The natural atypical scrapie phenotype is preserved on experimental transmission and sub-passage in PRNP homologous sheep

A single amino acid residue in bank vole prion protein drives permissiveness to Nor98/atypical scrapie and the emergence of multiple strain variants

Prions are infectious agents that replicate through the autocatalytic misfolding of the cellular prion protein (PrP^C) into infectious aggregates (PrP^Sc) causing fatal neurodegenerative diseases in humans and animals. Prions exist as strains, which are encoded by conformational variants of PrP^Sc. The transmissibility of prions depends on the PrP^C sequence of the recipient host and on the incoming prion strain, so that some animal prion strains are more contagious than others or are transmissible to new species, including humans. Nor98/atypical scrapie (AS) is a prion disease of sheep and goats reported in several countries worldwide. At variance with classical scrapie (CS), AS is considered poorly contagious and is supposed to be spontaneous in origin. The zoonotic potential of AS, its strain variability and the relationships with the more contagious CS strains remain largely unknown. We characterized AS isolates from sheep and goats by transmission in ovinised transgenic mice (tg338) and in two genetic lines of bank voles, carrying either methionine (BvM) or isoleucine (BvI) at PrP residue 109. All AS isolates induced the same pathological phenotype in tg338 mice, thus proving that they encoded the same strain, irrespective of their geographical origin or source species. In bank voles, we found that the M109I polymorphism dictates the susceptibility to AS. BvI were susceptible and faithfully reproduced the AS strain, while the transmission in BvM was highly inefficient and was characterized by a conformational change towards a CS-like prion strain. Sub-passaging experiments revealed that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible animals. These findings add new clues for a better comprehension of strain selection dynamics in prion infections and have wider implications for understanding the origin of contagious prion strains, such as CS.

Prions are transmissible agents responsible for fatal neurodegenerative diseases in humans and animals. Prions exist as strains, exhibiting distinct disease phenotypes and transmission properties. Some prion diseases occur sporadically with a supposedly spontaneous origin, while others are contagious and give rise to epidemics, mainly in animals. We investigated the strain properties of Nor98/atypical scrapie (AS), a sporadic prion disease of small ruminants. We found that AS was faithfully reproduced not only in a homologous context, i.e. ovinised transgenic mice, but also in an unrelated animal species, the bank vole. A natural polymorphism of the bank vole prion protein, coding for methionine (BvM) or for isoleucine (BvI) at codon 109, dictated the susceptibility of voles to AS, with BvI being highly susceptible to AS and BvM rather resistant. Most importantly, the M109I polymorphism mediated the emergence of AS-derived mutant prion strains resembling classical scrapie (CS), a contagious prion disease. Finally, by sub-passages in bank voles, we found that the main strain component of AS is accompanied by minor CS-like strain components, which can be positively selected during replication in both AS-resistant or AS-susceptible vole lines. These findings allow a better understanding of strain selection dynamics and suggest a link between sporadic and contagious prion diseases.

PrPres in placental tissue following experimental transmission of atypical scrapie in ARR/ARR sheep is not infectious by Tg338 mouse bioassay

Nor98-like atypical scrapie is a sporadic disease that affects the central nervous system of sheep and goats that, in contrast to classical scrapie, is not generally regarded as naturally transmissible. However, infectivity has been demonstrated via bioassay not only of brain tissue but also of certain peripheral nerves, lymphoid tissues, and muscle. This study examines placental tissue, a well characterized route of natural transmission for classical scrapie. Further, this study was conducted in sheep homozygous for the classical scrapie resistant ARR genotype and is the first to characterize the transmission of Nor98-like scrapie between homozygous-ARR sheep. Nor98-like scrapie isolated from a United States ARR/ARR sheep was transmitted to four ARR/ARR ewes via intracerebral inoculation of brain homogenate. These ewes were followed and observed to 8 years of age, remained non-clinical but exhibited progression of infection that was consistent with Nor98-like scrapie, including characteristic patterns of PrPSc accumulation in the brain and a lack of accumulation in peripheral lymphoid tissues as detected by conventional methods. Immunoblots of placental tissues from the infected ewes revealed accumulation of a distinct conformation of PrPres, particularly as the animals aged; however, the placenta showed no infectivity when analyzed via ovinized mouse bioassay. Taken together, these results support a low risk for natural transmission of Nor98-like scrapie in ARR/ARR sheep.

Scientific report on the analysis of the 2-year compulsory intensified monitoring of atypical scrapie

The European Commission asked EFSA whether the scientific data on the 2-year intensified monitoring in atypical scrapie (AS) outbreaks (2013-2020) provide any evidence on the contagiousness of AS, and whether they added any new knowledge on the epidemiology of AS. An ad hoc data set from intensified monitoring in 22 countries with index case/s of AS in sheep and/or goats (742 flocks from 20 countries, 76 herds from 11 countries) was analysed. No secondary cases were confirmed in goat herds, while 35 secondary cases were confirmed in 28 sheep flocks from eight countries. The results of the calculated design prevalence and of a model simulation indicated that the intensified monitoring had limited ability to detect AS, with no difference between countries with or without secondary cases. A regression model showed an increased, but not statistically significant, prevalence (adjusted by surveillance stream) of secondary cases in infected flocks compared with that of index cases in the non-infected flocks (general population). A simulation model of within-flock transmission, comparing a contagious (i.e. transmissible between animals under natural conditions) with a non-contagious scenario, produced a better fit of the observed data with the non-contagious scenario, in which each sheep in a flock had the same probability of developing AS in the first year of life. Based on the analyses performed, and considering uncertainties and data limitations, it was concluded that there is no new evidence that AS can be transmitted between animals under natural conditions, and it is considered more likely (subjective probability range 50-66%) that AS is a non-contagious, rather than a contagious disease. The analysis of the data of the EU intensified monitoring in atypical scrapie infected flocks/herds confirmed some of the known epidemiological features of AS but identified that major knowledge gaps still remain.

