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

Impact of vCJD on blood supply

Variant Creutzfeldt-Jakob disease (vCJD) is an at present inevitably lethal neurodegenerative disease which can only be diagnosed definitely post mortem. The majority of the approximately 200 victims to date have resided in the UK where most contaminated beef materials entered the food chain. Three cases in the UK demonstrated that vCJD can be transmitted by blood transfusion. Since BSE and vCJD have spread to several countries outside the UK, it appears advisable that specific risk assessments be carried out in different countries and geographic areas. This review explains the approach adopted by Germany in assessing the risk and considering precautionary measures. A fundamental premise is that the feeding chain of cattle and the food chain have been successfully and permanently cleared from contaminated material. This raises the question of whether transmissions via blood transfusions could have the potential to perpetuate vCJD in mankind. A model calculation based on actual population data showed, however, that this would not be the case. Moreover, an exclusion of transfusion recipients from blood donation would add very little to the safety of blood transfusions, but would have a considerable impact on blood supply. Therefore, an exclusion of transfusion recipients was not recommended in Germany.

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

The dose–response of cattle exposed to the bovine spongiform encephalopathy (BSE) agent is an important component of modelling exposure risks for animals and humans and thereby, the modulation of surveillance and control strategies for BSE. In two experiments calves were dosed orally with a range of amounts of a pool of brainstems from BSE-affected cattle. Infectivity in the pool was determined by end-point titration in mice. Recipient cattle were monitored for clinical disease and, from the incidence of pathologically confirmed cases and their incubation periods (IPs), the attack rate and IP distribution according to dose were estimated. The dose at which 50 % of cattle would be clinically affected was estimated at 0.20 g brain material used in the experiment, with 95 % confidence intervals of 0.04–1.00 g. The IP was highly variable across all dose groups and followed a log-normal distribution, with decreasing mean as dose increased. There was no evidence of a threshold dose at which the probability of infection became vanishingly small, with 1/15 (7 %) of animals affected at the lowest dose (1 mg).

Estimating incidence of the French BSE infection using a joint analysis of both asymptomatic and clinical BSE surveillance data

Bovine Spongiform Encephalopathy (BSE) clinical surveillance data were the main source of information to perform back-calculation of BSE infection incidence. Since 2001, systematic BSE screening tests enhanced the clinical surveillance and allowed to detect some preclinical, i.e. asymptomatic, cases of BSE. We propose a method to incorporate additional information provided by screening tests. It was the first time that a back-calculation model was developed for a full BSE clinical surveillance. In the spirit, our approach resembles what it was done in the Acquired Immune Deficiency Syndrome (AIDS) epidemic to incorporate the Human Immunodeficiency Virus (HIV) diagnosis. Nevertheless, in the BSE epidemic, we had to consider different surveillance systems, their peculiarity, and the phenomenon of communicating vessels between these surveillance systems. In addition, both the preclinical sensitivity of tests and the status of BSE cases, asymptomatic or clinical, were not precisely known. We applied the model to the French BSE epidemic in order to obtain an updated estimate of the incidence of BSE infection. Our back-calculation model fitted very well the observed data of each surveillance system. We detected a lengthening of the incubation period and estimated that the number of infections was very small in the late 1990s and zero in July 2001.

Update on human prion disease

The recognition that variant Creutzfeldt-Jakob disease (vCJD) is caused by the same prion strain as bovine spongiform encephalopathy in cattle has dramatically highlighted the need for a precise understanding of the molecular biology of human prion diseases. Detailed clinical, pathological and molecular data from a large number of human prion disease patients indicate that phenotypic diversity in human prion disease relates in part to the propagation of disease-related PrP isoforms with distinct physicochemical properties. Incubation periods of prion infection in humans can exceed 50 years and therefore it will be some years before the extent of any human vCJD epidemic can be predicted with confidence.

Elevated manganese levels in blood and central nervous system occur before onset of clinical signs in scrapie and bovine spongiform encephalopathy

Prion diseases, or transmissible spongiform encephalopathies, are neurodegenerative diseases that can only be accurately diagnosed by analysis of central nervous system tissue for the presence of an abnormal isoform of the prion protein known as PrP(Sc). Furthermore, these diseases have long incubation periods during which there are no clear symptoms but where the infectious agent could still be present in the tissues. Therefore, the development of diagnostic assays to detect a surrogate marker for the presence of prion disease is essential. Previous studies on mice experimentally infected with scrapie, an ovine spongiform encephalopathy, suggested that changes in the levels of Mn occur in the blood and brain before the onset of symptoms of the disease. To assess whether these findings have relevance to the animal diseases scrapie and bovine spongiform encephalopathy, tissues from bovine spongiform encephalopathy- and scrapie-infected cattle and sheep were analyzed for their metal content and compared with values for noninfected animals. In field cases and experimentally infected animals, elevated Mn was associated with prion infection. Although some central nervous system regions showed elevated Mn, other regions did not. The most consistent finding was an elevation of Mn in blood. This change was present in experimentally infected animals before the onset of symptoms. In scrapie-infected sheep, elevated Mn levels occurred regardless of the genotype of the sheep and were even detected in scrapie-resistant sheep in which no symptoms of disease were detected. These findings suggest that elevated blood Mn could be a potential diagnostic marker for prion infection even in the absence of apparent clinical disease.

QUANTIFICATION OF PRION GENE EXPRESSION IN BRAIN AND PERIPHERAL ORGANS OF GOLDEN HAMSTER BY REAL-TIME RT-PCR

Determination of tissue-specific expression of cellular prion protein (PrPc) is essential for understanding its poorly explained role in organisms. Herein we report on quantification of PrP mRNA in golden hamsters, a popular experimental model for studying mechanisms of transmissible spongiform encephalopathies (TSE), by real-time RT-PCR. Total RNA was isolated from four different regions of the brain and six peripheral organs of eight golden hamsters. PrP mRNA copy numbers were determined using absolute standard curve method with real-time quantitative PCR instrument. It was found that high mRNA levels were present in all four regions of the brain examined, including obex, neocortex, cerebellum, and thalamus. In peripheral organs examined, inguinal lymph node showed high level of the expression similar to that in overall brain; spleen, heart, liver, and lung showed moderate levels of the expression; and kidney showed the lowest expression. Our result is consistent with the potential involvement of different organs in prion diseases and offers essential data for further study of TSE mechanism in this animal model.

