhibited brain infection and/or disease when the olfactory epithelium was disrupted prior to intranasal inoculation. A time course analysis of prion deposition in the brain following loss of the olfactory epithelium in models of neuron-restricted prion replication suggests that neuroinvasion from the olfactory mucosa is via the olfactory nerve or brain stem associated cranial nerves. We propose that induction of neurogenesis after damage to the olfactory epithelium can lead to prion infection of immature olfactory sensory neurons and accelerate prion spread to the brain.

The evolution of the prevalence of classical scrapie in sheep in Great Britain using surveillance data between 2005 and 2012

After the decline of the Bovine Spongiform Encephalopathy (BSE) epidemic in Great Britain (GB), scrapie remains the most prevalent animal Transmissible Spongiform Encephalopathy (TSE) present in GB. A number of control measures have been implemented for classical scrapie, and since 2005 there has been a large reduction in the number of observed cases. The objective of this study is to estimate two measures of disease frequency using up to date surveillance data collected during and after the implementation of different control measures established since 2004, and breeding for resistance schemes that ran from 2001 until 2009. This would enable an assessment of the effectiveness of both the breeding for resistance programme and the compulsory eradication measures in reducing the prevalence of scrapie in GB. Evaluation of the sensitivity of the rapid post-mortem test for scrapie indicated that it detected scrapie in the last 25% of the incubation period. A back-calculation model was developed to estimate the prevalence of infection at animal and flock-level. The results of the model indicated a mean drop of infection prevalence of 31% each year, leading to a 90% drop in infection prevalence between 2005, with an estimate of 5737 infected sheep in GB in 2012. The risks of classical scrapie infection in animals with genotypes of National Scrapie Plan Types I-IV (all other genotypes), relative to Type V (all genotypes containing V136 R154 Q171 and not A136 R154 R171), were estimated to be: 0, 0.0008, 0.07, and 0.21 respectively. The model estimated a very low rate of reporting of clinical suspects and a large decline from 2007 of the probability of a sheep being reported as a clinical suspect. The model also estimated that the expected number of sheep holdings with classical scrapie in 2012 was 215 (95% confidence interval: 33-437), out of a total of approximately 72,000 sheep holdings in GB. Model estimates indicate that the prevalence in 2012 has dropped to 10% of that in 2005, showing the effectiveness of the control measures. It also shows a bias in the destination of infected animals, with the majority of infected animals being detected in the fallen stock surveillance stream, and an extremely low proportion of animals detected as clinical suspects; this is very important in terms of the design of surveillance schemes for classical scrapie.

Foodborne transmission of bovine spongiform encephalopathy to non-human primates results in preclinical rapid-onset obesity

Obesity has become one of the largest public health challenges worldwide. Recently, certain bacterial and viral pathogens have been implicated in the pathogenesis of obesity. In the present study, we retrospectively analyzed clinical data, plasma samples and post-mortem tissue specimens derived from a risk assessment study in bovine spongiform encephalopathy (BSE)-infected female cynomolgus monkeys (Macaca fascicularis). The original study design aimed to determine minimal infectious doses after oral or intracerebral (i.c.) infection of macaques to assess the risk for humans. High-dose exposures resulted in 100% attack rates and a median incubation time of 4.7 years as described previously. Retrospective analyses of clinical data from high-dosed macaques revealed that foodborne BSE transmission caused rapid weight gain within 1.5 years post infection (β = 0.915; P<0.0001) which was not seen in age- and sex-matched control animals or i.c. infected animals. The rapid-onset obesity was not associated with impaired pancreatic islet function or glucose metabolism. In the early preclinical phase of oral transmission associated with body weight gain, prion accumulation was confined to the gastrointestinal tract. Intriguingly, immunohistochemical findings suggest that foodborne BSE transmission has a pathophysiological impact on gut endocrine cells which may explain rapid weight gain. To our knowledge, this is the first experimental model which clearly demonstrates that foodborne pathogens can induce obesity.

Prion diseases as transmissible zoonotic diseases

Prion diseases, also called transmissible spongiform encephalopathies (TSEs), lead to neurological dysfunction in animals and are fatal. Infectious prion proteins are causative agents of many mammalian TSEs, including scrapie (in sheep), chronic wasting disease (in deer and elk), bovine spongiform encephalopathy (BSE; in cattle), and Creutzfeldt-Jakob disease (CJD; in humans). BSE, better known as mad cow disease, is among the many recently discovered zoonotic diseases. BSE cases were first reported in the United Kingdom in 1986. Variant CJD (vCJD) is a disease that was first detected in 1996, which affects humans and is linked to the BSE epidemic in cattle. vCJD is presumed to be caused by consumption of contaminated meat and other food products derived from affected cattle. The BSE epidemic peaked in 1992 and decreased thereafter; this decline is continuing sharply owing to intensive surveillance and screening programs in the Western world. However, there are still new outbreaks and/or progression of prion diseases, including atypical BSE, and iatrogenic CJD and vCJD via organ transplantation and blood transfusion. This paper summarizes studies on prions, particularly on prion molecular mechanisms, BSE, vCJD, and diagnostic procedures. Risk perception and communication policies of the European Union for the prevention of prion diseases are also addressed to provide recommendations for appropriate government policies in Korea.

