Pre-clinical and clinical diagnosis of scrapie by detection of PrP protein in tissues of sheep

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

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

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

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

Molecular dynamics studies on the NMR and X-ray structures of rabbit prion proteins

Prion diseases, traditionally referred to as transmissible spongiform encephalopathies (TSEs), are invariably fatal and highly infectious neurodegenerative diseases that affect a wide variety of mammalian species, manifesting as scrapie in sheep and goats, bovine spongiform encephalopathy (BSE or mad-cow disease) in cattle, chronic wasting disease in deer and elk, and Creutzfeldt-Jakob diseases, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, and kulu in humans, etc. These neurodegenerative diseases are caused by the conversion from a soluble normal cellular prion protein (PrP(C)) into insoluble abnormally folded infectious prions (PrP(Sc)), and the conversion of PrP(C) to PrP(Sc) is believed to involve conformational change from a predominantly α-helical protein to one rich in β-sheet structure. Such a conformational change may be amenable to study by molecular dynamics (MD) techniques. For rabbits, classical studies show that they have a low susceptibility to be infected by PrP(Sc), but recently it was reported that rabbit prions can be generated through saPMCA (serial automated Protein Misfolding Cyclic Amplification) in vitro and the rabbit prion is infectious and transmissible. In this paper, we first do a detailed survey on the research advances of rabbit prion protein (RaPrP) and then we perform MD simulations on the NMR and X-ray molecular structures of rabbit prion protein wild-type and mutants. The survey shows to us that rabbits were not challenged directly in vivo with other known prion strains and the saPMCA result did not pass the test of the known BSE strain of cattle. Thus, we might still look rabbits as a prion resistant species. MD results indicate that the three α-helices of the wild-type are stable under the neutral pH environment (but under low pH environment the three α-helices have been unfolded into β-sheets), and the three α-helices of the mutants (I214V and S173N) are unfolded into rich β-sheet structures under the same pH environment. In addition, we found an interesting result that the salt bridges such as ASP201-ARG155, ASP177-ARG163 contribute greatly to the structural stability of RaPrP.

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.

Utilização de biópsias da terceira pálpebra e mucosa retal em ovinos para diagnóstico de scrapie em uma propriedade da região sul do Brasil

Scrapie é uma encefalopatia espongiforme transmissível (EET) que causa lesões cerebrais degenerativas em ovinos e caprinos. Caracteriza-se pelo acúmulo, no tecido encefálico e linforreticular, da forma anormal da proteína priônica (PrP Sc) que provoca a morte maciça de neurônios e células gliais, além de vacuolização intensa no tecido afetado. Esse trabalho descreve a utilização da técnica de imuno-histoquímica (IHQ) para proteína priônica em tecido linforreticular de biópsias de terceira pálpebra e mucosa retal, como método diagnóstico de scrapie em ovinos. Realizaram-se exames de IHQ para scrapie em amostras de uma propriedade de origem de um ovino com diagnóstico dessa enfermidade. Utilizaram-se anticorpos monoclonais antipríon para diagnóstico ante mortem pela técnica de IHQ. Nas 318 amostras de biópsias analisadas, encontrou-se 19 resultados positivos para PrP Sc nos folículos de terceira pálpebra e não foi obtida marcação no tecido linfático de mucosa retal em nenhuma das amostras coletadas. Realizaram-se 18 necropsias dos animais positivos anteriormente por biópsia e 21 necropsias de ovinos parentes dos positivos de scrapie. Confirmou-se o resultado de scrapie pela IHQ após a necropsia dos animais positivos nas biópsias de terceira pálpebra. Nesses animais, os órgãos com maior número de cortes positivos foram a terceira pálpebra (18/18) e a tonsila (8/18). Nos ovinos com parentesco com os positivos, nenhum resultado de scrapie ocorreu. A utilização de tecidos linfoides no diagnóstico de scrapie por IHQ através de biópsias mostrou-se um método viável e eficaz para o diagnóstico pré-clínico.