Classical and Atypical Scrapie in Sheep and Goats. Review on the Etiology, Genetic Factors, Pathogenesis, Diagnosis, and Control Measures of Both Diseases

Prion diseases, such as scrapie, are neurodegenerative diseases with a fatal outcome, caused by a conformational change of the cellular prion protein (PrP^C), originating with the pathogenic form (PrP^Sc). Classical scrapie in small ruminants is the paradigm of prion diseases, as it was the first transmissible spongiform encephalopathy (TSE) described and is the most studied. It is necessary to understand the etiological properties, the relevance of the transmission pathways, the infectivity of the tissues, and how we can improve the detection of the prion protein to encourage detection of the disease. The aim of this review is to perform an overview of classical and atypical scrapie disease in sheep and goats, detailing those special issues of the disease, such as genetic factors, diagnostic procedures, and surveillance approaches carried out in the European Union with the objective of controlling the dissemination of scrapie disease.

Transmission of the atypical/Nor98 scrapie agent to Suffolk sheep with VRQ/ARQ, ARQ/ARQ, and ARQ/ARR genotypes

Scrapie is a transmissible spongiform encephalopathy that occurs in sheep. Atypical/Nor98 scrapie occurs in sheep that tend to be resistant to classical scrapie and it is thought to occur spontaneously. The purpose of this study was to test the transmission of the Atypical/Nor98 scrapie agent in three genotypes of Suffolk sheep and characterize the distribution of misfolded prion protein (PrP^Sc). Ten sheep were intracranially inoculated with brain homogenate from a sheep with Atypical/Nor98 scrapie. All sheep with the ARQ/ARQ and ARQ/ARR genotypes developed Atypical/Nor98 scrapie confirmed by immunohistochemistry, and one sheep with the VRQ/ARQ genotype had detectable PrP^Sc consistent with Atypical/Nor98 scrapie at the experimental endpoint of 8 years. Sheep with mild early accumulations of PrP^Sc in the cerebellum had concomitant retinal PrP^Sc. Accordingly, large amounts of retinal PrP^Sc were identified in clinically affected sheep and sheep with dense accumulations of PrP^Sc in the cerebellum.

The Scrapie Prevalence in a Goat Herd Is Underestimated by Using a Rapid Diagnostic Test

Current European surveillance regulations for scrapie, a naturally occurring transmissible spongiform encephalopathy (TSE) or prion disease in sheep and goats, require testing of fallen stock or healthy slaughter animals, and outline measures in the case of confirmation of disease. An outbreak of classical scrapie in a herd with 2500 goats led to the culling of the whole herd, providing the opportunity to examine a subset of goats, take samples, and examine them for the presence of disease-associated prion protein (PrPSc) to provide further information on scrapie test sensitivity, pathology, and association with prion protein genotype. Goats were examined clinically prior to cull, and the brains examined post mortem by Bio-Rad ELISA, a rapid screening test used for active surveillance in sheep and goats, and two confirmatory tests, Western blot and immunohistochemistry. Furthermore, up to 10 lymphoid tissues were examined by immunohistochemistry. Of 151 goats examined, three (2.0%) tested positive for scrapie by ELISA on brain, confirmed by confirmatory tests, and a further five (3.3%) were negative by ELISA but positive by at least one of the confirmatory tests. Only two of these, both positive by ELISA, displayed evident signs of scrapie. In addition, 10 (6.6%) goats, which also included two clinical suspects, were negative on brain examination but had detectable PrPSc in lymphoid tissue. PrPSc was detected most frequently in the medial retropharyngeal lymph node (LN; 94.4% of all 18 cases) and palatine tonsil (88.9%). Abnormal behavior and circling or loss of balance when blindfolded were the best clinical discriminators for scrapie status. None of the goats that carried a single allele in the prion protein gene associated with increased resistance to scrapie (Q211, K222, S146) were scrapie-positive, and the percentage of goats with these alleles was greater than expected from previous surveys. Significantly more goats that were scrapie-positive were isoleucine homozygous at codon 142 (II142). The results indicate that the sensitivity of the applied screening test is poor in goats compared to the confirmatory tests as gold standard, particularly for asymptomatic animals. Sensitivity of surveillance could be improved by testing retropharyngeal LN or palatine tonsil in addition to brain.

