Preclinical test for prion diseases

Clinical, pathological, and molecular features of classical and L-type atypical-BSE in goats

Monitoring of small ruminants for transmissible spongiform encephalopathies (TSEs) has recently become more relevant after two natural scrapie suspected cases of goats were found to be positive for classical BSE (C-BSE). C-BSE probably established itself in this species unrecognized, undermining disease control measures. This opens the possibility that TSEs in goats may remain an animal source for human prion diseases. Currently, there are no data regarding the natural presence of the atypical BSE in caprines. Here we report that C-BSE and L-type atypical BSE (L-BSE) isolates from bovine species are intracerebrally transmissible to goats, with a 100% attack rate and a significantly shorter incubation period and survival time after C-BSE than after L-BSE experimental infection, suggesting a lower species barrier for classical agentin goat. All animals showed nearly the same clinical features of disease characterized by skin lesions, including broken hair and alopecia, and abnormal mental status. Histology and immunohistochemistry showed several differences between C-BSE and L-BSE infection, allowing discrimination between the two different strains. The lymphoreticular involvement we observed in the C-BSE positive goats argues in favour of a peripheral distribution of PrPSc similar to classical scrapie. Western blot and other currently approved screening tests detected both strains in the goats and were able to classify negative control animals. These data demonstrate that active surveillance of small ruminants, as applied to fallen stock and/or healthy slaughter populations in European countries, is able to correctly identify and classify classical and L-BSE and ultimately protect public health.

Effectiveness of Capillary Electrophoresis Fluoroimniunoassay of Blood PrPSc for Evaluation of Scrapie Pathogenesis in Sheep

Management of prion diseases in livestock would benefit greatly from availability of a validated blood test. A promising immunocapillary electrophoresis technique (also known as capillary electrophoresis fluoroimmunoassay) to detect abnormal prion protein in blood from live sheep is evaluated here. Capillary electrophoresis fluoroimmunoassay was applied to analysis of extracted blood from scrapie-exposed sheep ( n = 87; 347 samples) at various stages of incubation, and to control sheep ( n = 194; 489 samples). Overall, test values for the control and test populations were not significantly different, and a similar proportion of control (7%) and test (10%) sheep were classified as positive. Over 2−3 month intervals from birth until clinical disease, test specificity and sensitivity ranged from 66.7% to 100% and 0% to 66.7%, respectively, indicating poor diagnostic performance at all stages of pathogenesis. In routine application, in its present form, the capillary electrophoresis fluoroimmunoassay procedure proved to be insufficiently robust for use as a blood test for scrapie diagnosis.

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

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

Modelling neurodegeneration in prion disease - applications for drug development

Prion diseases are a group of neurodegenerative diseases that affect mammals, including humans and ruminants such as sheep. They are believed to be caused by the conversion of the prion protein (PrP), a host expressed protein, into a toxic form (PrP(sc)). PrP(sc) accumulates in the brain, resulting in neuronal loss and the typical spongiform appearance of the brain. So far, there are no effective therapies available for prion diseases. This review discusses possible therapies for prion diseases and the models available for advancing research into the disease.

Salivary prions in sheep and deer

Scrapie of sheep and chronic wasting disease (CWD) of cervids are transmissible prion diseases. Milk and placenta have been identified as sources of scrapie prions but do not explain horizontal transmission. In contrast, CWD prions have been reported in saliva, urine and feces, which are thought to be responsible for horizontal transmission. While the titers of CWD prions have been measured in feces, levels in saliva or urine are unknown. Because sheep produce ~17 L/day of saliva, and scrapie prions are present in tongue and salivary glands of infected sheep, we asked if scrapie prions are shed in saliva. We inoculated transgenic (Tg) mice expressing ovine prion protein, Tg(OvPrP) mice, with saliva from seven Cheviot sheep with scrapie. Six of seven samples transmitted prions to Tg(OvPrP) mice with titers of -0.5 to 1.7 log ID₅₀ U/ml. Similarly, inoculation of saliva samples from two mule deer with CWD transmitted prions to Tg(ElkPrP) mice with titers of -1.1 to -0.4 log ID₅₀ U/ml. Assuming similar shedding kinetics for salivary prions as those for fecal prions of deer, we estimated the secreted salivary prion dose over a 10-mo period to be as high as 8.4 log ID₅₀ units for sheep and 7.0 log ID₅₀ units for deer. These estimates are similar to 7.9 log ID₅₀ units of fecal CWD prions for deer. Because saliva is mostly swallowed, salivary prions may reinfect tissues of the gastrointestinal tract and contribute to fecal prion shedding. Salivary prions shed into the environment provide an additional mechanism for horizontal prion transmission.

Atypical transmissible spongiform encephalopathies in ruminants: a challenge for disease surveillance and control

Since 1987, when bovine spongiform encephalopathy (BSE) emerged as a novel disease in cattle, enormous efforts were undertaken to monitor and control the disease in ruminants worldwide. The driving force was its high economic impact, which resulted from trade restrictions and the loss of consumer confidence in beef products, the latter because BSE turned out to be a fatal zoonosis, causing variant Creutzfeldt-Jakob disease in human beings. The ban on meat and bone meal in livestock feed and the removal of specified risk materials from the food chain were the main measures to successfully prevent infection in cattle and to protect human beings from BSE exposure. However, although BSE is now under control, previously unknown, so-called atypical transmissible spongiform encephalopathies (TSEs) in cattle and small ruminants have been identified by enhanced disease surveillance. This report briefly reviews and summarizes the current level of knowledge on the spectrum of TSEs in cattle and small ruminants and addresses the question of the extent to which such atypical TSEs have an effect on disease surveillance and control strategies.

Breeding with resistant rams leads to rapid control of classical scrapie in affected sheep flocks

Susceptibility to scrapie, a transmissible spongiform encephalopathy in sheep, is modulated by the genetic make-up of the sheep. Scrapie control policies, based on selecting animals of resistant genotype for breeding, have recently been adopted by the Netherlands and other European countries. Here we assess the effectiveness of a breeding programme based on selecting rams of resistant genotype to obtain outbreak control in classical scrapie-affected sheep flocks under field conditions. In six commercially-run flocks following this breeding strategy, we used genotyping to monitor the genotype distribution, and tonsil biopsies and post-mortem analyses to monitor the occurrence of scrapie infection. The farmers were not informed about the monitoring results until the end of the study period of six years. We used a mathematical model of scrapie transmission to analyze the monitoring data and found that where the breeding scheme was consistently applied, outbreak control was obtained after at most four years. Our results also show that classical scrapie control can be obtained before the frequency of non-resistant animals is reduced to zero in the flock. This suggests that control at the national scale can be obtained without a loss of genetic polymorphisms from any of the sheep breeds.