Detection of PrPSc in lung and mammary gland is favored by the presence of Visna/maedi virus lesions in naturally coinfected sheep

There are few reports on the pathogenesis of scrapie (Sc) and Visna/maedi virus (VMV) coinfections. The aim of this work was to study in vivo as well as post mortem both diseases in 91 sheep. Diagnosis of Sc and VMV infections allowed the distribution of animals into five groups according to the presence (+) or absence (−) of infection by Sc and VMV: Sc−/VMV−, Sc−/VMV+, Sc+/VMV− and Sc+/VMV+. The latter was divided into two subgroups, with and without VMV-induced lymphoid follicle hyperplasia (LFH), respectively. In both the lung and mammary gland, PrP^Sc deposits were found in the germinal center of hyperplasic lymphoid follicles in the subgroup of Sc+/VMV+ having VMV-induced LFH. This detection was always associated with (and likely preceded by) PrP^Sc observation in the corresponding lymph nodes. No PrP^Sc was found in other VMV-associated lesions. Animals suffering from scrapie had a statistically significantly lower mean age than the scrapie free animals at the time of death, with no apparent VMV influence. ARQ/ARQ genotype was the most abundant among the 91 ewes and the most frequent in scrapie-affected sheep. VMV infection does not seem to influence the scrapie risk group distribution among animals from the five groups established in this work. Altogether, these data indicate that certain VMV-induced lesions can favor PrP^Sc deposits in Sc non-target organs such as the lung and the mammary gland, making this coinfection an interesting field that warrants further research for a better comprehension of the pathogenesis of both diseases.

High incidence of subclinical infection of lymphoid tissues in scrapie-affected sheep flocks

Prion diseases are characterized by a long incubation period. In scrapie, sheep may incubate and spread the infection for several years before clinical signs evolve. We have previously studied the occurrence of subclinical infection in the brain. Now, we have studied the occurrence of subclinical infection in the brain and several lymphoid tissues in two scrapie-affected Icelandic sheep flocks by immunohistochemistry for PrP(Sc), a molecular marker for infectivity, and correlated this with results of PrP genotyping. At culling, one flock had one confirmed scrapie case, while the other flock had two. Analysis of 106 asymptomatic sheep by immunostaining for PrP(Sc) revealed that the incidence of subclinical infection was 58.3% in one flock and 42.5% in the other. PrP(Sc) was only detected in lymphoid tissues. The youngest positive sheep were 4 months old. PrP genotyping showed that over 90% of the sheep were of a genotype which is moderately sensitive to infection and may delay neuroinvasion. Our results show that asymptomatic sheep may spread the infection during the long incubation period of several years, which constitutes an important obstacle in the eradication of scrapie. Our findings indicate that contamination of the environment plays an important part in sustaining the infection.

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.

PrPSc accumulation in fetal cotyledons of scrapie-resistant lambs is influenced by fetus location in the uterus

Placentae from scrapie-infected ewes have been shown to accumulate PrPSc when the genotype of the fetus is of a susceptible genotype (VRQ/VRQ, ARQ/VRQ or ARQ/ARQ). Cotyledons from fetuses of genotypes ARR/ARR, ARQ/ARR and ARQ/VRR have previously been shown to be resistant to PrPSc accumulation. By using ewes from a naturally infected scrapie flock, cotyledons from fetuses of multiple births of different genotypes were examined. PrPSc was detected in fetal cotyledons of genotype ARQ/ARQ, but not in cotyledons from their dizygotic twin of genotype ARQ/ARR. This confirms earlier reports of single fetuses of these genotypes, but is the first description of such a finding in twin fetuses, one of each genotype. It is also demonstrated that cotyledons from sibling fetuses of genotypes ARQ/VRQ and ARQ/ARQ have different patterns of PrPSc accumulation depending on whether the dam is of genotype ARQ/ARQ or ARQ/VRQ. Lastly, it is shown that cotyledons from fetuses with resistant genotypes are weakly positive for PrPSc when they have shared the same pregnant uterine horn with a fetus of a susceptible genotype with cotyledons positive for the detection of PrPSc. Additionally, a PCR product for the Sry gene, a product specific to males, was found in cotyledons from female fetuses that had shared a uterine horn with a male fetus. This indicates that some sharing of fetal blood occurs between placentomes and fetuses residing in the same uterine horn, which can result in PrPSc accumulation in cotyledons with resistant genotypes.