The emergence of classical BSE from atypical/Nor98 scrapie

Atypical/Nor98 scrapie (AS) is a prion disease of small ruminants. Currently there are no efficient measures to control this form of prion disease, and, importantly, the zoonotic potential and the risk that AS might represent for other farmed animal species remains largely unknown. In this study, we investigated the capacity of AS to propagate in bovine PrP transgenic mice. Unexpectedly, the transmission of AS isolates originating from 5 different European countries to bovine PrP mice resulted in the propagation of the classical BSE (c-BSE) agent. Detection of prion seeding activity in vitro by protein misfolding cyclic amplification (PMCA) demonstrated that low levels of the c-BSE agent were present in the original AS isolates. C-BSE prion seeding activity was also detected in brain tissue of ovine PrP mice inoculated with limiting dilutions (endpoint titration) of ovine AS isolates. These results are consistent with the emergence and replication of c-BSE prions during the in vivo propagation of AS isolates in the natural host. These data also indicate that c-BSE prions, a known zonotic agent in humans, can emerge as a dominant prion strain during passage of AS between different species. These findings provide an unprecedented insight into the evolution of mammalian prion strain properties triggered by intra- and interspecies passage. From a public health perspective, the presence of c-BSE in AS isolates suggest that cattle exposure to small ruminant tissues and products could lead to new occurrences of c-BSE.

Dysregulation of autophagy in the central nervous system of sheep naturally infected with classical scrapie

Autophagy is a dynamic cellular mechanism involved in protein and organelle turnover through lysosomal degradation. Autophagy regulation modulates the pathologies associated with many neurodegenerative diseases. Using sheep naturally infected with scrapie as a natural animal model of prion diseases, we investigated the regulation of autophagy in the central nervous system (CNS) during the clinical phase of the disease. We present a gene expression and protein distribution analysis of different autophagy-related markers and investigate their relationship with prion-associated lesions in several areas of the CNS. Gene expression of autophagy markers ATG5 and ATG9 was downregulated in some areas of scrapie brains. In contrast, ATG5 protein accumulates in medulla oblongata and positively correlates with prion deposition and scrapie-related lesions. The accumulation of this protein and p62, a marker of autophagy impairment, suggests that autophagy is decreased in the late phases of the disease. However, the increment of LC3 proteins and the mild expression of p62 in basal ganglia and cerebellum, primarily in Purkinje cells, suggests that autophagy machinery is still intact in less affected areas. We hypothesize that specific cell populations of the CNS may display neuroprotective mechanisms against prion-induced toxicity through the induction of PrP^Sc clearance by autophagy.

Raccoons accumulate PrPSc after intracranial inoculation of the agents of chronic wasting disease or transmissible mink encephalopathy but not atypical scrapie

Prion diseases are neurodegenerative diseases characterized by the accumulation of misfolded prion protein (PrP^Sc) in the brain and other tissues. Animal prion diseases include scrapie in sheep, chronic wasting disease (CWD) in cervids, and transmissible mink encephalopathy (TME) in ranch-raised mink. We investigated the susceptibility of raccoons to various prion disease agents and compared the clinicopathologic features of the resulting disease. Raccoon kits were inoculated intracranially with the agents of raccoon-passaged TME (TME^Rac), bovine-passaged TME (TME^Bov), hamster-adapted drowsy (TME^DY) or hyper TME (TME^HY), CWD from white-tailed deer (CWD^Wtd) or elk (CWD^Elk), or atypical (Nor98) scrapie. Raccoons were euthanized when they developed clinical signs of prion disease or at study endpoint (<82 mo post-inoculation). Brain was examined for the presence of spongiform change, and disease-associated PrP^Sc was detected using an enzyme immunoassay, western blot, and immunohistochemistry. All raccoons inoculated with the agents of TME^Rac and TME^Bov developed clinical disease at ~6.6 mo post-inoculation, with widespread PrP^Sc accumulation in central nervous system tissues. PrP^Sc was detected in the brain of 1 of 4 raccoons in each of the CWD^Wtd-, CWD^Elk-, and TME^HY-inoculated groups. None of the raccoons inoculated with TME^DY or atypical scrapie agents developed clinical disease or detectable PrP^Sc accumulation. Our results indicate that raccoons are highly susceptible to infection with raccoon- and bovine-passaged TME agents, whereas CWD isolates from white-tailed deer or elk and hamster-adapted TME^HY transmit poorly. Raccoons appear to be resistant to infection with hamster-adapted TME^DY and atypical scrapie agents.

Animal prion diseases: the risks to human health

Transmissible spongiform encephalopathies (TSEs) or prion diseases of animals notably include scrapie in small ruminants, chronic wasting disease (CWD) in cervids and classical bovine spongiform encephalopathy (C-BSE). As the transmission barrier phenomenon naturally limits the propagation of prions from one species to another, and the lack of epidemiological evidence for an association with human prion diseases, the zoonotic potential of these diseases was for a long time considered negligible. However, in 1996, C-BSE was recognized as the cause of a new human prion disease, variant Creutzfeldt-Jakob disease (vCJD), which triggered an unprecedented public health crisis in Europe. Large-scale epidemio-surveillance programs for scrapie and C-BSE that were implemented in the EU after the BSE crisis revealed that the distribution and prevalence of prion diseases in the ruminant population had previously been underestimated. They also led to the recognition of new forms of TSEs (named atypical) in cattle and small ruminants and to the recent identification of CWD in Europe. At this stage, the characterization of the strain diversity and zoonotic abilities associated with animal prion diseases remains largely incomplete. However, transmission experiments in nonhuman primates and transgenic mice expressing human PrP clearly indicate that classical scrapie, and certain forms of atypical BSE (L-BSE) or CWD may have the potential to infect humans. The remaining uncertainties about the origins and relationships between animal prion diseases emphasize the importance of the measures implemented to limit human exposure to these potentially zoonotic agents, and of continued surveillance for both animal and human prion diseases.