Central Nervous System Tissue in Meat Products: An Evaluation of Risk, Prevention Strategies, and Testing Procedures

Since the outbreak of bovine spongiform encephalopathy (BSE) in the United Kingdom in 1986 and its subsequent link to the human neurological disorder variant Creutzfeldt-Jakob disease (vCJD), presence of tissues from the central nervous system (CNS) in meat products has been considered a public health concern and, thus, has been banned from entering the human food chain in many countries. Despite this, potential can exist during harvesting to contaminate or cross-contaminate edible meat products with CNS tissue that is designated as a specified risk material (SRM) in many countries. Methods used to detect CNS tissue in meat products vary greatly in their sensitivity, specificity, cost, labor and expertise needed, ease of completion, and type of results given (qualitative vs quantitative) and, within these constraints, appropriate testing methods must be selected to monitor or verify that meat products system controls are effective in removing CNS tissue from the human food chain. The extent to which monitoring procedures are needed should be based on the public health risk of CNS tissue in meat products as determined by each sovereign nation and/or third-party international organizations such as the World Organization for Animal Health (OIE). Risk associated with consumption of CNS tissue should be estimated by sovereign nations by establishing prevalence of BSE within their borders. Using this information, science-based decisions may guide international policy and trade. Using available scientific information, appropriate testing methods for monitoring or verification, and prevalence information, nations can estimate and reduce, to the extent deemed necessary, the public health risk of vCJD.

Prions and peripheral nerves: A deadly rendezvous

The infectious particle causing transmissible spongiform encephalopathy (TSE), a fatal neurodegenerative disease of humans and animals, has been termed prion. Its major component is an aggregated variant of the cellular prion protein, PrP(C). The main target of prion pathology is the central nervous system (CNS), yet most prion diseases are initiated or accompanied by prion replication at extracerebral locations, including secondary lymphoid organs, muscle and, in some instances, blood. How do prions travel from the periphery into the CNS? Is this an active or a passive process and does neuronal prion transport explain the long incubation times in prion diseases? Alternatively, if prion infectivity arises spontaneously in the CNS, as believed from sporadic Creutzfeldt-Jakob patients, how do prions manage to travel from the CNS into the periphery (e.g., spleen, muscle) of the infected host? The mechanisms of neuronal prion transport from the periphery into the CNS or vice versa are heavily investigated and debated but poorly understood. Although research in the past has accumulated knowledge on prion progression from the periphery to the brain, we are far from understanding the molecular mechanisms of neuronal prion transport.

Cellular prion protein in ovine milk

Cellular prion protein, PrP(C), is essential for the development of prion diseases where it is considered to be a substrate for the formation of the disease-associated conformer, PrP(Sc). In sheep, PrP(C) is abundant in neuronal tissue and is also found at lower concentrations in a range of non-neuronal tissues, including mammary gland. Here, we demonstrate the presence of soluble PrP(C) in the non-cellular, non-lipid fraction of clarified ovine milk. Compared with brain-derived PrP(C), ovine milk PrP(C) displays an increased electrophoretic mobility. Ovine milk PrP(C) is mainly present as three species that differ in the extent of their N-linked glycosylation, with glycoform profiles varying among animals. Similar PrP(C) species are also present in fresh and commercial homogenised/pasteurised bovine milk, with additional N-terminal PrP(C) fragments detectable in ruminant milk and commercial milk products.

Patterns of PrPCWD accumulation during the course of chronic wasting disease infection in orally inoculated mule deer (Odocoileus hemionus)

Patterns of abnormal prion protein (PrP) accumulation during the course of chronic wasting disease (CWD) infection were studied and the distribution and timing of disease-associated PrP (PrP(CWD)) deposition and lesions in 19 mule deer (Odocoileus hemionus) 90-785 days after oral inoculation were described. PrP(CWD) deposition occurred relatively rapidly and widely in lymphoid tissues, later in central and peripheral nervous tissues and sporadically in a variety of tissues and organs in terminal disease stages. Development of spongiform encephalopathy lagged behind PrP(CWD) deposition in the central nervous system (CNS), but occurred in the same neuroanatomical locations. PrP(CWD) deposition in the lymphatic and nervous systems tended to be consistent and progressive in specific organs and tissues. Locations of PrP(CWD) deposition were similar between deer of two PrP genotypes (225SS and 225SF), but the time course differed between genotypes: in 225SF deer, PrP(CWD) accumulated more slowly in lymphatic tissues than in 225SS animals, but that disparity was small in comparison to the disparity between genotypes in timing of deposition in CNS tissue. These data confirm retropharyngeal lymph node and medulla oblongata at the level of the obex as early sites of PrP(CWD) accumulation in mule deer with CWD. Data on the relative time frames for and genetic influences on PrP(CWD) accumulation may also offer insights about epidemic dynamics and potential control strategies.

Potential Risk Factors Associated with Bovine Spongiform Encephalopathy in Cattle from Schleswig-Holstein, Germany

Since the first detection of bovine spongiform encephalopathy (BSE) in homebred cattle in Germany on 26 November 2000, 382 cases have been confirmed until 30 September 2005. Thirty-two of these cases were reported from the Federal State of Schleswig-Holstein (SH). There are hypotheses on the routes of infection for German cattle, but only few efforts have been made to assess potential risk factors by epidemiological studies. The purpose of this study was to identify potential risk factors at the farm level for the occurrence of BSE in cattle in SH. By applying the method of indirect standardization, the prevalence of various structural and management parameters of BSE-affected farms in SH was compared with the prevalence of these parameters in a standard population of cattle farms from SH. The data describing the standard population were obtained by a cross-sectional study performed in SH in 2003. Data of the BSE case population were available from the central German BSE case database. A possible association of the occurrence of BSE with the feeding of milk replacers to calves was observed. There was a clear indication that the occurrence of BSE was associated with the presence of pigs and/or poultry on the farm. This finding suggests that cross-contamination of feed or cross-exposition may have occurred in SH. The results obtained using the indirect standardization analysis were validated by multiple logistic regression. This study indicates that the feedborne path has been the principal route of transmission for the BSE agent in SH.