Detection of PrPSc in Rectal Biopsy and Necropsy Samples from Sheep with Experimental Scrapie

Scrapie diagnosis is based on the demonstration of disease-associated prion protein (PrP(Sc)) in brain or, in the live animal, in readily accessible peripheral lymphoid tissue. Lymphatic tissues present at the rectoanal line were readily obtained from sheep without the need for anaesthesia. The presence of PrP(Sc) in such tissue was investigated in sheep infected orally with scrapie-infected brain material. The methods used consisted of immunohistochemistry and histoblotting on biopsy and post-mortem material. PrP(Sc) was detected in animals with PrP genotypes associated with high susceptibility to scrapie from 10 months after infection, i.e., from about the time of appearance of early clinical signs. In the rectal mucosa, PrP(Sc) was found in lymphoid follicles and in cells scattered in the lamina propria, often near and sometimes in the crypt epithelium. By Western blotting, PrP(Sc) was detected in rectal biopsy samples of sheep with the PrP genotype VRQ/VRQ, after electrophoresis of material equivalent to 8 mg of tissue. This study indicated that rectal biopsy samples should prove useful for the diagnosis of scrapie in 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.

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.

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.

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).

Detection of disease-specific PrP in the distal ileum of cattle exposed orally to the agent of bovine spongiform encephalopathy

The immunohistochemical localisation of the disease-specific protein, PrP(Sc), was examined in the distal ileum of cattle up to 40 months after they had been exposed orally to the agent of bovine spongiform encephalopathy (BSE), in the intestines and mesenteric lymph nodes of an additional group of cattle, killed six months after a similar exposure, and in the distal ileum of naturally occurring clinical cases of BSE. PrP(Sc) was detected, mainly in macrophages, in a small proportion of the follicles of Peyer's patches in the distal ileum of the experimentally exposed cattle throughout much of the course of the disease. The observations are in agreement with the infectivity data derived from mouse bioassays of the distal ileum. At the later stages of the disease, the proportion of immunostained follicles increased as the total number of follicles decreased with age. In the additional experimental group of cattle, PrP(Sc) was confined to the Peyer's patches in the distal ileum. No immunostaining was detected in the lymphoid tissue of the distal ileum of naturally occurring clinical cases of BSE. In some of the clinically affected experimentally induced and naturally occurring cases of BSE there was sparse immunostaining of the neurons of the distal ileal myenteric plexus.

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.

Prp-c and Prp-Sc at the fetal-maternal interface

Scrapie is a naturally occurring prion (PrP) disease causing a fatal neurodegenerative disorder in sheep and goats. Previous studies suggest that scrapie is transmitted naturally through exposure to the scrapie agent in wasted placentas of infected ewes. This study determined the distribution and biochemical properties of PrP cellular (PrP-C) and the distribution of PrP scrapie (PrP-Sc) in reproductive, placental, and selected fetal tissues and fetal fluids in sheep. Glycosylated, N-terminally truncated, proteinase K-sensitive PrP-C with apparent molecular masses of 23-37 kDa was present in reproductive, placental, and fetal tissues and fetal fluids. PrP-C was low or undetectable in intercotyledonary chorioallantois, amnion, urachus, amniotic fluid, and fetal urine. In pregnant ewes, cotyledonary chorioallantois, allantoic fluid, and caruncular endometrium contained higher levels of PrP-C than did intercaruncular endometrium, myometrium, oviduct, ovary, fetal bladder, or fetal kidney. Caruncular endometrial PrP-C was up-regulated during pregnancy. Despite the wide distribution of PrP-C in reproductive, placental, and selected fetal tissues and fetal fluid, PrP-Sc was detected only in caruncular endometrium and cotyledonary chorioallantois of pregnant scrapie-infected ewes. The embryo/fetus may not be exposed to scrapie in utero because it is separated physically from PrP-positive allantois and chorioallantois by PrP-negative amnion.

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.