Determining the Relative Susceptibility of Four Prion Protein Genotypes to Atypical Scrapie

Atypical scrapie is a sheep prion (PrP^Sc) disease whose epidemiology is consistent with a sporadic origin and is associated with specific polymorphisms of the normal cellular prion protein (PrP^C). To determine the relative amounts of PrP polymorphisms present in atypical scrapie, total PrP was digested with chymotrypsin to generate characteristic peptides spanning relevant polymorphisms at positions 136, 141, 154, 171, and 172 of sheep PrP^C. A multiple reaction monitoring method (MRM), employing ¹⁵N-labeled internal standards, was used to detect and quantify these polymorphisms present in both the PrP^Sc and PrP^C from heterozygous (ALRRY and ALHQY or ALRQD or AFRQY) atypical scrapie-infected or uninfected control sheep. Both polymorphisms of the full length and truncated (C1) natively expressed PrP^C are produced in equal amounts. The overall amount of PrP^C present in the infected or uninfected animals was similar. PrP^Sc isolated from heterozygotes was composed of significant amounts of both PrP polymorphisms, including the ALRRY polymorphism which is highly resistant to classical scrapie. Thus, an atypical scrapie infection does not result from an overexpression of sheep PrP^C. The replication of all atypical scrapie prions occurs at comparable rates, despite polymorphisms at positions 141, 154, 171, or 172.

Effect of Polymorphisms at Codon 146 of the Goat PRNP Gene on Susceptibility to Challenge with Classical Scrapie by Different Routes

This report presents the results of experimental challenges of goats with scrapie by both the intracerebral (i.c.) and oral routes, exploring the effects of polymorphisms at codon 146 of the goat PRNP gene on resistance to disease. The results of these studies illustrate that while goats of all genotypes can be infected by i.c. challenge, the survival distribution of the animals homozygous for asparagine at codon 146 was significantly shorter than those of animals of all other genotypes (chi-square value, 10.8; P = 0.001). In contrast, only those animals homozygous for asparagine at codon 146 (NN animals) succumbed to oral challenge. The results also indicate that any cases of infection in non-NN animals can be detected by the current confirmatory test (immunohistochemistry), although successful detection with the rapid enzyme-linked immunosorbent assay (ELISA) was more variable and dependent on the polymorphism. Together with data from previous studies of goats exposed to infection in the field, these data support the previously reported observations that polymorphisms at this codon have a profound effect on susceptibility to disease. It is concluded that only animals homozygous for asparagine at codon 146 succumb to scrapie under natural conditions.IMPORTANCE In goats, like in sheep, there are PRNP polymorphisms that are associated with susceptibility or resistance to scrapie. However, in contrast to the polymorphisms in sheep, they are more numerous in goats and may be restricted to certain breeds or geographical regions. Therefore, eradication programs must be specifically designed depending on the identification of suitable polymorphisms. An initial analysis of surveillance data suggested that such a polymorphism in Cypriot goats may lie in codon 146. In this study, we demonstrate experimentally that NN animals are highly susceptible after i.c. inoculation. The presence of a D or S residue prolonged incubation periods significantly, and prions were detected in peripheral tissues only in NN animals. In oral challenges, prions were detected only in NN animals, and the presence of a D or S residue at this position conferred resistance to the disease. This study provides an experimental transmission model for assessing the genetic susceptibility of goats to scrapie.

Case-control study on the use of pituitary-derived hormones from sheep as a potential risk factor for the occurrence of atypical scrapie in Great Britain

A case-control study was conducted in 2013 to investigate the use of pituitary-derived hormones from sheep as a potential risk factor for the presence of atypical scrapie in Great Britain sheep holdings. One hundred and sixty-five holdings were identified as cases. Two equal sets of controls were selected: no case of scrapie and cases of classical scrapie. A total of 495 holdings were selected for the questionnaire survey, 201 responses were received and 190 (38.3 per cent) were suitable for analysis. The variables 'use-of-heat-synchronisation/superovulation' and 'flock size' were significantly associated with the occurrence of atypical scrapie. Farms with atypical cases were less likely (OR 0.25, 95 per cent CI 0.07 to 0.89) to implement heat synchronisation/superovulation in the flock than the control group. Atypical cases were 3.3 times (95 per cent CI 1.38 to 8.13) more likely to occur in large holdings (>879 sheep) than in small flocks (<164 sheep). If the 'use-of-heat-synchronisation/superovulation' is a proxy for the use of pituitary-derived hormones, the significant negative association between having a case of atypical scrapie and the use of these practices rules out the initial hypothesis that using these drugs is a risk factor for the occurrence of atypical scrapie. Flock size was a significant risk factor for atypical scrapie, consistent with a previous generic case-control study.

Altered trafficking of abnormal prion protein in atypical scrapie: prion protein accumulation in oligodendroglial inner mesaxons

AIMS

Prion diseases exist in classical and atypical disease forms. Both forms are characterized by disease-associated accumulation of a host membrane sialoglycoprotein known as prion protein (PrPd ). In classical forms of prion diseases, PrPd can accumulate in the extracellular space as fibrillar amyloid, intracellularly within lysosomes, but mainly on membranes in association with unique and characteristic membrane pathology. These membrane changes are found in all species and strains of classical prion diseases and consist of spiral, branched and clathrin-coated membrane invaginations on dendrites. Atypical prion diseases have been described in ruminants and man and have distinct biological, biochemical and pathological properties when compared to classical disease. The purpose of this study was to determine whether the subcellular pattern of PrPd accumulation and membrane changes in atypical scrapie were the same as those found in classical prion diseases.