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.

Kuru in the 21st century--an acquired human prion disease with very long incubation periods

BACKGROUND

Kuru provides the principal experience of epidemic human prion disease. Its incidence has steadily fallen after the abrupt cessation of its route of transmission (endocannibalism) in Papua New Guinea in the 1950s. The onset of variant Creutzfeldt-Jakob disease (vCJD), and the unknown prevalence of infection after the extensive dietary exposure to bovine spongiform encephalopathy (BSE) prions in the UK, has led to renewed interest in kuru. We investigated possible incubation periods, pathogenesis, and genetic susceptibility factors in kuru patients in Papua New Guinea.

METHODS

We strengthened active kuru surveillance in 1996 with an expanded field team to investigate all suspected patients. Detailed histories of residence and exposure to mortuary feasts were obtained together with serial neurological examination, if possible.

FINDINGS

We identified 11 patients with kuru from July, 1996, to June, 2004, all living in the South Fore. All patients were born before the cessation of cannibalism in the late 1950s. The minimum estimated incubation periods ranged from 34 to 41 years. However, likely incubation periods in men ranged from 39 to 56 years and could have been up to 7 years longer. PRNP analysis showed that most patients with kuru were heterozygous at polymorphic codon 129, a genotype associated with extended incubation periods and resistance to prion disease.

INTERPRETATION

Incubation periods of infection with human prions can exceed 50 years. In human infection with BSE prions, species-barrier effects, which are characteristic of cross-species transmission, would be expected to further increase the mean and range of incubation periods, compared with recycling of prions within species. These data should inform attempts to model variant CJD epidemiology.

Prion infection of oral and nasal mucosa

Centrifugal spread of the prion agent to peripheral tissues is postulated to occur by axonal transport along nerve fibers. This study investigated the distribution of the pathological isoform of the protein (PrP(Sc)) in the tongues and nasal cavities of hamsters following intracerebral inoculation of the HY strain of the transmissible mink encephalopathy (TME) agent. We report that PrP(Sc) deposition was found in the lamina propria, taste buds, and stratified squamous epithelium of fungiform papillae in the tongue, as well as in skeletal muscle cells. Using laser scanning confocal microscopy, PrP(Sc) was localized to nerve fibers in each of these structures in the tongue, neuroepithelial taste cells of the taste bud, and, possibly, epithelial cells. This PrP(Sc) distribution was consistent with a spread of HY TME agent along both somatosensory and gustatory cranial nerves to the tongue and suggests subsequent synaptic spread to taste cells and epithelial cells via peripheral synapses. In the nasal cavity, PrP(Sc) accumulation was found in the olfactory and vomeronasal epithelium, where its location was consistent with a distribution in cell bodies and apical dendrites of the sensory neurons. Prion spread to these sites is consistent with transport via the olfactory nerve fibers that descend from the olfactory bulb. Our data suggest that epithelial cells, neuroepithelial taste cells, or olfactory sensory neurons at chemosensory mucosal surfaces, which undergo normal turnover, infected with the prion agent could be shed and play a role in the horizontal transmission of animal prion diseases.

BSE immunohistochemical patterns in the brainstem: a comparison between UK and Italian cases

The continuous monitoring of bovine spongiform encephalopathy (BSE) cases is an integral component of European research and surveillance programmes, to ensure that any changes in the presentation of transmissible spongiform encephalopathies (TSE) in cattle can be detected and defined. Monitoring is generally limited to the brainstem at the level of the obex, for reasons of practicality, safety and cost. Demonstration of disease-specific prion protein (PrP(d)) by immunohistochemistry is currently the most widely used confirmatory tool for both active and passive surveillance. This study assessed PrP(d) immunostaining in the brainstems (obex) of cattle with BSE in the UK and Italy. Immunoreactivity 'profiles' were created for each case based on the nature of the immunostaining, its relative intensity and precise neuroanatomical location. This study compares the obex immunostaining patterns of Italian cases (only active surveillance) and two UK groups (both active and passive surveillance). The neuroanatomical distribution and relative intensity of PrP(d) was highly reproducible in all cases. The overall staining intensity varied widely but was generally stronger in the active than in the passive surveillance populations. The conclusion to be drawn from this comparative study is that the pattern of immunopathology in these routine screening samples for BSE diagnosis and surveillance is the same in the UK and Italy, whether or not the animal was displaying typical, or indeed any, clinical signs at the time of sampling. This indicates that the current confirmatory diagnostic strategy remains appropriate for active surveillance applications.

Analysis of clinical signs associated with bovine spongiform encephalopathy in casualty slaughter cattle

Clinical signs associated with bovine spongiform encephalopathy (BSE) were studied in 1008 casualty slaughter cattle over 30 months of age to compare the results with the BSE status as determined by postmortem tests. The clinical BSE status was assessed using seven different criteria based on various publications. Only one (0.10%) out of 997 casualty slaughter cattle with a matching postmortem test result was positive for BSE. The BSE case was identified by only two case definitions tailored specifically to recumbent cases. The variety and often equivocal definition of clinical signs associated with BSE is reflected by the difference in the criteria that usually identified different animals as BSE suspects. The BSE status may be more difficult to assess in recumbent animals that do not allow a full clinical examination, and BSE may not be suspected if another disease is present that may mask signs of BSE.