Immunohistochemical detection of scrapie prion proteins in clinically normal sheep in Pennsylvania

Following diagnosis of scrapie in a clinically suspect Suffolk sheep, 7 clinically normal flockmates were purchased by the Pennsylvania Department of Agriculture to determine their scrapie status using an immunohistochemical procedure. Two of the 7 euthanized healthy sheep had positive immunohistochemical staining of the prion protein of scrapie (PrP-Sc) in their brains, nictitating membranes, and tonsils. The PrP-Sc was localized in the areas of the brain where, histopathologically, there was neurodegeneration and astrocytosis. The PrP-Sc occurred within germinal centers of the affected nictitating membranes and tonsils and was located in the cytoplasm of the dendrite-like cells, lymphoid cells, and macrophages. These results confirm that immunohistochemical examination of the nictitating membrane can be used as a screen for the presence of scrapie infection in clinically normal sheep at a capable veterinary diagnostic laboratory. In sheep with a PrP-Sc-positive nictitating membrane, the diagnosis of scrapie should be confirmed by histopathology and immunohistochemical examination of the brain following necropsy. Following full validation, immunohistochemistry assays for detection of PrP-Sc in nictitating membrane lymphoid tissues can improve the effectiveness of the scrapie control and eradication program by allowing diagnosis of the disease in sheep before the appearance of clinical signs.

Protease-resistant prion protein in brain and lymphoid organs of sheep within a naturally scrapie-infected flock

The hallmark of transmissible spongiform encephalopathies (TSE), such as scrapie in sheep, is the accumulation in tissues of an insoluble and protease resistant form (PrPres) of the cellular prion protein. In this study, we evaluated whether the diversity in both the clinical pattern and the PrP genotypes of scrapied sheep from the same flock was connected with different levels and/or glycoform patterns of the PrPres in the brain and lymphoid organs of the animals. Whereas the PrPres levels in spleen, lymph nodes and tonsils from sheep of different PrP genotypes and clinical status appeared comparable, they were highly variable in brain, particularly in the brain stem and the cerebellum. PrPres was only detected in sheep bearing at least one VRQ allele, including three asymptomatic sheep and the highest PrPres load was found in the cerebellum of VRQ/VRQ animals. All together, levels of PrPres in brain did not necessarily correlate with the severity of the clinical disease but might depend on the PrP genotype of the animals. Different brain regions from a given sheep displayed a similar glycopattern of PrPres, whereas the apparent molecular sizes of the unglycosylated and diglycosylated forms of the protein differed between brain and lymphoid tissues. We did not find any notifiable differences in the glycopattern of PrPres in brain from sheep of different PrP genotypes or different clinical status and this PrPres glycotype was also similar to that found in brain from four cattle BSE.

Scrapie transmission in Britain: a recipe for a mathematical model

Responses to an anonymous postal survey concerning scrapie are analysed. Risk factors associated with farms that have had scrapie are identified as size, geographical region, lambing practices and holding of certain breeds. Further analysis of farms that have scrapie only in bought-in animals reveals that such farms tend to breed a smaller proportion of their replacement animals than farms without scrapie. Farms that have had scrapie in home-bred animals have attributes associated with breeding many animals: large numbers of rams bought, few ewes bought, and many animals that are home-bred. The demography of British sheep farms as described by size, breeds, purchasing behaviour, age structure and proportion of animals that are home-bred is summarized. British farms with scrapie reveal certain special features: they have more sheep that are found dead, more elderly ewes and more cases of scab.

The immunodetection of the abnormal isoform of prion protein

Transmissible spongiform encephalopathies such as scrapie in sheep and goats, Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cattle, are neurodegenerative disorders. A proposed causative agent for these diseases is an infectious protein, the so called 'prion'. An abnormal isoform of prion protein (PrPSc) can be detected according to the prion propagation method used. As PrPSc appears to constitute the main, if not the only, infectious entity its detection for the diagnosis of prion diseases is important. Immunodetection methods for PrPSc analysis are popular tools for diagnosis and research studies. In this paper, a review of the present knowledge concerning immunodetection is presented and the enhancement of the immunoreactivity of antisera to mouse and hamster prion protein peptides using the techniques of Western blotting and immunohistochemistry is summarized.

Immunolocalization of the prion protein in scrapie affected rodent retinas

Some mouse and hamster scrapie models are known to replicate infectivity in the retina, and this can be associated with photoreceptor atrophy. We used immuno-labelling to identify the cellular localization of the prion protein (PrP) in the retina and correlated this with infectivity titre. It is only with the 263K scrapie strain in hamsters that disease-associated PrP (PrP(Sc)) staining was always easily detectable. Very little PrP(Sc) immunolabelling was observed in any of the mouse models, even in those demonstrating scrapie-induced retinopathy in which high titres of infective agent are known to occur in the retina.