Prions: Protein Aggregation and Infectious Diseases

Transmissible spongiform encephalopathies (TSEs) are inevitably lethal neurodegenerative diseases that affect humans and a large variety of animals. The infectious agent responsible for TSEs is the prion, an abnormally folded and aggregated protein that propagates itself by imposing its conformation onto the cellular prion protein (PrPC) of the host. PrPCis necessary for prion replication and for prion-induced neurodegeneration, yet the proximal causes of neuronal injury and death are still poorly understood. Prion toxicity may arise from the interference with the normal function of PrPC, and therefore, understanding the physiological role of PrPCmay help to clarify the mechanism underlying prion diseases. Here we discuss the evolution of the prion concept and how prion-like mechanisms may apply to other protein aggregation diseases. We describe the clinical and the pathological features of the prion diseases in human and animals, the events occurring during neuroinvasion, and the possible scenarios underlying brain damage. Finally, we discuss potential antiprion therapies and current developments in the realm of prion diagnostics.

Adaptation and evaluation of a rapid test for the diagnosis of sheep scrapie in samples of rectal mucosa

In recent publications, it was shown that disease-associated prion protein (PrP(d)) accumulates in the lymphoid tissue of the rectal mucosa of a high proportion of scrapie-infected sheep at clinical and preclinical stages, regardless of several host factors; PrP(d) can also be detected in biopsy specimens of rectal mucosa, with an increased probability proportional to age or incubation period and with an efficiency almost identical to that of tonsil biopsies. Rectal biopsies have the advantages of providing higher numbers of lymphoid follicles and of being simpler to perform, which makes them suitable for scrapie screening in the field. In biopsy samples, PrP(d) could be demonstrated by immunohistochemical (IHC) and Western immunoblotting methods, and the purpose of the present study was to optimize and evaluate a "rapid test" for the diagnosis of scrapie in rectal biopsy samples. The HerdChek CWD (chronic wasting disease) antigen EIA (enzyme immunoassay) test was chosen and, once optimized, provided specificity and sensitivity figures of 99.2% and 93.5%, respectively, compared with IHC results in the same samples obtained at a postmortem. The sensitivity of the assay increased from 82.1%, when a single rectal mucosa sample was tested to 99.4% for those sheep in which 3 or more samples were analyzed. Similarly, sensitivity values of the HerdChek CWD antigen EIA test on biopsy samples increased from 95% to 100% for sheep subjected to 1 or 2 sequential biopsies 4 months apart, respectively. Thus, a preclinical diagnosis of scrapie in live sheep can be achieved by a combination of a simple sampling procedure, which can be repeated several times with no detrimental effect for the animals, and a rapid and efficient laboratory method.

Classical sheep transmissible spongiform encephalopathies: pathogenesis, pathological phenotypes and clinical disease

Scrapie is a prion disease or transmissible spongiform encephalopathy (TSE) of sheep, goats and moufflon. As with its human counterparts, pathology consists of vacuolation, gliosis and accumulations of abnormal forms of a host prion protein (PrPd) in the brain of affected individuals. Immunohistochemical methods can be used to identify both the intracellular truncation sites of PrPd in different cell types (PrPd epitope mapping) and the different morphological patterns of accumulation (PrPd profiling). Differences in the inferred truncation sites of PrPd are found for different strains of sheep TSEs and for different infected cell types within individual strains. Immunochemical methods of characterizing strains broadly correspond to PrPd mapping discriminatory results, but distinct PrPd profiles, which provide strain- and source-specific information on both the cell types which sustain infection (cellular tropisms) and the cellular processing of PrPd, have no immunoblotting counterparts. The cause of neurological dysfunction in human is commonly considered to be neuronal loss secondary to a direct or indirect effect of the accumulation of PrPd. However, in sheep scrapie there is no significant neuronal loss, and relationships between different magnitudes, topographical and cytological forms of PrPd accumulation and clinical signs are not evident. PrPd accumulation also occurs in lymphoid tissues, for which there is indirect evidence of a pathological effect, in the peripheral nervous system and in other tissues. It is generally assumed that neuroinvasion results from infection of the enteric nervous system neurones subsequent to amplification of infectivity in lymphoid tissues and later spread via sympathetic and parasympathetic pathways. The evidence for this is, however, circumstantial. Accumulation of PrPd and presence of infectivity in tissues other than the nervous and lymphoreticular systems gives insights on the ways of transmission of infection and on food safety.

Dynamics and genetics of PrPSc placental accumulation in sheep

Placentae from scrapie-affected ewes are an important source of contamination. This study confirmed that scrapie-incubating ewes bearing susceptible genotypes could produce both abnormal prion protein (PrPSc)-positive and -negative placentae, depending only on the PRP genotype of the fetus. The results also provided evidence indicating that scrapie-incubating ARR/VRQ ewes may be unable to accumulate prions in the placenta, whatever the genotype of their progeny. Multinucleated trophoblast cells appeared to play a key role in placental PrPSc accumulation. PrPSc accumulation began in syncytiotrophoblasts before disseminating to uninucleated trophoblasts. As these result from trophoblast/uterine epithelial cell fusion, syncytiotrophoblast cells expressed maternal and fetal PrPC, whilst uninucleated trophoblast cells only expressed fetal PrPC. In ARR/VRQ scrapie-infected ewes, expression of the ARR allele by syncytiotrophoblasts appeared to prevent initiation of PrPSc placental deposition. The absence of prions in affected ARR/VRQ sheep placentae reinforces strongly the interest in ARR selection for scrapie control.