METHODS

Immunogold electron microscopy was used to examine brains of atypical scrapie-affected sheep and Tg338 mice.

RESULTS

Classical prion disease-associated membrane lesions were not found in atypical scrapie-affected sheep, however, white matter PrPd accumulation was localized mainly to the inner mesaxon and paranodal cytoplasm of oligodendroglia. Similar lesions were found in myelinated axons of atypical scrapie Tg338-infected mice. However, Tg338 mice also showed the unique grey matter membrane changes seen in classical forms of disease.

CONCLUSIONS

These data show that atypical scrapie infection directs a change in trafficking of abnormal PrP to axons and oligodendroglia and that the resulting pathology is an interaction between the agent strain and host genotype.

Pathology of Animal Transmissible Spongiform Encephalopathies (TSEs)

Pathology is the study of the structural and functional changes produced by diseases or - more specifically - the lesions they cause. To achieve this pathologists employ various approaches. These include description of lesions that are visible to the naked eye which are the subject of anatomic pathology and changes at the cellular level that are visible under the microscope, the subject of histopathology. Changes at the molecular level which are identified by probes that target specific molecules - mainly proteins that are detected using immunohistochemistry (IHC). As transmissible spongiform encephalopathies (TSEs) do not cause visible lesions anatomic pathology is not applicable to their study. For decades the application of histopathology to detect vacuoles or plaques was the only means of confirming TSE disease. The subsequent discovery of the cellular prion protein (PrP^C) and its pathogenic isoform, PrP^Sc, which is a ubiquitous marker of TSEs, led to the production of anti-PrP antibodies, and enabled the development of PrP^Sc detection techniques such as immunohistochemistry, Histoblot and PET-blot that have evolved in parallel with similar biochemical methods such as Western blot and ELISA. These methods offer greater sensitivity than histopathology in TSE diagnosis and crucially they can be applied to analyze various phenotypic aspects of single TSE sources increasing the amount of data and offering higher discriminatory power. The above principles are applied to diagnose and define TSE phenotypes which form the basis of strain characterisation.

Atypical scrapie in Australia

BACKGROUND

Since its initial detection in Norway in 1998, atypical scrapie ('atypical/Nor98 scrapie') has been reported in sheep in the majority of European countries (including in regions free of classical scrapie) and in the Falkland Islands, the USA, Canada, New Zealand and Australia.

CASE SERIES

The diagnosis in Australia of atypical scrapie in four Merino and one Merino-cross sheep showing clinical signs of neurological disease was based on the detection of grey matter neuropil vacuolation (spongiform change) in the brain (particularly in the molecular layer of the cerebellar cortex) and associated abnormal prion protein (PrP^Sc ) deposition in both grey and white matter. Changes were minimal in the caudal brainstem, the predilection site for lesions of classical scrapie.

CONCLUSION

The distinctive lesion profile of atypical scrapie in these five sheep highlights the diagnostic importance of routine histological evaluation of the cerebellum for evidence of neuropil vacuolation and associated PrP^Sc deposition in adult sheep with suspected neurological disease.

Abnormalities in Brainstem Auditory Evoked Potentials in Sheep with Transmissible Spongiform Encephalopathies and Lack of a Clear Pathological Relationship

Scrapie is transmissible spongiform encephalopathy (TSE), which causes neurological signs in sheep, but confirmatory diagnosis is usually made postmortem on examination of the brain for TSE-associated markers like vacuolar changes and disease-associated prion protein (PrP(Sc)). The objective of this study was to evaluate whether testing of brainstem auditory evoked potentials (BAEPs) at two different sound levels could aid in the clinical diagnosis of TSEs in sheep naturally or experimentally infected with different TSE strains [classical and atypical scrapie and bovine spongiform encephalopathy (BSE)] and whether any BAEP abnormalities were associated with TSE-associated markers in the auditory pathways. BAEPs were recorded from 141 clinically healthy sheep of different breeds and ages that tested negative for TSEs on postmortem tests to establish a reference range and to allow comparison with 30 sheep clinically affected or exposed to classical scrapie (CS) without disease confirmation (test group 1) and 182 clinically affected sheep with disease confirmation (test group 2). Abnormal BAEPs were found in 7 sheep (23%) of group 1 and 42 sheep (23%) of group 2. The proportion of sheep with abnormalities did not appear to be influenced by TSE strain or PrP(Sc) gene polymorphisms. When the magnitude of TSE-associated markers in the auditory pathways was compared between a subset of 12 sheep with and 12 sheep without BAEP abnormalities in group 2, no significant differences in the total PrP(Sc) or vacuolation scores in the auditory pathways could be found. However, the data suggested that there was a difference in the PrP(Sc) scores depending on the TSE strain because PrP(Sc) scores were significantly higher in sheep with BAEP abnormalities infected with classical and L-type BSE, but not with CS. The results indicated that BAEPs may be abnormal in sheep infected with TSEs but the test is not specific for TSEs and that neither vacuolation nor PrP(Sc) accumulation appears to be responsible for the clinical abnormalities.