Prion diseases: current understanding of epidemiology and pathogenesis, and therapeutic advances

The bovine spongiform encephalopathy (BSE) epidemic, along with the related threat to human health posed by the transmission of the BSE agent to humans, has highlighted the importance of prion diseases. These fatal neurodegenerative diseases are characterised by spongiform changes in the CNS, and comprise a wide spectrum of clinicopathological entities in humans and animals, such as Creutzfeldt-Jakob disease (CJD) and its emerging new variant (vCJD) in humans, and BSE and scrapie in animals. This article reviews the geographical distribution and the temporal trends of CJD and vCJD; the major events in the pathogenesis of prion diseases; the risk factors for sporadic CJD and vCJD; and the possible strategies for treating them. Worldwide statistics indicate that sporadic CJD has a stable incidence of one case per million people per year; in contrast, the incidence of vCJD appears to have increased exponentially from its characterisation in 1994 to a peak in 2000. As of December 2005, 183 definite or probable cases of vCJD had been reported worldwide. The crucial event in the pathogenesis of prion diseases is the conversion of the normally occurring cellular prion protein (PrP(c)) into a pathogenic form, called protease-resistant PrP (PrP(res)) or scrapie PrP (PrP(sc)). Pathogenetic studies in rodent models have shown that PrP(sc) is found in the enteric nervous system and in the gut-associated lymphoid tissue following oral scrapie ingestion. The role of the lymphoreticular system in the pathogenesis of TSE seems to be related to the strains of agents and the host genotype. Therapeutic approaches to vCJD are mainly based on the inhibition or prevention of the pathological change that creates PrP(sc). Derivatives of acridine (such as mepacrine [quinacrine]) and the phenothiazine psychotropics have been proposed as possible therapies because of their activity in cellular models; however, neither class was able to affect the protease resistance of preexisting PrP fibrils. More encouragingly, in animal models of prion disease, tetracyclines were found to reduce prion infectivity by direct inactivation of PrP(sc). While these findings are promising, the suitability of these compounds for clinical use is still limited by their low efficacy once symptoms are apparent. Treatments based on the vaccination approach have also produced positive results, but further investigations are necessary to establish their clinical application.

Assessment of the impact of cattle testing strategies on human exposure to BSE agents in Japan

In Japan, cattle screening tests for BSE are conducted at slaughterhouses for surveillance purposes and as a meat safety measure, but the public health impacts of such testing and the subsequent removal of positive animals from the food chain have not been quantitatively assessed. We evaluated the influence of removing specified risk materials and the alternation of age limits for testing cattle at the slaughterhouse on human exposure to the BSE agent in Japan by constructing a probabilistic risk model. A stochastic model using Monte Carlo simulation was constructed in order to estimate the BSE infectivity destined for the food chain from a single BSE-infected animal at slaughter. The impact of different testing strategies and risk material removal were then compared. Murine intra-cerebral ID50 (m.i.c. ID50) units were used as units for BSE infectivity. Sensitivity analysis was conducted for key input variables by changing values within plausible ranges. The expected fraction of BSE-infected cattle presented for slaughter that would be detected by screening tests was 20%, even if all slaughtered cattle were tested. The removal of risk materials reduced the median value estimate of infectivity destined for human consumption by 95%. Cattle screening tests reduced the infectivity further, but reduction efficacy did not differ among the various testing strategies. Sensitivity analysis indicated that the characteristics of BSE infectivity accumulation during the incubation period, extension of the incubation period, and lowering the detection limit of screening tests had no significant impact on relative infectivity reduction, which remained stable irrespective of testing strategy or changes in these parameters. This study suggests that the impact of changing the age limit for testing cattle on beef safety is small, provided that the removal of risk materials is conducted properly.

Diagnosis of transmissible spongiform encephalopathies in animals: a review

Transmissible spongiform encephalopathies (TSEs) in animals include, among others, bovine spongiform encephalopathy (BSE), scrapie, chronic wasting disease, and atypical forms of prion diseases. Diagnosis of TSEs is based on identification of characteristic lesions or on detection of the abnormal prion proteins in tissues, often by use of their partial proteinase K resistance property. Correctly sampling of target tissues is of utmost importance as this has a considerable effect on test sensitivity. Most of the rapid or screening tests are based on ELISA or Western immunoblot (WB) analysis, and many are officially approved. Confirmatory testing is normally performed by use of histologic examination, immunohistochemical analysis, certain WB protocols, or detection of prion fibrils by use of electron microscopy (scrapie-associated fibril). The discriminatory methods for diagnostic use are mostly based on WB technology and provide initial identification of the prion strain, particularly for differentiation of BSE from scrapie in small ruminants. Definitive prion strain characterization is performed by use of bioassays, usually in mice. A burgeoning number of transgenic mice have been developed for TSE studies. Development of new tests with higher sensitivity and of more reliable diagnostic applications for live animals tested for food safety reasons is a rapidly developing field. Ultimately, the choice of a test for TSE diagnosis depends on the rationale for the testing.

Immunohistochemical features of PrP(d) accumulation in natural and experimental goat transmissible spongiform encephalopathies

Scrapie is a transmissible spongiform encephalopathy (TSE) or prion disease, which naturally affects sheep and goats. Immunohistochemical epitope mapping of abnormal PrP accumulations (PrP(d)) in brain can help in characterizing sheep TSE sources or strains and in identifying potential bovine spongiform encephalopathy (BSE) infections of sheep. Natural and experimental TSE infections of goats were examined to determine whether the epitope mapping approach could also be applied to aid recognition of BSE infection in goats. Goats experimentally infected with the SSBP/1 or CH1641 sheep scrapie strains or with cattle BSE, together with four field cases of natural TSE in goats, were examined immunohistochemically with six different antibodies. CH1641 and SSBP/1 infections in goats, as in sheep, showed PrP(d) accumulations which were mainly intracellular. Some differences in targeting, particularly of Purkinje cells, was evident in inter-species comparisons of CH1641 and SSBP/1. PrP(d) labelling of goat BSE experimental cases showed extensive intracellular and extracellular accumulations, also similar to those in sheep BSE. Intra-neuronal PrP(d) in both goat and sheep BSE was labelled only by antibodies recognizing epitopes located C-terminally of residue His99, whereas in natural sheep TSE sources, and in sheep and goat SSBP/1, PrP(d) was also detected by antibodies to epitopes located between residues Trp93 and His99. Testing of four natural goat TSE samples showed one case in which epitope mapping characteristics and the overall patterns of PrP(d) accumulation was identical with those of experimental goat BSE. The four natural goat scrapie cases examined showed some degree of immunohistochemical phenotype variability, suggesting that multiple strains exist within the relatively small UK goat population.