Sensitivity of the Western blot detection of prion protein PrPres in natural sheep scrapie

The sensitivity of Western blot detection of PrPres using two different extraction procedures on brain material of 30 scrapie-affected sheep was compared. Whereas PrPres could be detected in all sheep after extraction with the first method, 30% did not give any signal after extraction with the second method. However, the second method, when positive, permitted the detection of PrPres from smaller amounts of infected brain tissue. When used with the ruminant specific monoclonal antibody p4, the second method gave positive signals corresponding to less than 12.5 microg of scrapie-infected brain, that, up to now, is the highest sensitivity described for PrPres detection from naturally infected ruminant brains. The overall results showed highly variable levels of PrPres between sheep and are presented in relation to breed, survival time and animal genotype data. Further progress can thus be expected for PrPres detection in prion diseases, if more efficient extraction procedures and more sensitive immunological reagents are used. Such technical improvements could contribute to more accurate diagnosis in animals affected naturally.

The distribution of scrapie-associated fibrils in neural and non-neural tissues of advanced clinical cases of natural scrapie in sheep

The distribution of scrapie-associated fibrils (SAFs) throughout four brain regions, the pituitary gland, along the whole length of the spinal cord and in the sciatic nerve was assessed in 10 sheep terminally affected by scrapie and in four control sheep. Tonsils, retropharyngeal, broncho-mediastinal and mesenteric lymph nodes, the distal ileum, proximal colon and spleen were also examined for fibrils in all 14 sheep. Fibrils were detected in all four brain regions and throughout the length of the spinal cord in nine of the scrapie affected sheep. SAFs were not detectable in any of the sciatic nerve samples tested. In one of the 10 clinically affected sheep only minimal lesions were found by histopathology and fibrils were detected only from the cerebrum and one spinal cord region (taken at the C1 C2 vertebrae). Fibrils were not detected in the tonsils or retropharyngeal lymph nodes but were detected in other non-neural tissues of some of the scrapie-affected sheep. These tissues included pituitary gland, broncho-mediastinal and mesenteric portal lymph nodes, distal ileum, proximal colon and spleen. Fibrils could not be detected in any of the tissues taken from the four control sheep.

An immunological approach to prion diseases

Ovine scrapie and bovine spongiform encephalopathy are genetic diseases, presenting probably autoimmunity transmissible by the oral route. The absence of immune response in prion diseases indicates a tolerant state for PrP(C) and PrP(SC). The tolerant state against these diseases should be overcome before immunizing animals. We suggest that an early diagnosis may be possible using polyclonal and monoclonal antibodies specific for either ovine or bovine PrP(SC). Such reagents could be obtained by immunizing corresponding animals with peptides from beta sheet molecules bound to a linker or with the complete molecule (27-30 kDa).

Control of scrapie eventually possible?

After a brief description of the scrapie situation in the Netherlands, the technical progress made in aspects of scrapie diagnosis is reported. Emphasis is placed on the use of immuno-histochemistry (IHC) in the post-mortem histological diagnosis and on the recently published preclinical test for scrapie, in which IHC is applied to tonsillar biopsies. These two approaches use the same IHC technique and enable us to confirm suspected scrapie in individual animals, and for certain genotypes even in the preclinical phase. The tonsillar biopsy method could eventually lead to an infection- or PrPSc-detection method at flock level. Further work is required, including validation of the assay for various breeds, genotypes, and strains of the agent, and the conversion of the test into a more practical assay. The article continues with a discussion of several scrapie control strategies, describing briefly schemes tried in various countries, and elaborates on a proposed scrapie control scheme that could be suitable for the Netherlands. This scheme is essentially based on breeding for resistance, based on PrP genotyping.

Alternative usage of exon 1 of bovine PrP mRNA

Here we report two types of bovine prion protein (PrP) mRNA that possessed different lengths of the 5'-untranslated region and were expressed in various bovine tissues. The two mRNA species were transcribed from identical positions but differed in the usage of the splice site for exon 1/intron. One mRNA possessed exon 1 consisting of 53 nucleotides and the other possessed exon 1 consisting of 168 nucleotides. Usage of exons 2 and 3 was identical for the two mRNA species. The two mRNA species were detected in all but spleen tissue; the mRNA possessing 168-nt exon 1 was not detected in bovine spleen. This is the first report on the tissue-specific alternative splicing of PrPc mRNA in any other species. Only a low level of PrPc appeared to be present in bovine spleen. These results suggested the possibility that the mRNA possessing 53-nt exon 1 was inefficiently translated into Prp; however, in vitro translation analysis showed no marked difference in translational efficiency between the two mRNA species.