Transmission of atypical scrapie to homozygous ARQ sheep

Two Cheviot ewes homozygous for the A₁₃₆L₁₄₁R₁₅₄Q₁₇₁ (AL₁₄₁RQ) prion protein (PrP) genotype were exposed intracerebrally to brain pools prepared using four field cases of atypical scrapie from the United Kingdom. Animals were clinically normal until the end of the experiment, when they were culled 7 years post-inoculation. Limited accumulation of disease-associated PrP (PrP^Sc) was observed in the cerebellar molecular layer by immunohistochemistry, but not by western blot or enzyme-linked immunosorbent assay. In addition, PrP^Sc was partially localized in astrocytes and microglia, suggesting that these cells have a role in PrP^Sc processing, degradation or both. Our results indicate that atypical scrapie is transmissible to AL₁₄₁RQ sheep, but these animals act as clinically silent carriers with long incubation times.

Review: A review on classical and atypical scrapie in caprine: Prion protein gene polymorphisms and their role in the disease

Scrapie is a naturally occurring transmissible spongiform encephalopathy in sheep and goat. It has been known for ~250 years and is characterised by the accumulation of an abnormal isoform of a host-encoded prion protein that leads to progressive neurodegeneration and death. Scrapie is recognised in two forms, classical and atypical scrapie. The susceptibility to both types of scrapie is influenced by polymorphisms of the prion protein gene (PRNP). Sheep susceptibility or resistance to classical scrapie is strongly regulated by the polymorphisms at codons 136, 154 and 171 of the PRNP. The genetic role in atypical scrapie in sheep has been defined by polymorphisms at codons 141, 154 and 171, which are associated with different degrees of risk in the occurrence of the ovine disease. Progress has been achieved in the prevention of scrapie in sheep due to efficient genetic breeding programmes based on eradication and control of the disease. In Europe, the success of these programmes has been verified by applying eradication and genetic selection plans. In general terms, the ovine selection plans aim to eliminate and reduce the susceptible allele and to enrich the resistant allele ARR. During outbreaks all susceptible animals are slaughtered, only ARR/ARR resistant rams and sheep and semi-resistant females are preserved. In the occurrence of scrapie positive goats a complete cull of the flock (stamping out) is performed with great economic loss and severe risk of extinction for the endangered breeds. The ability to select scrapie-resistant animals allows to define new breeding strategies aimed to boost genetic progress while reducing costs during scrapie outbreaks. Allelic variants of PRNP can be protective for caprine scrapie, and the knowledge of their distribution in goats has become very important. Over the past few years, the integration of genetic information on goat populations could be used to make selection decisions, commonly referred to as genetic selection. The objective of this review was to summarise the main findings of polymorphisms of the caprine prion protein (PrP) gene and to discuss the possible application of goat breeding schemes integrating genetic selection, with their relative advantages and limitations.

Archival search for historical atypical scrapie in sheep reveals evidence for mixed infections

Natural scrapie in sheep occurs in classical and atypical forms, which may be distinguished on the basis of the associated neuropathology and properties of the disease-associated prion protein on Western blots. First detected in 1998, atypical scrapie is known to have occurred in UK sheep since the 1980s. However, its aetiology remains unclear and it is often considered as a sporadic, non-contagious disease unlike classical scrapie which is naturally transmissible. Although atypical scrapie tends to occur in sheep of prion protein (PRNP) genotypes that are different from those found predominantly in classical scrapie, there is some overlap so that there are genotypes in which both scrapie forms can occur. In this search for early atypical scrapie cases, we made use of an archive of fixed and frozen sheep samples, from both scrapie-affected and healthy animals (∼1850 individuals), dating back to the 1960s. Using a selection process based primarily on PRNP genotyping, but also on contemporaneous records of unusual clinical signs or pathology, candidate sheep samples were screened by Western blot, immunohistochemistry and strain-typing methods using tg338 mice. We identified, from early time points in the archive, three atypical scrapie cases, including one sheep which died in 1972 and two which showed evidence of mixed infection with classical scrapie. Cases with both forms of scrapie in the same animal as recognizable entities suggest that mixed infections have been around for a long time and may potentially contribute to the variety of scrapie strains.

Animal models for prion-like diseases

Prion diseases or Transmissible Spongiform Encephalopathies (TSEs) are a group of fatal neurodegenerative disorders affecting several mammalian species being Creutzfeldt-Jacob Disease (CJD) the most representative in human beings, scrapie in ovine, Bovine Spongiform Encephalopathy (BSE) in bovine and Chronic Wasting Disease (CWD) in cervids. As stated by the "protein-only hypothesis", the causal agent of TSEs is a self-propagating aberrant form of the prion protein (PrP) that through a misfolding event acquires a β-sheet rich conformation known as PrP(Sc) (from scrapie). This isoform is neurotoxic, aggregation prone and induces misfolding of native cellular PrP. Compelling evidence indicates that disease-specific protein misfolding in amyloid deposits could be shared by other disorders showing aberrant protein aggregates such as Alzheimer's Disease (AD), Parkinson's Disease (PD), Amyotrophic lateral sclerosis (ALS) and systemic Amyloid A amyloidosis (AA amyloidosis). Evidences of shared mechanisms of the proteins related to each disease with prions will be reviewed through the available in vivo models. Taking prion research as reference, typical prion-like features such as seeding and propagation ability, neurotoxic species causing disease, infectivity, transmission barrier and strain evidences will be analyzed for other protein-related diseases. Thus, prion-like features of amyloid β peptide and tau present in AD, α-synuclein in PD, SOD-1, TDP-43 and others in ALS and serum α-amyloid (SAA) in systemic AA amyloidosis will be reviewed through models available for each disease.