Recent Developments in Prion Disease Research: Diagnostic Tools and In Vitro Cell Culture Models

After prion infection, an abnormal isoform of prion protein (PrP(Sc)) converts the cellular isoform of prion protein (PrP(C)) into PrP(Sc). PrP(C)-to-PrP(Sc) conversion leads to PrP(Sc) accumulation and PrP(C) deficiency, contributing etiologically to induction of prion diseases. Presently, most of the diagnostic methods for prion diseases are dependent on PrP(Sc) detection. Highly sensitive/accurate specific detection of PrP(Sc) in many different samples is a prerequisite for attempts to develop reliable detection methods. Towards this goal, several methods have recently been developed to facilitate sensitive and precise detection of PrP(Sc), namely, protein misfolding cyclic amplification, conformation-dependent immunoassay, dissociation-enhanced lanthanide fluorescent immunoassay, capillary gel electrophoresis, fluorescence correlation spectroscopy, flow microbead immunoassay, etc. Additionally, functionally relevant prion-susceptible cell culture models that recognize the complexity of the mechanisms of prion infection have also been pursued, not only in relation to diagnosis, but also in relation to prion biology. Prion protein (PrP) gene-deficient neuronal cell lines that can clearly elucidate PrP(C) functions would contribute to understanding of the prion infection mechanism. In this review, we describe the trend in recent development of diagnostic methods and cell culture models for prion diseases and their potential applications in prion biology.

Rapid and discriminatory diagnosis of scrapie and BSE in retro-pharyngeal lymph nodes of sheep

BACKGROUND

Diagnosis based on prion detection in lymph nodes of sheep and goats can improve active surveillance for scrapie and, if it were circulating, for bovine spongiform encephalopathy (BSE). With sizes that allow repetitive testing and a location that is easily accessible at slaughter, retropharyngeal lymph nodes (RLN) are considered suitable organs for testing. Western blotting (WB) of brain homogenates is, in principle, a technique well suited to both detect and discriminate between scrapie and BSE. In this report, WB is developed for rapid diagnosis in RLN and to study biochemical characteristics of PrPres.

RESULTS

Optimal PrPres detection in RLN by WB was achieved by proper tissue processing, antibody choice and inclusion of a step for PrPresconcentration. The analyses were performed on three different sheep sources. Firstly, in a study with preclinical scrapie cases, WB of RLN from infected sheep of VRQ/VRQ genotype--VRQ represents, respectively, polymorphic PrP amino acids 136, 154, and 171--allowed a diagnosis 14 mo earlier compared to WB of brain stem. Secondly, samples collected from sheep with confirmed scrapie in the course of passive and active surveillance programmes in the period 2002-2003 yielded positive results depending on genotype: all sheep with genotypes ARH/VRQ, VRQ/VRQ, and ARQ/VRQ scored positive for PrPres, but ARQ/ARQ and ARR/VRQ were not all positive. Thirdly, in an experimental BSE study, detection of PrPres in all 11 ARQ/ARQ sheep, including 7 preclinical cases, was possible. In all instances, WB and IHC were almost as sensitive. Moreover, BSE infection could be discriminated from scrapie infection by faster electrophoretic migration of the PrPres bands. Using dual antibody staining with selected monoclonal antibodies like 12B2 and L42, these differences in migration could be employed for an unequivocal differentiation between BSE and scrapie. With respect to glycosylation of PrPres, BSE cases exhibited a greater diglycosylated fraction than scrapie cases. Furthermore, a slight time dependent increase of diglycosylated PrPres was noted between individual sheep, which was remarkable in that it occurred in both scrapie and BSE study.

CONCLUSION

The present data indicate that, used in conjunction with testing in brain, WB of RLN can be a sensitive tool for improving surveillance of scrapie and BSE, allowing early detection of BSE and scrapie and thereby ensuring safer sheep and goat products.

Identification of an allelic variant of the goat PrP gene associated with resistance to scrapie

The association between PrP gene variations and scrapie susceptibility was studied in a single herd of Ionica breed goats. The entire herd comprised 100 animals, 11 of which were clinically affected and showed pathological prion protein (PrPSc) deposition in both their central nervous system (CNS) and lymphoreticular system (LRS). Among asymptomatic goats, nine harboured PrPSc in both CNS and LRS, 19 showed PrPSc only at the LRS level and 61 animals had no PrPSc deposition. Genetic analysis of the PrP gene coding sequence revealed the presence of several polymorphisms, namely G37V, T110P, H143R, R154H, Q222K and P240S. Silent polymorphisms were also found at codons 42, 138, 219 and 232. The effect of PrP polymorphism on scrapie susceptibility was assessed by comparing the genotype distribution at each locus among animals with different pathogenetic and clinical disease stages. Significant differences in the distribution of genotypes were observed for codons 154 and 222, with polymorphism at codon 154 modulating susceptibility to scrapie and lysine at codon 222 being associated with scrapie resistance. The allelic variant encoding lysine at position 222 could be a valuable candidate to select in the framework of appropriate breeding programmes for scrapie resistance in goats.

PrP(Sc) deposition in nervous tissues without lymphoid tissue involvement is frequently found in ARQ/ARQ Sarda breed sheep preclinically affected with natural scrapie

The pathogenesis of natural scrapie in Sarda breed sheep was investigated in 1050 asymptomatic and 49 sick sheep from scrapie-affected flocks. Central and peripheral nervous system, along with lymphoreticular system (LRS) tissues, were subjected to immunohistochemistry (IHC) and Western-blotting (WB) for detection of pathological isoform of the prion protein (PrP(Sc)). A total of 69 of the 1050 clinically healthy sheep were found to be infected with scrapie, with PrP(Sc) being detected in both the central nervous system (CNS) and enteric nervous system (ENS) plexuses of 60 of the sheep, while IHC and WB yielded evidence of (PrP(Sc)) deposition only in lymphoid tissues of the remaining 9 clinically healthy sheep. PrP(Sc) was also detected in the CNS, as well as in ENS plexuses from all of the 49 clinically affected sheep. Nevertheless, 18 of the 69 clinically healthy animals (26%, 17 ARQ/ARQ and 1 ARQ/AHQ sheep), along with 3 ARQ/ARQ sheep (6%) of the clinically affected group, showed no IHC or WB evidence of PrP(Sc) in lymphoid tissues, but PrP(Sc) could be still detected in their CNS and ENS plexuses. The study demonstrates dual CNS and ENS PrP(Sc) deposition in Sarda sheep with scrapie, in spite of an apparent lack of lymphoid tissue involvement in a number of cases.

vCJD prion acquires altered virulence through trans-species infection

Variant Creutzfeldt-Jakob disease (vCJD) appears to be caused by infection with the bovine spongiform encephalopathy (BSE) agent. To date, all patients with vCJD are homozygous for methionine at codon 129 of the PrP gene. To investigate the relationship between polymorphism at codon 129 and susceptibility to BSE or vCJD prions, we performed splenic follicular dendritic cell assay with humanized knock-in mice through peripheral infection. All humanized knock-in mice showed little or no susceptibility to BSE prions. Only the subset of humanized knock-in mice with codon 129 Met/Met genotype showed weak susceptibility by Western blotting. Surprisingly, we succeeded in the transmission of vCJD prions to humanized knock-in mice not only with codon 129 Met/Met but also with codon 129 Met/Val. Humanized knock-in mice with codon 129 Val/Val were not susceptible. The results suggest that human heterozygotes at codon 129 are also at risk for secondary infection with vCJD.