Sensitive enzyme-linked immunosorbent assay for detection of PrP(Sc) in crude tissue extracts from scrapie-affected mice

An enzyme-linked immunosorbent assay (ELISA) was developed that detects PrP(Sc) in crude extracts from brain and spleen tissue of scrapie-affected mice with high sensitivity and specificity. Brain tissue was homogenized in 8% Zwittergent 3-12 and 0.5% Sarkosyl. The homogenate was treated with collagenase and DNase I and then subjected to proteinase K digestion. Precipitates containing PrP(Sc) were obtained by ultracentrifugation. Spleen tissue was homogenized in 4% Triton X-100 and 0.5% Sarkosyl, and the homogenate was treated firstly with collagenase and DNase I, and secondly with proteinase K. PrP(Sc) was then extracted with 6.25% Sarkosyl and precipitated through salting-out with NaCl and by ultracentrifugation. When PrP(Sc) was dissolved in 3-4 M guanidine thiocyanate and adsorbed to microtiter plates, strong and specific reactions to the formation of antigen-antibody complexes could be detected by ELISA. The sensitivity of PrP(Sc)-detection for this ELISA, as measured by serial dilution of scrapie material in tissue homogenates from uninfected animals, was equal or higher than that attained by Western blot. This ELISA is more rapid than Western blot and seems to be more suitable for screening large numbers of animals. It also has potential application for the diagnosis of the transmissible spongiform encephalopathies.

Improvement of PrPSc-detection in mouse spleen early at the preclinical stage of scrapie with collagenase-completed tissue homogenization and Sarkosyl-NaCl extraction of PrPSc

Scrapie in sheep has recently become again a target of control measures and eradication programs. Crucial for the effectiveness of these measures is the detection of infected sheep during the long and potentially hazardous incubation period. However, routine-diagnosis is mostly limited to clinical examinations when disease becomes apparent, and to postmortem investigations. Through the detection of the scrapie-specific isoform of the prion protein (PrPSc) by Western blot in the spleen and lymph nodes from scrapie-infected mice and sheep, we have shown previously that diagnosis during the preclinical stage is possible. We introduce here an improved method for the diagnosis of mouse scrapie shortly after infection. Through a homogenization procedure that includes a collagenase digestion step, and through extraction and salting-out of PrPSc by Sarkosyl and NaCl, respectively, we were able to detect PrPSc in spleen tissue of intraperitoneally infected mice seven days postinfection. Moreover, the new protocol makes sample-handling easier and reduces the hands-on time. We also successfully enriched PrPSc from spleen tissue through immobilized metal affinity chromatography (IMAC); however, for the diagnosis at the earliest stage of infection, extraction of PrPSc by Sarkosyl and NaCl was more effective.

Bovine spongiform encephalopathy--the beginning of the end?

The incidence of bovine spongiform encephalopathy (BSE) in the United Kingdom is now declining at a significant rate, indicating that the 1988 ban on feeding ruminant-derived meat and bone meal to cattle is having the anticipated effect. The question now is whether or not BSE can be completely eradicated. At present there is no evidence of lateral or maternal transmission, the occurrence of which would complicate the eradication process. Eradication therefore seems to be achievable, especially now that meat and bone meal has been recently banned from the diets of all farmed animals in the UK. In this review the aetiological role of meat and bone meal in the causation of BSE is discussed together with the epidemiological data and the results of studies on genetic susceptibility. The controversial theories relating to the nature of the causal agent, and strain-typing studies on BSE agent, are described. Current information on pathogenesis and diagnosis is presented. The occurrence of BSE in cattle outside the United Kingdom, and BSE-related disease in species other than cattle is also discussed.