Phenotype shift from atypical scrapie to CH1641 following experimental transmission in sheep

The interactions of host and infecting strain in ovine transmissible spongiform encephalopathies are known to be complex, and have a profound effect on the resulting phenotype of disease. In contrast to classical scrapie, the pathology in naturally-occurring cases of atypical scrapie appears more consistent, regardless of genotype, and is preserved on transmission within sheep homologous for the prion protein (PRNP) gene. However, the stability of transmissible spongiform encephalopathy phenotypes on passage across and within species is not absolute, and there are reports in the literature where experimental transmissions of particular isolates have resulted in a phenotype consistent with a different strain. In this study, intracerebral inoculation of atypical scrapie between two genotypes both associated with susceptibility to atypical forms of disease resulted in one sheep displaying an altered phenotype with clinical, pathological, biochemical and murine bioassay characteristics all consistent with the classical scrapie strain CH1641, and distinct from the atypical scrapie donor, while the second sheep did not succumb to challenge. One of two sheep orally challenged with the same inoculum developed atypical scrapie indistinguishable from the donor. This study adds to the range of transmissible spongiform encephalopathy phenotype changes that have been reported following various different experimental donor-recipient combinations. While these circumstances may not arise through natural exposure to disease in the field, there is the potential for iatrogenic exposure should current disease surveillance and feed controls be relaxed. Future sheep to sheep transmission of atypical scrapie might lead to instances of disease with an alternative phenotype and onward transmission potential which may have adverse implications for both public health and animal disease control policies.

Prion protein genotypes of sheep as determined from 3343 samples submitted from Ontario and other provinces of Canada from 2005 to 2012

This study analyzed sheep prion protein (PrP) genotypes of samples submitted from Ontario and other provinces of Canada to the Animal Health Laboratory at the University of Guelph, Guelph, Ontario, between 2005 and 2012. In Ontario, the proportion of scrapie-resistant sheep increased from 2005 to 2012 as evidenced by an increase in the ARR haplotype. When Canadian provinces (Alberta, Ontario, Quebec, and Nova Scotia) were compared from 2008 to 2012, a high proportion of scrapie-resistant sheep was found in all the provinces. The proportions of resistant sheep were lower in Alberta and Quebec than in Ontario and Nova Scotia. Alberta had higher proportions of susceptible sheep and a higher frequency of VRQ alleles, and Quebec had a higher frequency of the ARQ allele.

Clinical examination protocol to detect atypical and classical scrapie in sheep

The diagnosis of scrapie, a transmissible spongiform encephalopathy (TSEs) of sheep and goats, is currently based on the detection of disease-associated prion protein by post mortem tests. Unless a random sample of the sheep or goat population is actively monitored for scrapie, identification of scrapie cases relies on the reporting of clinical suspects, which is dependent on the individual's familiarization with the disease and ability to recognize clinical signs associated with scrapie. Scrapie may not be considered in the differential diagnosis of neurological diseases in small ruminants, particularly in countries with low scrapie prevalence, or not recognized if it presents as nonpruritic form like atypical scrapie. To aid in the identification of clinical suspects, a short examination protocol is presented to assess the display of specific clinical signs associated with pruritic and nonpruritic forms of TSEs in sheep, which could also be applied to goats. This includes assessment of behavior, vision (by testing of the menace response), pruritus (by testing the response to scratching), and movement (with and without blindfolding). This may lead to a more detailed neurologic examination of reporting animals as scrapie suspects. It could also be used in experimental TSE studies of sheep or goats to evaluate disease progression or to identify clinical end-point.

Heart rate variability analysis in sheep affected by transmissible spongiform encephalopathies

Background

The function of the autonomic nervous system can be assessed by determining heart rate variability (HRV), which is impaired in some brainstem diseases in humans. Transmissible spongiform encephalopathies (TSEs) in sheep are diseases characterised by accumulation of disease-associated prion protein in the brainstem, including nuclei of the parasympathetic nervous system. This study was undertaken to assess whether analysis of HRV can be used as an aid in the diagnosis of TSEs in clinically affected, naturally or experimentally infected sheep.

Findings

When HRV indices were compared between 41 clinical TSE cases (18 sheep infected with scrapie and 23 sheep infected with bovine spongiform encephalopathy), 11 control sheep and six sheep reported as scrapie suspects or dosed with BSE brain homogenate, which were not confirmed as TSE cases by postmortem tests, no significant differences were found between the groups. Median heart rate was significantly different but only when sheep were grouped by gender: it was higher in female TSE cases than in control sheep and higher in female than castrated male ovine classical BSE cases.

Conclusions

HRV analysis was not useful as a diagnostic aid for TSEs of sheep.

Atypical scrapie isolates involve a uniform prion species with a complex molecular signature

The pathobiology of atypical scrapie, a prion disease affecting sheep and goats, is still poorly understood. In a previous study, we demonstrated that atypical scrapie affecting small ruminants in Switzerland differs in the neuroanatomical distribution of the pathological prion protein (PrP^d). To investigate whether these differences depend on host-related vs. pathogen-related factors, we transmitted atypical scrapie to transgenic mice over-expressing the ovine prion protein (tg338). The clinical, neuropathological, and molecular phenotype of tg338 mice is similar between mice carrying the Swiss atypical scrapie isolates and the Nor98, an atypical scrapie isolate from Norway. Together with published data, our results suggest that atypical scrapie is caused by a uniform type of prion, and that the observed phenotypic differences in small ruminants are likely host-dependant. Strikingly, by using a refined SDS-PAGE technique, we established that the prominent proteinase K-resistant prion protein fragment in atypical scrapie consists of two separate, unglycosylated peptides with molecular masses of roughly 5 and 8 kDa. These findings show similarities to those for other prion diseases in animals and humans, and lay the groundwork for future comparative research.