Clinical and pathological features of experimental scrapie in Irish Blackface Mountain sheep

There have been no reports of natural scrapie in Irish Blackface Mountain (BM) sheep which account for approximately 16% of the Irish national sheep flock. The aim of this study was to determine if Irish BM sheep had unusual clinical and/or pathological features of scrapie which would account for failure to diagnose the disease in this breed. BM (n=7), Texel (n=3) and Suffolk sheep (n=1) of scrapie-susceptible PrP genotypes (ARQ/ARQ and VRQ/ARQ) were orally challenged with scrapie-infected brain inoculum. The incubation period, clinical signs, pathology and distribution of disease specific prion protein (PrP(d)) in scrapie-affected BM sheep were similar to scrapie in the Texel and Suffolk sheep. It was concluded that there was no evidence to suggest that scrapie in BM sheep differs clinicopathologically from scrapie in other breeds of sheep.

Approaches to Scrapie diagnosis by applying immunohistochemistry and rapid tests on central nervous and lymphoreticular systems

Comparative studies evaluating the performance of Transmissible Spongiform Encephalopathies (TSE) rapid tests (validated for Bovine Spongiform Encephalopathy samples) on Scrapie samples have not been reported widely, particularly those dealing with lymphoreticular system tissues to a much lesser extent. The main objective of this study was to compare the ability of two current rapid tests (Western blot and Luminiscence Immunoassay Prionics-Check; WB and LIA, respectively) to detect PrPsc using central nervous system as well as lymphoreticular system samples corresponding to naturally infected animals. Thirty-four Scrapie-affected sheep, 26 with clinical signs of the disease, were included in the study. Tonsil, retropharyngeal lymph node and medulla oblongata were assessed by three tests: immunohistochemistry (confirmatory test), WB and LIA (rapid tests). The conclusion which can be drawn from this study is the fact that all animals involved in the study, including those at a preclinical stage, could be diagnosed regardless of the test used (with immunohistochemistry consistently showing higher sensitivity) only when the analyses of both the central nervous system and the lymphoreticular system were considered. The choice of these tissues for routine diagnosis is, therefore, proposed as a valuable tool to highly reduce the number of undetected positive cases.

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.

Biology of PrPsc accumulation in two natural scrapie-infected sheep flocks

Sheep scrapie is a prion disease that requires interaction of exogenous prions with host prion protein (PrP) supporting prion formation. Disease is associated with deposition of a host-generated conformational variant of PrP, PrPsc, in a variety of tissues, including brain, resulting in fatal spongiform encephalopathy. Efficiency of PrPsc formation is determined by polymorphisms in the PrP-coding sequence. This article adds to previous data of natural sheep scrapie, concentrating on the effect of host genotype and age on PrPsc accumulation patterns during preclinical and clinical disease. Two entire scrapie-infected, predominantly Suffolk-cross, sheep flocks euthanized for regulatory purposes were genotyped and analyzed for PrPsc deposition in various tissues using single- and dual-label immunohistochemistry. Scrapie, as defined by PrPsc deposition, occurred in 13/80 sheep. Preclinical disease was evident in nearly 70% of infected sheep, ranging in age from 14 months to 7 years. PrPsc accumulated systemically in the nervous tissue, various lymphoid tissues, both alimentary tract related and non-alimentary tract related, and the placenta. Clinical neurological illness was always associated with spongiform encephalopathy and PrPsc deposition in the brain. Only 6 of 9 sheep with preclinical scrapie had PrPsc deposition in the brain but widespread PrPsc deposition in peripheral lymphoid tissue, supporting previous data showing peripheral PrPsc accumulation preceding deposition in the brain. PrPsc colocalized with a marker for follicular dendritic cells throughout the lymphoid system. PrPsc also accumulated in the peripheral nervous system, particularly the nervous supply of the gastrointestinal tract. Abundant PrPsc was evident in trophoblast cells of placentomes but not in the endometrium, myometrium, or associated nervous plexus. PrPsc deposits were not observed in the mammary parenchyma or bone marrow. Scrapie susceptibility was defined genetically by PrP codon 171: PrPsc deposition was restricted to PrP genotype AA136RR154QQ171 in 12/13 cases or AV136RR154QQ171 in 1/13 cases. The earliest accumulation was observed in the single VRQ/ARQ heterozygous animal, consistent with the reported high scrapie susceptibility and brief incubation period observed in breeds with predominance of the V136R154Q171 allele. Disease occurred within, as well as independent of, mother-daughter lines, suggesting both maternal and nonmaternal transmission in the flocks.

Raman analysis of prion protein in blood cell membranes from naturally affected scrapie sheep

At present, there are no efficient antemortem diagnostic tests for transmissible spongiform encephalopathys (TSEs), particularly in blood. New strains of prion PrPSc, which causes TSEs, are currently appearing, and researchers remain concerned that if prion variants continue to emerge, some of them may escape detection by existing immunoassay tests. Because a common structural feature of PrPSc strains is their high content of beta sheets, Raman spectroscopy has proven to be a suitable technique to analyze a key membranous fraction of blood containing PrPSc. In this fraction, a significant increase in beta sheets has been correlated with the worsening of this TSE in naturally scrapie-infected animals in comparison with healthy controls. Since sensitivity and specificity were found to be 100% for each, this test may lead to a new and alternative diagnosis for prion diseases.