Ovine Infection with the Agents of Scrapie (CH1641 Isolate) and Bovine Spongiform Encephalopathy: Immunochemical Similarities can be Resolved by Immunohistochemistry

Immunochemical ("rapid") tests, which recognize a partly protease-resistant conformer of the prion protein (PrP(res)) are now widely used in Europe for the diagnosis of transmissible spongiform encephalopathies (TSEs). Some of these tests can be used to distinguish natural scrapie from experimental bovine spongiform encephalopathy (BSE) in sheep, on the basis of migration pattern differences of PrP(res) in Western immunoblots. However, PrP(res) from sheep inoculated with CH1641 scrapie gives an immunoblot profile similar to that of sheep inoculated with BSE. Therefore, field scrapie strains similar to CH1641 might be misclassified as ovine BSE in the rapid tests currently employed. This study confirmed that the Western blot similarities (size of the unglycosylated band and distinct reactivity with 6H4 and P4 antibodies) between CH1641 and BSE remained consistent regardless of the PrP genotype of the sheep, but the two infections resulted in accumulation of disease-associated PrP (PrP(d)) that could easily be distinguished by the immunohistochemical "peptide mapping" method. This method, which reveals conformational differences of PrP(d) by the use of a panel of antibodies, indicated that PrP(d) from the CH1641 isolate was truncated further upstream in the N terminus than was PrP(d) from other ovine TSEs, including experimental BSE. In addition, the immunohistochemical "PrP(d) profile method", which defines the phenotype of PrP(d) accumulation in the brain of affected sheep, showed that CH1641 infection leads to much more intra-neuronal and considerably less extracellular PrP(d) than does experimental BSE. The overall results demonstrate that a combined Western blotting and immunohistochemical approach is required to discriminate between different TSE strains in sheep.

Accumulation of prion protein in the peripheral nervous system in human prion diseases

After the finding that anti-prion antibodies stain sensory and sympathetic ganglia in variant Creutzfeldt-Jakob disease (vCJD), it was suggested that this localization supported the oral route of entry. However, prion accumulation subsequently also appeared in the peripheral nervous system (PNS) in sporadic cases. This study aims at evaluating the extent of prion protein accumulation in the PNS in all clinicopathologic subgroups of the disorder, with the exception of the familial and sporadic forms of fatal insomnia. Patients included 2 vCJD cases, 2 Gerstmann-Sträussler-Scheinker (GSS), 2 iatrogenic (iCJD), and 16 sporadic CJD (sCJD) cases. Gasserian (17) and spinal (9), celiac (2) and thoracic sympathetic (one) ganglia, spinal cord and medulla of one vCJD, 2 GSS, one iCJD, and 5 sCJD cases were examined. Immunostained sensory ganglia were seen in both vCJD, both iCJD, one GSS, and 10 sCJD cases; the celiac ganglion was positive in one of two sCJD cases, and the spinal dorsal horn and the medullary sensory nuclei were positive in one patient with vCJD, one with iCJD, and 3 with sCJD. Western blot demonstrated presence of PrP in the gasserian ganglion of one patient with sCJD. Accumulation of prion in ganglia (including autonomic) of the PNS, shared by all subgroups of spongiform encephalopathy, and in the dorsal horns and medullary sensory nuclei, shows that the sensory route is involved in the trafficking of this protein.

Extraneural prion neuroinvasion without lymphoreticular system infection

While prion infection of the lymphoreticular system (LRS) is necessary for neuroinvasion in many prion diseases, in bovine spongiform encephalopathy and atypical cases of sheep scrapie there is evidence to challenge that LRS infection is required for neuroinvasion. Here we investigated the role of prion infection of LRS tissues in neuroinvasion following extraneural inoculation with the HY and DY strains of the transmissible mink encephalopathy (TME) agent. DY TME agent infectivity was not detected in spleen or lymph nodes following intraperitoneal inoculation and clinical disease was not observed following inoculation into the peritoneum or lymph nodes, or after oral ingestion. In contrast, inoculation of the HY TME agent by each of these peripheral routes resulted in replication in the spleen and lymph nodes and induced clinical disease. To clarify the role of the LRS in neuroinvasion, the HY and DY TME agents were also inoculated into the tongue because it is densely innervated and lesions on the tongue, which are common in ruminants, increase the susceptibility of hamsters to experimental prion disease. Following intratongue inoculation, the DY TME agent caused prion disease and was detected in both the tongue and brainstem nuclei that innervate the tongue, but the prion protein PrP(Sc) was not detected in the spleen or lymph nodes. These findings indicate that the DY TME agent can spread from the tongue to the brain along cranial nerves and neuroinvasion does not require agent replication in the LRS. These studies provide support for prion neuroinvasion from highly innervated peripheral tissues in the absence of LRS infection in natural prion diseases of livestock.

BSE safety standards: An evaluation of public health policies of Japan, Europe, and USA

Since the advent of bovine spongiform encephalopathy (BSE) in the United Kingdom in 1986, new BSE cases have recently become rare. However, in Japan and the United States, positive cases have started to be seen recently. The rise in BSE cases paved the way for the human form of this disease, the variant Creutzfeldt-Jakob disease (vCJD). The observed trends in the UK may be attributed to effective implementation of public health policies coupled with increased vigilance through advancement in science and technology, or they may well be a reflection of the natural disease progression. We aim to discuss the BSE chronology of events, and compare examination methods, costs and cost-efficiency, management, and public policies of Japan, Europe, and the USA.

[Bovine spongiform encephalopathy]

The identification of variant Creutzfeldt-Jakob disease (vCJD) in human strongly reinforced the perception of risks associated with the infectious agent involved in Bovine Spongiform Encephalopathy (BSE). The development of rapid tests for the diagnosis of BSE by the detection of the abnormal prion protein allowed a huge increase in surveillance of the cattle disease. This first revealed a higher prevalence of the infection than previously believed. However, food safety measures, mainly based on the ban of the use of meat and bone meal in ruminants and the elimination of specified risk materials from the food chain, already allowed significant progress in the control of the cattle disease, especially in the United Kingdom. Nevertheless, the diagnosis can still not be obtained in the live animal, while the disease only appears following a several years incubation period. Another major issue is the identification of the BSE agent when it has been transmitted to another species. This question not only arises in veterinary medicine, with the major question of a possible infection of small ruminants by the BSE agent, but also in human in which the existence of other forms of the disease linked to the BSE agent but possibly differing from Creutzfeldt-Jakob disease cannot be excluded.