PrP expression level and sensitivity to prion infection

Mice overexpressing the prion protein (PrP) sequence from various host species are widely used for measuring infectious titers in prion disease. However, the impact that the transgene expression level might have on the susceptibility to infection raises some concerns about the final biological relevance of these models. Here we report that endpoint titration of a sheep scrapie isolate in sheep and in mice overexpressing the ovine PrP results in similar estimates of the infectious titer.

A comparative study of modified confirmatory techniques and additional immuno-based methods for non-conclusive autolytic bovine spongiform encephalopathy cases

Background

In the framework of the Bovine Spongiform Encephalopathy (BSE) surveillance programme, samples with non-conclusive results using the OIE confirmatory techniques have been repeatedly found. It is therefore necessary to question the adequacy of the previously established consequences of this non-conclusive result: the danger of failing to detect potentially infected cattle or erroneous information that may affect the decision of culling or not of an entire bovine cohort. Moreover, there is a very real risk that the underreporting of cases may possibly lead to distortion of the BSE epidemiological information for a given country.

In this study, samples from bovine nervous tissue presenting non-conclusive results by conventional OIE techniques (Western blot and immunohistochemistry) were analyzed. Their common characteristic was a very advanced degree of autolysis. All techniques recommended by the OIE for BSE diagnosis were applied on all these samples in order to provide a comparative study. Specifically, immunohistochemistry, Western blotting, SAF detection by electron microscopy and mouse bioassay were compared. Besides, other non confirmatory techniques, confocal scanning microscopy and colloidal gold labelling of fibrils, were applied on these samples for confirming and improving the results.

Results

Immunocytochemistry showed immunostaining in agreement with the positive results finally provided by the other confirmatory techniques. These results corroborated the suitability of this technique which was previously developed to examine autolysed (liquified) brain samples. Transmission after inoculation of a transgenic murine model TgbovXV was successful in all inocula but not in all mice, perhaps due to the very scarce PrPsc concentration present in samples.

Electron microscopy, currently fallen into disuse, was demonstrated to be, not only capable to provide a final diagnosis despite the autolytic state of samples, but also to be a sensitive diagnostic alternative for resolving cases with low concentrations of PrPsc.

Conclusions

Demonstration of transmission of the disease even with low concentrations of PrPsc should reinforce that vigilance is required in interpreting results so that subtle changes do not go unnoticed. To maintain a continued supervision of the techniques which are applied in the routine diagnosis would prove essential for the ultimate eradication of the disease.

A quantitative assessment of the amount of prion diverted to Category 1 materials and wastewater during processing

In this article the development and parameterization of a quantitative assessment is described that estimates the amount of TSE infectivity that is present in a whole animal carcass (bovine spongiform encephalopathy [BSE] for cattle and classical/atypical scrapie for sheep and lambs) and the amounts that subsequently fall to the floor during processing at facilities that handle specified risk material (SRM). BSE in cattle was found to contain the most oral doses, with a mean of 9864 BO ID50 s (310, 38840) in a whole carcass compared to a mean of 1851 OO ID50 s (600, 4070) and 614 OO ID50 s (155, 1509) for a sheep infected with classical and atypical scrapie, respectively. Lambs contained the least infectivity with a mean of 251 OO ID50 s (83, 548) for classical scrapie and 1 OO ID50 s (0.2, 2) for atypical scrapie. The highest amounts of infectivity falling to the floor and entering the drains from slaughtering a whole carcass at SRM facilities were found to be from cattle infected with BSE at rendering and large incineration facilities with 7.4 BO ID50 s (0.1, 29), intermediate plants and small incinerators with a mean of 4.5 BO ID50 s (0.1, 18), and collection centers, 3.6 BO ID50 s (0.1, 14). The lowest amounts entering drains are from lambs infected with classical and atypical scrapie at intermediate plants and atypical scrapie at collection centers with a mean of 3 × 10(-7) OO ID50 s (2 × 10(-8) , 1 × 10(-6) ) per carcass. The results of this model provide key inputs for the model in the companion paper published here.

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.