Abnormal prion protein in genetically resistant sheep from a scrapie-infected flock

The central molecular event in transmissible spongiform encephalopathies, such as scrapie in sheep, is the accumulation in tissues of an abnormal isoform of the cellular prion protein. A previous investigation of 26 sheep showed that the accumulation of PrP(res) in brain correlated more with the prnp genotype than with the severity of the clinical disease. Here, the ability of a sandwich ELISA to detect PrP(res) distribution in the brain was demonstrated. Immunohistochemistry also strongly supported the hypothesis that the dorsal motor nucleus of the vagus nerve is the possible entry site in the brain for the scrapie agent. Remarkably, three asymptomatic (or possibly asymptomatic for scrapie) sheep carrying an allele known to be associated with clinical scrapie resistance (ARR), which were negative for the detection of PrP(res) by Western blotting and immunohistochemistry, were positive for the presence of PrP(res) by ELISA, raising the possibility of carriers resistant to the disease and possibly contributing to the persistence of scrapie in certain flocks.

Diagnosing prion diseases: needs, challenges and hopes

Prion diseases are among the most intriguing infectious diseases and are associated with unconventional proteinaceous infectious agents known as prions. Prions seem to lack nucleic acid and propagate by transmission of protein misfolding. The nature of prions and their unique mode of transmission present challenges for early diagnosis of prion diseases. In this article, state-of-the-art prion diagnostic techniques, together with the new strategies that are being used to develop sensitive, early and non-invasive diagnoses for these diseases are reviewed.

Demonstration of lateral transmission of scrapie between sheep kept under natural conditions using lymphoid tissue biopsy

Scrapie free adult sheep were introduced to a sheep flock specifically maintained to maximise scrapie infection. Native born sheep of the highly susceptible VRQ/VRQ genotype in this flock show highly efficient transmission, evidenced by 100% infection, with an age at death of less than 2 years. Infection in introduced sheep was identified by biopsy of tonsilar and nictitating membrane lymphoid tissue. Progeny of these sheep were monitored and clinical disease confirmed by examination of the brain using routine diagnostic methods. Naïve sheep of New Zealand origin introduced to the flock in adulthood became infected, demonstrating that lateral transmission had occurred. Lambs born to introduced ewes became infected and died at the same age as lambs born to native ewes, consistent with lateral transmission of scrapie to lambs. Although maternal transmission cannot be totally excluded for the lambs in this study, the data are consistent with lateral transmission being the most important means of spread leading to the high incidence of scrapie observed in this flock.

Detection of PrPsc on lymphoid tissues from naturally affected scrapie animals: comparison of three visualization systems

We assessed three different visualization systems used routinely in research and diagnosis of transmissable spongiform encephalopathies (TSEs) to demonstrate whether the methodology applied to immunohistochemical (IHC) examination may alter the results concerning detection of prion protein (PrPsc) in the lymphoreticular system (LRS): avidin-biotin-peroxidase (Vectastain ABC kit; Vector), Envision (DAKO), and catalyzed signal amplification (CSA; DAKO). The study aimed to determine which of these showed the highest sensitivity, with the hope of providing an accurate tool for pathogenesis and preclinical diagnosis research in TSEs. Histological sections from palatine tonsils, spleen, GALT (ileum and ileocecal valve), and lymph nodes from sheep belonging to a Spanish scrapie-positive flock were processed by IHC using L42 as primary antibody. As substrate chromogen, diaminobenzidine was used, and all slides were subjectively assessed by light microscopy. A further study using an image analyzer software system was carried out to confirm that the conclusion provided by microscopic examination was objective. The CSA system showed the highest sensitivity in all cases, increasing both variables assessed: the number of follicles that were PrPsc-positive was detected as well as the intensity of immunostaining in each of them.

Mammalian prion biology: one century of evolving concepts

Prions have been responsible for an entire century of tragic episodes. Fifty years ago, kuru decimated the population of Papua New Guinea. Then, iatrogenic transmission of prions caused more than 250 cases of Creutzfeldt-Jakob disease. More recently, transmission of bovine spongiform encephalopathy to humans caused a widespread health scare. On the other hand, the biology of prions represents a fascinating and poorly understood phenomenon, which may account for more than just diseases and may represent a fundamental mechanism of crosstalk between proteins. The two decades since Stanley Prusiner's formulation of the protein-only hypothesis have witnessed spectacular advances, and yet some of the most basic questions in prion science have remained unanswered.

Prospects for the development of pre-mortem laboratory diagnostic tests for Creutzfeldt-Jakob disease

At present the diagnosis of Creutzfeldt-Jakob disease (CJD) and related transmissible spongiform encephalopathies in humans is based on clinical criteria and (at post-mortem) the histopathological and immunological examination of brain tissue. The misfolded prion protein, PrPSc, is the single most significant marker, but its recognition by standard serological methods is complicated by its antigenic similarity to the normal prion protein, PrPC. Although there are commercial diagnostic assays available for bovine spongiform encephalopathy using brain specimens taken at slaughter, there are no suitable pre-mortem assays for cattle and none either for pre-mortem human disease. Especially in view of the recent report of variant CJD transmission by blood transfusion, it is important that tests for pre-symptomatic infections are developed. This will safeguard the blood supply and, for example, prevent the transmission of CJD in neurosurgery. This paper reviews the current and prospective approaches to the pre-mortem diagnosis of CJD, in particular its variant form.

Monte Carlo simulation of surveillance strategies for scrapie in Norwegian sheep

Our aim was to compare the efficiency of different surveillance strategies for detecting scrapie-infected sheep flocks in the Norwegian population using simulation modelling. The dynamic Monte Carlo simulation model has the flock as the unit. The input parameters include properties of the sheep population (number of flocks, flock size, age distribution, reasons for culling, breeds, prion protein-allele distribution); properties of scrapie (genotype-dependent infection rate and incubation periods, and age- and genotype-dependent prevalence of scrapie); properties of the surveillance strategy (selection of sheep for examination, period in which infected sheep are detectable, and properties of the diagnostic tests). For simplification, the prion protein-alleles were grouped into three allele groups: VRQ, ARR, and ARQ' (ARQ' represents ARQ, ARH and AHQ). Through either abattoir surveillance or surveillance of fallen stock, <or=9% of scrapie flocks were detected. The necessary sample size for detecting any particular number of scrapie flocks was considerably lower using surveillance of fallen stock than abattoir surveillance. After increasing the diagnostic method's sensitivity, only the efficiency of abattoir surveillance increased. The prion protein-genotypes ARQ'/ARQ', VRQ/ARQ' and VRQ/VRQ were overrepresented both in the sampled infected sheep and in the detected sheep. Sheep with ARQ'/ARQ' constituted >70% of the detected sheep (compared to 33% in the underlying population). The model output was sensitive to the susceptibility of infection for the genotype ARQ'/ARQ'. The effect was large for abattoir surveillance (increased susceptibility increased the efficiency of abattoir surveillance).