Transmissible spongiform encephalopathy risk assessment: the UK experience

Transmissible spongiform encephalopathy (TSE) risk assessments undertaken in the United Kingdom have mainly had the objective of determining the risks posed to humans from exposure to the causal agents associated with bovine spongiform encephalopathy (BSE) and variant Creutzfeld-Jakob disease (vCJD). In this article, I examine 19 of these risk assessments published to date and consider how their results might be influenced by underlying model assumptions and methodology. Three separate aspects common to all the assessments are infective load estimation, exposure pathway identification, and risk estimation. These are each discussed in detail.

Immunohistochemical distinction between preclinical bovine spongiform encephalopathy and scrapie infection in sheep

Sheep are susceptible experimentally to bovine spongiform encephalopathy (BSE), the clinical signs being indistinguishable from those of scrapie. Because of the possibility of natural ovine BSE infection, laboratory tests are needed to distinguish between scrapie and BSE infection. The objectives of this study were to determine whether (1) PrPSc accumulates in biopsy samples of the tonsil or third eyelid, or both, of BSE-infected sheep before the appearance of clinical disease, and (2) such samples from BSE- and scrapie-infected sheep differ in respect of PrPSc accumulations. Homozygous ARQ sheep (n = 10) were dosed orally at 4-5 months of age with a brain homogenate from BSE-infected cattle. Third eyelid and tonsillar biopsy samples were taken at < or = 6 monthly intervals post-infection and examined immunohistochemically for PrPSc. Third eyelid protuberances were difficult to identify, resulting in many unsuitable samples; however, third eyelid samples shown to contain lymphoid follicles were invariably negative for PrPSc. In contrast, tonsillar biopsy samples became positive for PrPSc from 11 to 20 months post-infection. Consistent differences in the morphology of PrPSc granules in tingible body macrophages (TBMs) between BSE- and scrapie-infected sheep were detected with anti-peptide antibodies directed towards amino acids 93-106 of the ovine prion protein: thus, PrPSc appeared as single granules in TBMs of tonsillar sections from BSE-infected sheep, whereas clusters of PrPSc granules were observed within TBMs in the tonsils of scrapie-infected sheep. In contrast, antibodies against epitopes situated N- and C-terminally from the 93-106 region of the ovine prion protein revealed no differences between BSE- and scrapie-infected sheep in terms of PrPSc granules in TBMs.

Comparative study of the PrPBSE distribution in brains from BSE field cases using rapid tests

The distribution of PrP(BSE) in the brain of nine confirmed BSE field cases was analyzed using immunohistochemistry and compared to the levels of PrP(BSE) determined by two rapid tests (Prionics-Check WESTERN and Prionics-Check LIA). Each brain was dissected into 16 areas: spinal cord, medulla oblongata, pons, mesencephalon, thalamus, hippocampus, cerebellar vermis, cerebellar medulla, cerebellar hemispheres, occipital cortex, temporal cortex, parietal cortex, striatum, frontal cortex, piriform lobe and olfactory bulbs. The highest levels of PrP(BSE) were detected in the medulla oblongata, spinal cord and pons, and correspondingly both rapid tests showed 100% correlation with the immunohistochemistry with regard to sensitivity and specificity. Some inconsistencies between the levels of PrP(BSE) determined either by immunohistochemistry or by the rapid tests were found in brain areas with medium to low levels of PrP(BSE). These brain areas included the cerebellar hemisphere, olfactory bulb, and the temporal and parietal cortices. A brain PrP(BSE) distribution curve (BPDC) was designed by plotting the PrP(BSE) signals obtained from the two rapid tests versus the anatomical region along the caudal-rostral axis of the brain. Comparison of the BPDC of the nine BSE cases showed that all cases had a similar PrP(BSE) distribution in the brain but with variable intensities, which could be explained by different stages in the progression of the disease. We propose that the BPDC could be used as a tool to differentiate classical cases of BSE from the recently identified atypical BSE cases.

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.

Immunohistochemical characteristics of disease-associated PrP are not altered by host genotype or route of inoculation following infection of sheep with bovine spongiform encephalopathy

It has previously been reported that disease-associated prion protein (PrPd) derived from natural scrapie and from sheep infected experimentally with bovine spongiform encephalopathy (BSE) differed in respect of their immunohistochemical and immunoblotting properties. For BSE, however, these initial observations were restricted to orally challenged sheep of the ARQ/ARQ PrP genotype. Here, extended examinations were performed on 28 sheep that developed neurological signs after BSE experimental infection by one of three routes. Intracerebrally infected ARQ/ARQ sheep showed more widespread and abundant accumulations of PrPd in tissues of the lymphoreticular system (LRS) than VRQ/VRQ animals, whereas no peripheral PrPd was detected in ARR/ARR sheep. The intensity and dissemination of PrPd accumulation in LRS tissues were less than those found previously in orally dosed sheep. AHQ/AHQ sheep challenged orally and ARQ/AHQ and ARQ/ARQ animals infected intravenously showed similar LRS-tissue PrPd distributions and levels to those of ARQ/ARQ sheep infected intracerebrally. The patterns of intra- and extracellular immunoreactivity to different PrP antibodies in brain and LRS tissues and the immunoblotting characteristics of PrPres from brain samples remained constant, irrespective of the route of inoculation and the PrP genotype, and were the same as described previously for ARQ/ARQ sheep dosed orally with BSE. These results suggest that the intracellular truncation of BSE PrPd and the proteinase K cleavage site of BSE PrPres are not altered by PrP genotype or by route of inoculation and that, therefore, screening tests based on these properties can be applied to identify potential sheep BSE cases occurring naturally.

Involvement of the peripheral nervous system in human prion diseases including dural graft associated Creutzfeldt-Jakob disease

OBJECTIVE

To investigate abnormal prion protein (PrP) deposition in the peripheral nervous system (PNS) in human prion diseases.

METHODS

Eight patients with prion diseases were examined: three with sporadic Creutzfeldt-Jakob disease (sCJD), two with dural graft associated CJD (dCJD), one with Gerstmann-Straussler-Scheinker disease (GSS) with a PrP P102L mutation (GSS102), and two with a P105L mutation (GSS105). An atypical case of sCJD with PrP plaques in the brain presented clinically with peripheral neuropathy, and showed demyelination in 12% of the teased fibres of the sural nerve. The PNS was investigated by immunohistochemical and western blotting analyses of PrP.