Immunoreactivity of specific epitopes of PrPSc is enhanced by pretreatment in a hydrated autoclave

An abnormal protein (PrPSc) accumulates in animals affected with scrapie. Immunoblotting procedures have been used widely to detect PrPSc. Blotted membranes were subjected to pretreatment in a hydrated autoclave, and the subsequent immunoreactivity of PrPSc was examined. The immunoreactivity of PrPSc to antisera against the synthetic peptides of the mouse PrP amino acid sequences 199 to 208 and 213 to 226 was enhanced by the pretreatment. However, the reactivity to antisera of peptide sequences 100 to 115 and 165 to 174 was not affected. The antibody-binding ability of the specific epitopes which are located close to the C-terminal end of PrP27-30 the proteinase-resistant portion of PrPSc, was enhanced by pretreatment in a hydrated autoclave. This pretreatment increased the sensitivity of PrPSc, and it would be useful for diagnosis of scrapie.

Immunohistochemical detection of prion protein in lymphoid tissues of sheep with natural scrapie

The scrapie-associated form of the prion protein (PrPSc) accumulates in the brain and lymphoid tissues of sheep with scrapie. In order to assess whether detecting PrPSc in lymphoid tissue could be used as a diagnostic test for scrapie, we studied the localization and distribution of PrPSc in various lymphoid tissues collected at necropsy from 55 sheep with clinical scrapie. Samples collected from the spleen, palatine tonsil, ileum, and five different lymph nodes were immunohistochemically stained for PrPSc. PrPSc was found to be deposited in a reticular pattern in the center of both primary and secondary lymphoid follicles. In addition, granules of PrPSc were seen in the cytoplasm in macrophages associated with the lymphoid follicles. In 54 (98%) of the 55 scrapie-affected sheep, PrPSc was detected in the spleen, retropharyngeal lymph node, mesenteric lymph node, and the palatine tonsil. However, only in the palatine tonsils was PrPSc present in a consistently high percentage of the lymphoid follicles. PrP was not detected in any of the lymphoid tissues of 12 sheep that had no neurohistopathological signs of a scrapie infection. We conclude that the tonsils are the best-suited lymphoid tissue to be biopsied for the detection of PrPSc in the diagnosis of clinical scrapie in living sheep.

Animal spongiform encephalopathies--an update. Part 1. Scrapie and lesser known animal spongiform encephalopathies

The present article (part I) reviews recent developments in animal spongiform encephalopathies (SEs), with the exception of bovine spongiform encephalopathy (BSE), which is dealt with in part II. The article focuses on scrapie and describes epidemiological aspects and the prospects for a preclinical diagnosis. Up to now, confirmatory diagnosis of scrapie depended on histological examination of the brain, collected during post-mortem examination from sheep with clinical signs of the disease. An altered protein, PrPSc, can be detected in the brain of diseased animals. The demonstration of the same protein in the spleen and in peripheral lymph nodes of infected animals seems to offer interesting possibilities of arriving at a method for a preclinical diagnosis, and thus a diagnosis in the live animal. Progress has also been made in our understanding of the relationship between the genetic constitution and susceptibility of the host. Susceptibility is expressed as the survival time of sheep inoculated with scrapie. This was thought to be determined by a single genetic locus designated the Sip gene (scrapie incubation period gene). Putative markers for the two alleles of the Sip gene, sA and pA, have been discovered, consisting of restriction fragment length polymorphisms (RFLPs). In field tests, however, the link between these markers and the length of incubation time was far from consistent. These RFLPs were found to be situated outside the prion-protein-coding region of the ovine gene. In later studies, RFLPs were detected inside this region. These markers appear to be more informative, i.e. they correspond with a difference in the length of the scrapie incubation period.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection of PrPSc in sheep at the preclinical stage of scrapie and its significance for diagnosis of insidious infection

A field sheep insidiously infected with natural scrapie was diagnosed at the preclinical stage through detection of the core fragment of PrpSc (PrPcore) in peripheral lymph nodes by the biopsy method. Three out of 32 euthanized healthy sheep were found positive for PrPcore in the spleen and lymph nodes. Mice that were inoculated with spleen homogenate of one of these sheep showed clinical signs of scrapie and were positive for PrPcore in their brain samples. These results suggest that the detection of PrPcore is significant for diagnosis in the preclinical phase of scrapie.