Prions and the human transmissible spongiform encephalopathies

The last 5 years have seen the emergence of a new disease in humans (vCJD), mainly in the United Kingdom. This emergence has been accompanied by an explosion of scientific data on a novel group of the responsible infectious agents called prions and has profound implications for infection control and transfusion policies. Also of concern is the finding of prions in neural, gingival, pulpal, and salivary tissue in animal models and significant titers of infectivity from extraneural organs (particularly, in cases of vCJD, in lymphoreticular tissues). There is limited information on the presence of prion proteins in the oral tissues from human studies. Because of the differences in patterns of disease in animal models and in strains of prion protein, it is difficult to extrapolate directly these findings to humans, but it illustrates a potential for transmission by way of the dental route. High levels of infectivity may be present in tissues early in the incubation period and before clinical signs and symptoms. The dental profession must turn its attention to the routine decontamination of dental instruments to ensure that these procedures are performed to the highest regulatory standard. Clinicians and manufacturers must work closely together to develop instruments that are either single use or can be presented in a form that can be more easily decontaminated. Clinicians must pay close attention to manufacturers' decontamination instructions and must not reuse items designated as single use, such as endodontic files. Improvements in compliance with these requirements will not only reduce the risk of transmission of TSEs but also other less tenacious infectious agents.

Scrapie and chronic wasting disease

Scrapie and CWD share many features. There are marked similarities in the clinical presentations, the lesions, and the pathogenesis of these diseases, and some similarities in the epidemiology. Extrapolation from the scrapie model of TSE disease to CWD--which occurs in three different species, and should not be considered to be uniform in their response--may be erroneous, however. Such differences may influence diagnostics (e.g., the amount and distribution of PrPC in these different species), pathogenesis (e.g., the influence of genetics on susceptibility and resistance), and epidemiology (e.g., the mode and dynamics of transmission and influences of domestication). IHC is used widely for diagnostics and in the study of the pathogenesis of scrapie and CWD. This technique holds promise for antemortem diagnosis of infection in the peripheral lymphoid tissues such as lymphoid follicles of the nictitating membrane and the tonsil.

Post tonsillectomy morbidity: following the introduction of single-use-instruments

OBJECTIVE

Following the introduction of single-use-instruments (SUI) for tonsillectomy in January 2001 in UK, the Medical Devices Agency (MDA) received reports of significant increase in secondary haemorrhage in some centres. This was believed to be related to electrosurgical (diathermy) forceps. The aim of this study is to compare post tonsillectomy morbidity before (year 2000) and after (year 2001) the introduction of SUI in a unit where bipolar dissection is the most popular method of dissection.

METHOD

The case notes of all tonsillectomy patients between 20 April and 31 October for the years 2000 (145 patients) and 2001 (166 patients) were reviewed. The patients' age and sex, other concurrent operations, grade of surgeon, dissection and haemostasis methods, length of post operative hospital stay and the reason for prolonged stay (if more than one night) were noted. Readmitted cases were analysed to identify the cause and the management.

RESULTS

There was no case of reactionary haemorrhage in both the non SUI (year 2000) and the SUI (year 2001) groups. For the non SUI versus SUI group, 6.2 versus 4.8% stayed more than one night post operatively; 4.8 versus 5.4% were readmitted for secondary haemorrhage with 1.4 versus 2.4% return to theatre to control bleeding, respectively. The average hospital stay for the readmitted cases were 2.1 versus 1.6 days and the average time lapse between tonsillectomy and secondary haemorrhage were 6.7 versus 6 days respectively.

CONCLUSION

There was no significant difference in post tonsillectomy morbidity following the introduction of SUI.

Scrapie and experimental BSE in sheep

Scrapie is a natural disease of sheep, but it can also be successfully transmitted between sheep by experimental inoculation. Although BSE is primarily a disease of cattle, it has also infected humans (causing vCJD) and, in addition, can be transmitted orally to sheep bringing concerns that BSE might naturally have infected the UK sheep population. Because of this, scrapie and BSE are being compared and studied in detail in sheep. PrP genotype controls sheep susceptibility and resistance to scrapie and to BSE, and deposition of the disease-associated PrP(Sc), used as a marker of infection, has the potential to act as a means of identifying BSE-infected animals and describing different pathogenesis mechanisms. Sheep orally dosed with BSE show signs of infection in their blood and this model is of major importance in the study of the safety of blood products for use with human beings.

Occurrence and distribution of infection-specific PrP in tissues of clinical scrapie cases and cull sheep from scrapie-affected farms in Shetland

The prion protein (PrP) genotypes of all cull sheep originating from four scrapie-affected farms in Shetland in 1998-1999 were determined and a representative sample of the different genotypes was selected for necropsy. Samples of brain and selected viscera were removed from 159 such sheep aged 2-11 years. These samples were examined immunohistochemically and by Western blotting for infection-specific forms of PrP. None of the sheep bearing the following genotypes showed any evidence of PrP accumulation in brain, intestine, selected lymph nodes or the cranial mesenteric ganglia: ARQ/ARQ (n = 41), ARQ/ARH (n = 12), ARH/ARH (n = 2), ARQ/ARR (n = 24), ARR/ARR (n= 2). In five of 71 sheep bearing a single VRQ allele, PrP accumulation was detected immunohistochemically in viscera or brain, or both. These results suggested that only a small proportion of susceptible sheep showed evidence of infection (accumulation of PrP) on the farms studied, and that even sheep of the most susceptible genotype (VRQ/VRQ) did not invariably develop disease in an infected environment. Furthermore, there was no evidence that, in sheep of semi-resistant or fully resistant genotypes, infection could be sequestered within the lymphoreticular system or peripheral nervous system and thereby provide a possible "carrier" source of infection. Rather, the data suggested that some sheep, possibly because they had been exposed to a relatively low infective dose, became infected and accumulated the infective agent over a protracted pre-clinical phase of the disease. Such sheep might be potentially infective for many years. In two VRQ/ARR genotype sheep, PrP was confined to the brain. Infection-specific PrP was also confined to the brain in two of 24 clinical cases of VRQ/ARQ scrapie. Thus, direct neuroinvasion, apparently without a prior phase of replication in the lymphoreticular system, occurred in a proportion of VRQ/ARQ sheep. Possibly it may occur in all sheep of the VRQ/ARR genotype. The factors responsible for direct neuroinvasion are not understood. However, it cannot be attributed to genotype alone.