RESULTS

In immunohistochemical studies, granular PrP deposits were detected in some neurones of dorsal root ganglia and a few fibres of peripheral nerves and spinal posterior roots in one sCJD and two dCJD patients, but not in GSS102 or GSS105 patients. The atypical case of sCJD with peripheral neuropathy showed no obvious PrP deposition in the nerves. Western blotting analysis of the PNS from the dCJD patients revealed a small amount of protease K resistant PrP in the dorsal root ganglia and peripheral nerves.

CONCLUSIONS

Abnormal PrP deposition occurs in the dorsal root ganglia and peripheral nerves in sCJD and dCJD. The PrP deposits in the PNS are not correlated with clinical manifestation of peripheral neuropathy in CJD.

Chronic lymphocytic inflammation specifies the organ tropism of prions

Prions typically accumulate in nervous and lymphoid tissues. Because proinflammatory cytokines and immune cells are required for lymphoid prion replication, we tested whether inflammatory conditions affect prion pathogenesis. We administered prions to mice with five inflammatory diseases of the kidney, pancreas, or liver. In all cases, chronic lymphocytic inflammation enabled prion accumulation in otherwise prion-free organs. Inflammatory foci consistently correlated with lymphotoxin up-regulation and ectopic induction of FDC-M1+ cells expressing the normal cellular prion protein PrPC. By contrast, inflamed organs of mice lacking lymphotoxin-alpha or its receptor did not accumulate the abnormal isoform PrPSc, nor did they display infectivity upon prion inoculation. By expanding the tissue distribution of prions, chronic inflammatory conditions may act as modifiers of natural and iatrogenic prion transmission.

Tissue distribution of bovine spongiform encephalopathy infectivity in Romney sheep up to the onset of clinical disease after oral challenge

Sixty Romney sheep of three prion protein genotypes were dosed orally at six months of age with an inoculum prepared from the brains of cattle clinically affected with BSE, and 15 sheep were left undosed as controls. They were randomly assigned within genotype to groups and were sequentially euthanased and examined postmortem at intervals of six or 12 months, depending on their predicted susceptibility. Tissue pools prepared from the three, four or five dosed animals in each group were inoculated into groups of 20 RIII mice as a bioassay for infectivity. Separate inocula were prepared from the matched control sheep killed at each time. In the ARQ/ARQ sheep killed four months after inoculation, infectivity was detected in the Peyer's patch tissue pool, and at 10 months it was detected in the spleen pool; from 16 months, infectivity was detected in a range of nervous and lymphoreticular tissues, including the spinal cord pool, distal ileum excluding Peyer's patches, liver, Peyer's patches, mesenteric and prescapular lymph nodes, spleen, tonsil and cervical thymus. No infectivity was detected in the tissue pools from the ARQ/ARR and ARR/ARR sheep killed 10 months or 22 months after infection.

Population-level retrospective study of neurologically expressed disorders in ruminants before the onset of bovine spongiform encephalopathy (BSE) in Belgium, a BSE risk III country

A retrospective epidemiological study (n = 7,875) of neurologically expressed disorders (NED) in ruminants before the onset of the bovine spongiform encephalopathy epidemic (years studied, 1980 to 1997) was carried out in Belgium. The archives of all veterinary laboratories and rabies and transmissible spongiform encephalopathy (TSE) epidemiosurveillance networks were consulted. For all species, a significantly higher number of NED with virological causes (rabies) was reported south of the Sambre-Meuse Valley. During the period 1992 to 1997, for which the data were complete, (i) the predicted annual incidence of NED varied significantly as a function of species and area (higher numbers in areas where rabies was present) but was always above 100 cases per million, and (ii) the mean incidence of suspected TSE cases and, among them, those investigated by histopathological examination varied significantly as a function of species and area. The positive predictive value of a presumptive clinical diagnosis of NED ranged from 0.13 (game) to 0.63 (sheep). Knowledge of the positive predictive value permits the definition of a reference point before certain actions (e.g., awareness and training campaigns) are undertaken. It also shows the usefulness of a systematic necropsy or complementary laboratory tests to establish an etiological diagnosis. TSE analysis of a small, targeted historical sampling (n = 48) permitted the confirmation of one case and uncovered another case of scrapie. The results of the present study help to develop and maintain the quality of the worldwide clinical epidemiological networks for TSE, especially in countries that in the past imported live animals, animal products, and feedstuffs from countries with TSE cases.

Risk Analysis of Bovine Spongiform Encephalopathy in Korea

The occurrence of Bovine Spongiform Encephalopathy (BSE, so called mad cow diseases) that was first identified in England in 1986 was considered as being limited to only European countries, including England. However, the outbreak in Asia as well as North America since 2001 has amplified the fear that there isn't any nation in the world that is a safe area. In order to assess the risk of BSE outbreak in each country, the Office International des Epizooties (OIE) and EU have respectively established criteria, where OIE has set 5 levels and EU has set 4 levels. The Scientific Steering Committee (SSC) of the European Commission conducted a Geographical BSE Risk(GBR) assessment for 64 nations, such as the United States, etc., as of April 29, 2003. However, as of July 1, 2005, the duty of GBR assessment is expected to be transferred to a newly established body called EFSA (European Food Safety Authority, located in Parma, Italy). As Korea has not undergone a GBR assessment up to now, this study analyzed the risk of BSE outbreak in Korea by reviewing BSE prevention measures, etc., that have been put in place. This study shall be a barometer for estimating the GBR assessment level of Korea.

Clinical findings in 78 suspected cases of bovine spongiform encephalopathy in Great Britain

The clinical findings in 59 cows with bovine spongiform encephalopathy (BSE) were compared with those in 19 cattle that were submitted as BSE suspects but not confirmed by immunohistochemistry. Both groups were also compared with a control group of 20 healthy cows. Abnormalities in behaviour, temperament, mental status and activity, neurogenic disorders of gait and hyperreactivity to touch were frequently observed in the cattle with BSE. Not every animal with BSE displayed clinical signs in all these categories, and the severity of the signs was not always useful for differentiating them from the BSE suspects that were not confirmed by pathology. The neurological examination was better than passive observations for the clinical diagnosis of BSE. Tests of the animals' responses to sudden auditory, visual and tactile stimuli were very useful for distinguishing cases of BSE from unconfirmed BSE suspects if the cases did not display signs in all the categories.