Bovine spongiform encephalopathy: the effect of oral exposure dose on attack rate and incubation period in cattle - an update

BACKGROUND

To provide information on dose-response and aid in modelling the exposure dynamics of the BSE epidemic in the United Kingdom groups of cattle were exposed orally to a range of different doses of brainstem homogenate of known infectious titre from clinical cases of classical bovine spongiform encephalopathy (BSE). Interim data from this study was published in 2007. This communication documents additional BSE cases, which occurred subsequently, examines possible influence of the bovine prion protein gene on disease incidence and revises estimates of effective oral exposure.

FINDINGS

Following interim published results, two further cattle, one dosed with 100 mg and culled at 127 months post exposure and the other dosed with 10 mg and culled at 110 months post exposure, developed BSE. Both had a similar pathological phenotype to previous cases. Based on attack rate and incubation period distribution according to dose, the dose estimate at which 50% of confirmed cases would be clinically affected was revised to 0.15 g of the brain homogenate used in the experiment, with a 95% confidence interval of 0.03-0.79 g. Neither the full open reading frame nor the promoter region of the prion protein gene of dosed cattle appeared to influence susceptibility to BSE, but this may be due to the sample size.

CONCLUSIONS

Oral exposure of cattle to a large range of doses of a BSE brainstem homogenate produced disease in all dose groups. The pathological presentation resembled natural disease. The attack rate and incubation period were dependent on the dose.

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.

Geographical BSE risk assessment and its impact on disease detection and dissemination

Bovine Spongiform Encephalopathy (BSE) rapidly evolved into an issue of major public concern particularly when, in 1996, evidence was provided that this disease had crossed the species barrier and infected humans in the UK with what has become known as "variant Creutzfeldt Jakob Disease" (vCJD). The aim of this paper is to describe the European Geographical BSE risk assessment (GBR) that was successfully used for assessing the qualitative likelihood that BSE could be present in a country where it was not yet officially recognized. It also discusses how this can lead to risk-based and therefore preventive management of BSE at national and international levels. The basic assumption of the GBR method is that the BSE agent is initially introduced into a country's domestic cattle production system through the importation of contaminated feedstuffs or live cattle. This is referred to as an "external challenge". The ability of the system to cope with such a challenge is, in turn, referred to as its "stability": a stable system will not allow the BSE agent to propagate and amplify following its introduction, while an unstable system will. The BSE-status of a country assessed by this system was used by the European Commission as the basis for trade legislation rules for cattle and their products. The GBR was an invaluable tool in evaluating the potential global spread of BSE as it demonstrated how a disease could be transferred through international trade. This was shown to be a critical factor to address in reducing the spread and amplification of BSE throughout the world. Furthermore, GBR resulted in the implementation of additional measures and management activities both to improve surveillance and to prevent transmission within the cattle population.

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.

Alternative BSE risk assessment methodology for beef and beef offal imported into Japan

The Food Safety Commission (FSC) of Japan, established in July 2003, has its own initiative to conduct risk assessments on food stuffs known as "self-tasking assessment". Within this framework, the FSC decided to conduct a risk assessment of beef and beef offal imported into Japan from countries with no previous BSE reports; thus, a methodology was formed to suit to this purpose. This methodology was partly based on the previous assessments of Japanese domestic beef and beef imported from U.S.A./Canada, but some modifications were made. Other organizations' assessment methods, such as those used for BSE status assessment in live cattle by the OIE and EFSA's GBR, were also consulted. In this review, the authors introduce this alternative methodology, which reflects (1) the risk of live cattle in the assessed country including temporal risks of BSE invasion and domestic propagation, with the assessment results verified by surveillance data, and (2) the risk of beef and beef offal consisting of cumulative BSE risk by types of slaughtering and meat production processes implemented and the status of mechanically recovered meat production. Other possible influencing factors such as atypical BSE cases were also reviewed. The key characteristic of the current assessment is a combination of the time-sequential risk level of live cattle and qualitative risk level of meat production at present in an assessed country.

Conserved properties of human and bovine prion strains on transmission to guinea pigs