PrP(Sc) accumulation in placentas of ewes exposed to natural scrapie: influence of foetal PrP genotype and effect on ewe-to-lamb transmission

Placentas from scrapie-affected ewes are known to be infectious. Nevertheless, placenta infectivity in such ewes is not systematic. Maternal transmission to lambs is highly suspected but contamination of the foetus in utero has not been demonstrated. Using ewes from a naturally scrapie-infected flock, it was demonstrated that abnormal prion protein (PrP(Sc)) accumulation in the placenta (i) is controlled by polymorphisms at codons 136, 154 and 171 of the foetal PrP gene and (ii) is restricted mainly to placentome foetal trophoblastic cells. In order to go deeper into the role of the placenta in scrapie transmission, the pattern of PrP(Sc) dissemination was established in susceptible lambs (genotype VRQ/VRQ) sampled from 140 days post-insemination to the age of 4 months from either VRQ/VRQ ewes with PrP(Sc)-positive placentas or ARR/VRQ ewes with PrP(Sc)-negative placentas. In both VRQ/VRQ lamb groups, PrP(Sc) spatial and temporal accumulation patterns were similar, suggesting post-natal rather than in utero contamination.

Follicular dendritic cell of the knock-in mouse provides a new bioassay for human prions

Infectious prion diseases initiate infection within lymphoid organs where prion infectivity accumulates during the early stages of peripheral infection. In a mouse-adapted prion infection, an abnormal isoform (PrP(Sc)) of prion protein (PrP) accumulates in follicular dendritic cells within lymphoid organs. Human prions, however, did not cause an accumulation of PrP(Sc) in the wild type mice. Here, we report that knock-in mouse expressing humanized chimeric PrP demonstrated PrP(Sc) accumulations in follicular dendritic cells following human prion infections, including variant Creutzfeldt-Jakob disease. The accumulated PrP(Sc) consisted of recombinant PrP, but not of the inoculated human PrP. These accumulations were detectable in the spleens of all mice examined 30 days post-inoculation. Infectivity of the spleen was also evident. Conversion of humanized PrP in the spleen provides a rapid and sensitive bioassay method to uncover the infectivity of human prions. This model should facilitate the prevention of infectious prion diseases.

Evaluation of peripheral lymphoreticular biopsy techniques and their clinical side effects in sheep

Tissue samples were collected postmortem from 126 sheep at five lymphoreticular sites by different techniques. The three most successful combinations of sites and techniques were: the third eyelids, using a forceps and scissors, which provided a mean (se) of 5.32 (0.70) lymphoid follicles per 5 microm tissue section, a mandibular lymph node, using a Biopty gun, which gave 1.19 (0.26) lymphoid follicles per 5 microm tissue section, and tonsil, using a biopsy forceps, which gave 1.14 (0.27) lymphoid follicles per 5 microm tissue section. These three techniques were repeated once a month for five months on five sheep under general anaesthesia, and their clinical effects were compared with five control sheep which were restrained and anaesthetised in the same way but from which no biopsies were taken. Most lymphoid follicles (3.47 [0.58] per 5 pm tissue section) were obtained by using the third eyelid biopsy technique. There were no clinical side effects associated with the biopsy procedure. There were increases in the plasma concentration of cortisol in all the animals, suggesting that the restraint and anaesthesia were more stressful than the biopsy procedure.

Cellular prion protein status in sheep: tissue-specific biochemical signatures

Expression of the cellular prion protein PrP(C) is sine qua none for the development of transmissible spongiform encephalopathy and thus for the accumulation of the illness-associated conformer PrP(Sc). Therefore, the tissue distribution of PrP(C) at the protein level in both quantitative and qualitative terms was investigated. PrP(C) was quantified using a two-site enzyme immunometric assay which was calibrated with purified ovine recombinant prion protein (rPrP). The most PrP(C)-rich tissue was the brain, followed by the lungs, skeletal muscle, heart, uterus, thymus and tongue, which contained between 20- and 50-fold less PrP(C) than the brain. The PrP(C) content of these tissues seems to be comparable between sheep. Other organs, however, showed different, but low, levels of the protein depending on the animal examined. This was also the case for tissues from the gastrointestinal tract. The tissue containing the lowest concentration of PrP(C) was shown to be the liver, where PrP(C) was found to be between 564- and 16000-fold less abundant than in the brain. PrP(C) was concentrated from crude cellular extracts by immunoprecipitation using several monoclonal and polyclonal anti-ovine PrP antibodies. Interestingly, it was observed that the isoform profile of PrP(C) was tissue-specific. The most atypical electrophoretic profile of PrP(C) was found in the skeletal muscle, where two polypeptides of 32 and 35 kDa were detected.

Onset and distribution of tissue prp accumulation in scrapie-affected suffolk sheep as demonstrated by sequential necropsies and tonsillar biopsies

Tonsillar biopsies (single or multiple) or necropsies, or both, were performed on sheep taken from a Suffolk flock in which frequent cases of scrapie had occurred over a period of several years. Clinically affected sheep of the susceptible PrP(AQ/AQ)genotype had widespread disease-specific PrP accumulation in the central nervous system (CNS), lymphoreticular system and peripheral ganglia. In nine healthy PrP(AQ/AQ)Suffolk sheep between 4 and 7 years of age, PrP could not be demonstrated post mortem in any of the lymphoreticular tissues, or in the peripheral ganglia or CNS. Tonsillar biopsies taken from animals of the resistant PrP(AR/AR)and PrP(AR/AQ)genotypes at age 3, 8, 14, 20 or 26 months did not show PrP accumulation. Disease- specific PrP accumulation in tonsillar biopsies from PrP(AQ/AQ)sheep was not seen in 20 animals aged 3 months, but was found in two of 10 animals at age 8 months and in eight of 10 animals at age 20 months. The numbers of PrP-positive tonsillar biopsies obtained from sheep previously biopsied on more than one occasion was greater than the number of positive tonsils obtained from other susceptible sheep of comparable ages. The earliest disease-specific PrP accumulation seen was in tingible body macrophages within germinal centres and only later was it detected in cells resembling follicular dendritic cells. Fourteen PrP(AQ/AQ)sheep examined post mortem at up to 17 months of age and which had not previously been biopsied or were biopsied only once had no CNS or tonsillar PrP accumulations. Two of these sheep subjected to necropsy at 14 months had PrP accumulation in lymphoreticular tissue, where it was confined to the mesenteric lymph nodes. In susceptible sheep, only low levels of immunohistochemically detectable PrP were present in a minority of follicles from tonsillar biopsies of young lambs, but by 14 months of age widespread PrP accumulation, affecting many or even all follicles, was present. Although clinical cases had widespread PrP accumulations in viscera, susceptible survivors had no such accumulations in tissues of the lymphoreticular system, peripheral nervous system or CNS, suggesting that some animals were not exposed to infection or were exposed to a non-infectious dose.