Maternal transmission studies of BSE in sheep

If BSE (bovine spongiform encephalopathy) infected the UK sheep population concurrently with cattle, it would only now be maintained by transmission between sheep by routes which could include from mother to lamb either in utero or via perinatal close contact. In this study of experimental BSE, Cheviot ewes challenged orally with BSE cattle brain produced lambs of various PrP genotypes over the next 7 years. Of 72 surviving to >30 months of age, 29 are of the most susceptible PrP genotype (AQ/AQ) and born to mothers that were challenged with BSE. None of the progeny have shown any signs of disease. The results suggest that in these sheep, BSE could only transmit by the maternal route at a frequency of less than one in four (95 % confidence limit) from clinically affected ewes, a rate which if replicated in other breeds may not be sufficient to maintain BSE within the sheep population.

The modern landscape of transfusion-related iatrogenic Creutzfeldt–Jakob disease and blood screening tests

The idea that blood in naturally occurring transmissible spongiform encephalopathies is not infectious has imploded in the face of recent transmissions from the blood of naturally occurring scrapie in sheep and of variant Creutzfeldt-Jakob disease in humans. Although donor exclusion criteria ensure that the number of any further iatrogenic cases will be small, the risk of future blood-borne disease transmissions could be entirely eliminated by a diagnostic preclinical screening test. A variety of methodological approaches to blood testing are under development, with different levels of success, but no method has yet achieved the critical goal of discriminating transmissible spongiform encephalopathy-infected from healthy uninfected humans.

Antiprion immunotherapy: to suppress or to stimulate?

Although human prion diseases are rare, they are invariably fatal, and treatments remain elusive. Hundreds of iatrogenic prion transmissions have occurred in the past two decades, and the bovine spongiform encephalopathy epidemic has raised concerns about prion transmission from cattle to humans. Research into therapeutics for prion disease is being pursued in several centres and prominently includes immunological strategies. Currently, the options that are being explored aim either to mobilize the innate and adaptive immune systems towards prion destruction or to suppress or dedifferentiate the lymphoreticular compartments that replicate prions. This article reviews the pathophysiology of prion diseases in mouse models and discusses their relevance to immunotherapeutic and immunoprophylactic antiprion strategies.

Current concepts and controversies in prion immunopathology

Scrapie in sheep and new variant Creutzfeldt-Jakob disease in humans are typically initiated by extracerebral exposure to prions. Both exhibit early prion accumulation in sites of the peripheral lymphoreticular system, such as splenic or lymph nodal germinal centers. In germinal centers, follicular dendritic cells (FDCs), whose development and maintenance depend on lymphotoxin and tumor necrosis factor signaling, are believed to be the main cell type for efficient prion replication in the periphery. Here, we discuss the molecular requirements for prion replication competence in stromal and lymphoid compartments of lymphoid organs. In addition, we examine the preconditions of transepithelial passage of prions in the mucosal-associated lymphoid system. Our results suggest that under specific conditions, efficient prion replication in mesenteric and inguinal lymph nodes is possible in the absence of mature FDCs. M cells are a plausible candidate for the mucosal portal of prion infection.

Prion Diseases and the Spleen

Transmissible spongiform encephalopathies are fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans, bovine spongiform encephalopathy and scrapie in sheep and goats. Transmissible spongiform encephalopathies are thought by some to result from changes in the conformation of a membrane glycoprotein called PrPC (prion protein) into a pathogenic form, PrPSc, which constitutes the major component of an unprecedented type of infectious particle supposedly devoid of nucleic acid. Although there is no primary immunological response to the infectious agent, several lines of evidence indicate an involvement of the lymphoreticular system in the development of prion diseases. Studies in rodents have shown that after peripheral infection, uptake of the scrapie agent is followed by an initial phase of replication in the lymphoreticular system, particularly the spleen and lymph nodes. Moreover, infectivity titers in lymphoreticular organs reach a maximum relatively quickly, well before those in the brain, and then maintain a plateau for the remainder of the disease progression. The presence of PrPSc in peripheral lymphoid organs of all cases of variant Creutzfeldt-Jakob disease strongly underscores the importance of the lymphoreticular system. Thus, a better understanding of the cells participating in PrPSc replication and dissemination into the central nervous system is of particular interest. This review will therefore discuss the present knowledge of the role of the spleen in transmissible spongiform encephalopathies as well as the participation of the different spleen cell types in the disease process.

Characterization of two distinct prion strains derived from bovine spongiform encephalopathy transmissions to inbred mice

Distinct prion strains can be distinguished by differences in incubation period, neuropathology and biochemical properties of disease-associated prion protein (PrP(Sc)) in inoculated mice. Reliable comparisons of mouse prion strain properties can only be achieved after passage in genetically identical mice, as host prion protein sequence and genetic background are known to modulate prion disease phenotypes. While multiple prion strains have been identified in sheep scrapie and Creutzfeldt-Jakob disease, bovine spongiform encephalopathy (BSE) is thought to be caused by a single prion strain. Primary passage of BSE prions to different lines of inbred mice resulted in the propagation of two distinct PrP(Sc) types, suggesting that two prion strains may have been isolated. To investigate this further, these isolates were subpassaged in a single line of inbred mice (SJL) and it was confirmed that two distinct prion strains had been identified. MRC1 was characterized by a short incubation time (110+/-3 days), a mono-glycosylated-dominant PrP(Sc) type and a generalized diffuse pattern of PrP-immunoreactive deposits, while MRC2 displayed a much longer incubation time (155+/-1 days), a di-glycosylated-dominant PrP(Sc) type and a distinct pattern of PrP-immunoreactive deposits and neuronal loss. These data indicate a crucial involvement of the host genome in modulating prion strain selection and propagation in mice. It is possible that multiple disease phenotypes may also be possible in BSE prion infection in humans and other animals.

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.