Experimental oral transmission of atypical scrapie to sheep

Such transmission results in peripheral tissue infectivity that is not detectable by current surveillance screening methods.

To investigate the possibility of oral transmission of atypical scrapie in sheep and determine the distribution of infectivity in the animals’ peripheral tissues, we challenged neonatal lambs orally with atypical scrapie; they were then killed at 12 or 24 months. Screening test results were negative for disease-specific prion protein in all but 2 recipients; they had positive results for examination of brain, but negative for peripheral tissues. Infectivity of brain, distal ileum, and spleen from all animals was assessed in mouse bioassays; positive results were obtained from tissues that had negative results on screening. These findings demonstrate that atypical scrapie can be transmitted orally and indicate that it has the potential for natural transmission and iatrogenic spread through animal feed. Detection of infectivity in tissues negative by current surveillance methods indicates that diagnostic sensitivity is suboptimal for atypical scrapie, and potentially infectious material may be able to pass into the human food chain.

Atypical/Nor98 scrapie infectivity in sheep peripheral tissues

Atypical/Nor98 scrapie was first identified in 1998 in Norway. It is now considered as a worldwide disease of small ruminants and currently represents a significant part of the detected transmissible spongiform encephalopathies (TSE) cases in Europe. Atypical/Nor98 scrapie cases were reported in ARR/ARR sheep, which are highly resistant to BSE and other small ruminants TSE agents. The biology and pathogenesis of the Atypical/Nor98 scrapie agent in its natural host is still poorly understood. However, based on the absence of detectable abnormal PrP in peripheral tissues of affected individuals, human and animal exposure risk to this specific TSE agent has been considered low. In this study we demonstrate that infectivity can accumulate, even if no abnormal PrP is detectable, in lymphoid tissues, nerves, and muscles from natural and/or experimental Atypical/Nor98 scrapie cases. Evidence is provided that, in comparison to other TSE agents, samples containing Atypical/Nor98 scrapie infectivity could remain PrP^Sc negative. This feature will impact detection of Atypical/Nor98 scrapie cases in the field, and highlights the need to review current evaluations of the disease prevalence and potential transmissibility. Finally, an estimate is made of the infectivity loads accumulating in peripheral tissues in both Atypical/Nor98 and classical scrapie cases that currently enter the food chain. The results obtained indicate that dietary exposure risk to small ruminants TSE agents may be higher than commonly believed.

Following the bovine spongiform encephalopathy (BSE) crisis and the identification of its zoonotic properties, a sanitary policy has been implemented based on both eradication of transmissible spongiform encephalopathies (TSE) in food-producing animals and exclusion of known infectious materials from the food chain. Atypical/Nor98 scrapie is a prion disease of small ruminants identified worldwide. Currently it represents a significant part of the TSE cases detected in Europe. The restricted tissue distribution of Atypical/Nor98 scrapie agent in its natural host and the low detected prevalence of secondary cases in affected flocks meant that it is believed to be a poorly transmissible disease. This has led to the view that Atypical/Nor98 scrapie is a spontaneous disorder for which human and animal exposure risk remains low. In this study we demonstrate that in affected individuals, Atypical/Nor98 scrapie agent can disseminate in lymphoid tissues, nerves, and muscles, challenging the idea that it is a brain-restricted infectious agent. Evidence for the deficiencies in the current methods applied for monitoring Atypical/Nor98 scrapie is provided that would indicate an underestimation in the prevalence in the general population and in the affected flocks. These elements challenge the hypothesis on the biology of this recently identified TSE agent.

Atypical scrapie/Nor98 in a sheep from New Zealand

In a consignment of sheep brains from New Zealand, to be used in Europe as negative control material in scrapie rapid screening test evaluations, brain samples from 1 sheep (no. 1512) gave the following initially confusing results in various screening tests: the brainstem repeatedly produced negative results in 2 very similar screening kits (enzyme-linked immunosorbent assay [ELISA]-1, ELISA-2), a macerate made from brainstem and cerebellum returned a clearly positive result in ELISA-2, and the macerate and a brainstem sample gave negative results in a third screening test (ELISA-3). In subsequent testing, cerebellum tissue alone tested strongly positive in ELISA-1 and produced a banding pattern very similar to atypical scrapie/Nor98 in a confirmatory Western blot (WB). The macerate showed weak staining in the confirmatory WB but presented a staining pattern identical to atypical scrapie/Nor98 in the scrapie-associated fibril WB. The latter test confirmed conclusively the first case of atypical scrapie/Nor98 in a sheep from New Zealand. Other parts of the brain either tested negative or very weak positive in ELISA-2 and in WBs, or tested with negative results by histopathology and immunohistochemistry. It appears that sheep no. 1512 is a case of atypical scrapie/Nor98 in which the abnormal prion protein was detected mainly in the cerebellum. This case emphasizes the need to retain brainstem, and cerebral and cerebellar tissues, as frozen and fixed materials, for conclusive confirmatory testing. Furthermore, consideration should be given to which screening method to use.