Correlation between the results of a histopathological examination and the detection of abnormal brain fibrils in the diagnosis of bovine spongiform encephalopathy

A statistical comparison was made between the results of the statutory neurohistopathological method for the post mortem diagnosis of bovine spongiform encephalopathy (BSE) and the detection of abnormal brain fibrils (SAF). A total of 773 suspect cases was examined by both methods; it comprised 531 animals born before the feeding of ruminant-derived protein to ruminant species was prohibited and 242 born after the ban. The relative sensitivities and specificities of the methods were calculated for the diagnosis of clinically suspected BSE. The agreement between the results of the methods was excellent for 331 of the cases born before the ban and for all the cases born after it. In these cases the samples were not autolysed. For the 200 cases in which autolysis was recorded there was poor agreement between the diagnostic methods and this was attributed to an apparently reduced specificity of the histopathological diagnosis in the autolysed material. Despite the potentially greater specificity of fibril detection in the diagnosis of scrapie-like disease, this study indicates that a reliance on fibril detection alone may result in some false negative diagnoses, probably owing to the inadequate sampling of the tissues.

Characterisation of antisera raised against species-specific peptide sequences from scrapie-associated fibril protein and their application for post-mortem immunodiagnosis of spongiform encephalopathies

Transmissible spongiform encephalopathies (TSE), such as scrapie or Creutzfeldt-Jakob disease (CJD), are fatal neurodegenerative diseases of the central nervous system caused by a yet unidentified virus. They are accompanied by a brain specific amyloidosis, during which a host coded protein irreversibly aggregates to form the scrapie-associated fibrils. The diagnosis of TSE relies on histopathological detection of spongiform lesions, on electron microscopical detection of fibrils, or on the immunological detection of SAF protein, which is the most specific diagnostic marker. In order to improve the diagnosis of TSE, we developed a protocol for rapid tissue fractionation and enrichment of SAF protein which subsequently allows the specific detection of SAF protein by western blotting and immunodetection. Using some new antisera raised against synthetic peptides with sequences specific for the hamster, sheep, cattle and human SAF protein, several samples can be diagnosed for TSE within 24 hours, starting with only 10-100 mg of brain tissue from different species.

[Transmissible animal spongiform encephalopathies. Epidemiologic aspects]

This article presents an update of data about the transmissible spongiform encephalopathies which affect some animal species, particularly bovine spongiform encephalopathy in Great britain and in other countries where this disease is sporadic. The genetic susceptibility of scrapie in sheep and the natural transmission of these spongiform encephalopathies are discussed. The control measures for public health and animal health are presented.

Immunohistochemical detection of prion protein in sheep with scrapie

Prion protein (PrP), which is involved in the pathogenesis of scrapie, occurs in 2 forms. The form extracted from scrapie brain is protease resistant (PrP-res), whereas PrP from normal brain is protease sensitive (PrP-sen). This study examined whether PrP-res could be detected in brains of sheep with scrapie by immunohistochemistry (IHC). A suitable IHC procedure was developed using brain tissue from hamsters that had been inoculated with the transmissible mink encephalopathy agent. Tissue samples were fixed in PLP (periodate, lysine, paraformaldehyde) that contained paraformaldehyde at a concentration of 0.125%. Before application of the IHC technique, tissue sections were deparaffinized and treated with formic acid to simultaneously enhance PrP-res immunoreactivity and degrade PrP-sen. Primary antibody was obtained from a rabbit immunized to PrP-res extracted from brains of mice with experimentally induced scrapie. Brain from 21 sheep with histopathologically confirmed scrapie were examined by IHC. In all 21 brains, PrP-res was widely distributed throughout the brain stem. Staining was particularly intense in neuronal cell bodies and around blood vessels. The IHC technique successfully detected PrP-res in brain samples that had been frozen or that were severely autolyzed before fixation in PLP. Brains from 11 scrapie-suspect sheep that were not considered histologically positive were also examined by IHC. PrP-res was found in 4 of these brains. Sections of brains from 14 clinically normal sheep did not have detectable PrP-res. Results of this study indicate that IHC detection of PrP-res is equivalent, and perhaps superior, to histopathology for the diagnosis of scrapie in sheep. Furthermore, IHC is applicable to tissues that have autolytic changes or processing artifacts that prevent satisfactory histopathologic evaluation for lesions of scrapie.