Oral inoculation of sheep with the agent of bovine spongiform encephalopathy (BSE). 1. Onset and distribution of disease-specific PrP accumulation in brain and viscera

Sixty-three Romney sheep aged 6 months, consisting of three groups (PrP(ARQ/ARQ), PrP(ARQ/ARR), and PrP(ARR/ARR)genotypes) of 21 animals, were infected orally with brain tissue from BSE-infected cattle. Sub-groups of the 21 PrP(ARQ/ARQ) animals were killed, together with uninfected controls 4, 10, 16, 22 or 24-28 (after the development of full clinical disease) months post-inoculation (mpi). One sheep from each of the two groups of four killed at 4 or 10 mpi were shown by immunohistochemical examination to possess disease-specific PrP accumulations in single lymph nodes. At 16 mpi, such accumulations were detected in two of four infected sheep in some viscera and in the spinal cord and brain. At 22 mpi, three of five infected sheep had widespread disease-specific PrP accumulations in all tissues examined, but the remaining two animals gave positive results only in the central nervous system. Clinical disease appeared at 20-28 mpi. Three sheep killed with advanced clinical signs showed widespread PrP accumulation in brain, spinal cord and peripheral tissues. These results confirmed that PrP(ARQ/ARQ) Romney sheep are susceptible to experimental infection with the BSE agent. The different sites at which initial PrP accumulations were detected suggested that the point of entry of infection varied. Once established, however, infection appeared to spread rapidly throughout the lymphoreticular system. The results suggested that in some BSE-infected sheep neuroinvasion occurred in the absence of detectable PrP accumulations in the viscera or peripheral nervous system. In contrast to cattle with BSE, however, most sheep showed disease-specific PrP accumulations in the lymphoreticular system. In this respect, BSE-infected resembled scrapie-infected sheep; it is possible, however, that future research will reveal differences in respect of targeting of cell types within the lymphoreticular and peripheral nervous systems. The PrP(ARQ/ARR)and PrP(ARR/ARR)sheep were also killed in sub-groups at intervals after inoculation. Up to 24 mpi, however, none of these animals showed disease-specific PrP accumulations. Further results will be reported later.

Pathogenesis of natural scrapie in sheep

Although scrapie has been known for a long time as a natural disease of sheep and goats, the pathogenesis in its natural host still remains unclear. To study the pathogenesis of natural scrapie, we used immunohistochemistry to monitor the deposition of PrP(Sc) in various tissues, collected during a natural scrapie infection from sheep with the PrP(VRQ)/PrP(VRQ) genotype which were purposely bred for their short incubation period for natural scrapie. PrP(Sc) was present in the lymphoid tissues of all animals from the age of 5 months onwards. At this age, PrP(Sc) was detected in the neural tissues only in the enteric nervous system (ENS) at the level of the duodenum and ileum. At the age of 10 months, PrP(Sc) was not only found in the ENS but also in the ganglion mesentericum cranialis/coeliacum, the dorsal motor nucleus of the vagus, and the intermediolateral column of the thoracic segments T8-T10. PrP(Sc) was detected for the first time in the nucleus tractus solitarius and ganglion nodosus at 17 months of age and in the ganglion trigeminale and several spinal ganglia at 21 months of age. Since the scrapie agent consists largely, if not entirely of PrP(Sc), these results indicate that the ENS acts as a portal of entry to the neural tissues for the scrapie agent followed by centripetal and retrograde spread through sympathetic and parasympathetic efferent fibers of the autonomic nervous system to the spinal cord and medulla oblongata respectively. PrP(Sc) accumulation in sensory ganglia occurs after infection of the CNS and is therefore probably due to centrifugal and anterograde spread of the scrapie agent from the CNS through afferent nerve fibers.

Partial dissociation of PrP(Sc) deposition and vacuolation in the brains of scrapie and BSE experimentally affected goats

The diagnosis of transmissible spongiform encephalopathies (TSEs) depends on the detection of vacuolation in brain sections taken from affected individuals and/or the identification of the disease-associated isoform of the PrP (prion) protein (PrP(Sc)). During the course of an investigation, goats clinically affected following experimental infection with three different sources of TSE (SSBP/1, CH1641 and BSE) developed widespread vacuolar degeneration in the brain. With BSE, PrP(Sc) was clearly recognized in affected goat brain by immunocytochemistry (icc) and Western blotting, but in contrast the experimental scrapie sources SSBP/1 and CH1641 showed almost no or very little PrP(Sc) by icc. Western blot analysis of PrP(Sc) from BSE-affected and SSBP/1-affected goat brain showed that the protein was present in brain affected by both TSE sources, but could not be used to determine how much protein was present. It became clear that PrP(Sc) and vacuolation could be partially dissociated following challenge with two of the three TSE sources. Subtle differences in glycosylation patterns between BSE- and SSBP/1-associated PrP protein isoforms could also be recognized, although these experimentally generated results should not be regarded as a BSE/scrapie differential test. However, our study warns that the reliance on PrP(Sc) determination by icc alone as a means by which to diagnose TSE infection may generate false negative results.

The transmission dynamics of the aetiological agent of scrapie in a sheep flock

We formulate and investigate the properties of a model framework to mimic the transmission dynamics of the aetiological agent of scrapie in a sheep flock. We derive expressions for summary parameters that characterize transmission scenarios, notably the basic reproduction number R(0) and the mean generation time T(g). The timescale of epidemic outbreaks is expressed in terms of R(0) and cumulants of the generation time distribution. We discuss the relative contributions to the overall rate of transmission of horizontal and vertical routes during invasion and in endemicity. Simplified models are used to obtain analytical insight into the characteristics of the endemic state.