The first transmissions of human prion diseases to rodents used guinea pigs (Gps, Cavia porcellus). Later, transgenic mice expressing human or chimeric human/mouse PrP replaced Gps, but the small size of the mouse limits some investigations. To investigate the fidelity of strain-specific prion transmission to Gps, we inoculated 'type 1' and 'type 2' prion strains into Gps, and we measured the incubation times and determined the strain-specified size of the unglycosylated, protease-resistant (r) PrP(Sc) fragment. Prions passaged once in Gps from cases of sporadic (s) Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker (GSS) disease caused by the P102L mutation were used, as well as human prions from a variant (v) CJD case, bovine prions from bovine spongiform encephalopathy (BSE) and mouse-passaged scrapie prions. Variant CJD and BSE prions transmitted to all the inoculated Gps with incubation times of 367 ± 4 and 436 ± 28 days, respectively. On second passage in Gps, vCJD and BSE prions caused disease in 287 ± 4 and 310 ± 4 days, whereas sCJD and GSS prions transmitted in 237 ± 4 and 279 ± 19 days, respectively. Although hamster Sc237 prions transmitted to two of three Gps after 574 and 792 days, mouse-passaged RML and 301V prion strains, the latter derived from BSE prions, failed to transmit disease to Gps. Those Gps inoculated with vCJD or BSE prions exhibited 'type 2' unglycosylated, rPrP(Sc) (19 kDa), whereas those receiving sCJD or GSS prions displayed 'type 1' prions (21 kDa), as determined by western blotting. Such strain-specific properties were maintained in Gps as well as mice expressing a chimeric human/mouse transgene. Gps may prove particularly useful in further studies of novel human prions such as those causing vCJD.

Time and frequency domain analysis of heart rate variability in cattle affected by bovine spongiform encephalopathy

Background

Heart rate variability (HRV) analysis is a method to assess the function of the autonomic nervous system. Brainstem nuclei that influence HRV are affected by vacuolar changes and accumulation of disease-associated prion protein (PrP^d) in bovine spongiform encephalopathy (BSE) resulting in clinical signs suggestive of an increased parasympathetic tone. It was hypothesised that BSE in cattle causes changes in the autonomic nervous system; this was tested by comparing HRV indices derived from 1048 electrocardiograms, which were recorded from 51 naturally or experimentally infected cattle with BSE confirmed by postmortem tests, 321 clinical suspect cases or cattle inoculated with potentially infectious tissue without disease confirmation and 78 BSE-free control cattle.

Findings

Statistically significant differences were found for low or high frequency power, their normalised values and ratio when the last recording prior to cull or repeated recordings were compared but only between male and female cattle of the three groups and not between groups of the same gender, even though BSE cases of each gender appeared to be more nervous during the recording. The same findings were made for heart rate, deviation from the mean RR interval and vasovagal tonus index when repeated recordings were compared. BSE cases with severe vacuolar changes in the parasympathetic nucleus of the vagus nerve had a significantly lower low:high frequency power ratio but not a lower heart rate than BSE cases with mild vacuolation, whereas severity of vacuolar changes in the solitary tract nucleus or intensity of PrP^d accumulation in both nuclei did not appear to have any affect on either index. Abnormalities in the electrocardiogram were detected in 3% of the recordings irrespective of the BSE status; sinus arrhythmia was present in 93% of the remaining recordings.

Conclusions

HRV analysis was not useful to distinguish BSE-positive from BSE-negative cattle grouped by gender, and HRV indices appeared to be mainly influenced by gender. There is agreement with earlier studies that vacuolar changes in the brainstem may be associated with an increased parasympathetic tone in BSE and that abnormalities in an electrocardiogram can be detected in cattle without evidence of heart disease.

Experimental bovine spongiform encephalopathy: detection of PrP(Sc) in the small intestine relative to exposure dose and age

European regulations for the control of bovine spongiform encephalopathy (BSE) decree destruction of the intestines from slaughtered cattle, therefore producers have been obliged to import beef casings from countries with a negligible BSE risk. This study applies immunohistochemical and biochemical approaches to investigate the occurrence and distribution of disease-associated prion protein (PrP(Sc)) in the duodenum, jejunum and ileum of cattle orally exposed to a 1 g or 100 g dose of a titrated BSE brainstem homogenate. Samples were derived from animals at various times post exposure. Lymphoid follicles were counted and the frequency of affected follicles recorded. No PrP(Sc) was detected in the duodenum or jejunum of animals exposed to a 1 g dose or in the duodenum of animals receiving a 100 g dose. PrP(Sc) was detected in the lymphoid tissue of the ileum of 1/98 (1.0%) animals receiving the 1 g dose and in the jejunum and ileum of 8/58 (13.8%) and 45/99 (45.5%), respectively, of animals receiving the 100 g dose. The frequency of PrP(Sc)- positive follicles was less than 1.5% per case and biochemical tests appeared less sensitive than immunohistochemistry. The probability of detecting lymphoid follicles in the ileum declined with age and for the 100 g exposure the proportion of positive follicles increased, while the proportion of positive animals decreased with age. Detection of PrP(Sc) in intestinal neural tissue was rare. The results suggest that the jejunum and duodenum of BSE-infected cattle contain considerably less BSE infectivity than the ileum, irrespective of exposure dose. In animals receiving the low exposure dose, as in most natural cases of BSE, the rarity of PrP(Sc) detection compared with high-dose exposure, suggests a very low BSE risk from food products containing the jejunum and duodenum of cattle slaughtered for human consumption.