Susceptibility to scrapie and disease phenotype in sheep: cross-PRNP genotype experimental transmissions with natural sources

Disease phenotype of classical sheep scrapie is changed upon experimental passage through white-tailed deer

Prion agents occur in strains that are encoded by the structure of the misfolded prion protein (PrPSc). Prion strains can influence disease phenotype and the potential for interspecies transmission. Little is known about the potential transmission of prions between sheep and deer. Previously, the classical US scrapie isolate (No.13-7) had a 100% attack rate in white-tailed deer after oronasal challenge. The purpose of this study was to test the susceptibility of sheep to challenge with the scrapie agent after passage through white-tailed deer (WTD scrapie). Lambs of various prion protein genotypes were oronasally challenged with WTD scrapie. Sheep were euthanized and necropsied upon development of clinical signs or at the end of the experiment (72 months post-inoculation). Enzyme immunoassay, western blot, and immunohistochemistry demonstrated PrPSc in 4 of 10 sheep with the fastest incubation occurring in VRQ/VRQ sheep, which contrasts the original No.13-7 inoculum with a faster incubation in ARQ/ARQ sheep. Shorter incubation periods in VRQ/VRQ sheep than ARQ/ARQ sheep after passage through deer was suggestive of a phenotype change, so comparisons were made in ovinized mice and with sheep with known strains of classical sheep scrapie: No. 13-7 and x-124 (that has a more rapid incubation in VRQ/VRQ sheep). After mouse bioassay, the WTD scrapie and x-124 isolates have similar incubation periods and PrPSc conformational stability that are markedly different than the original No. 13-7 inoculum. Furthermore, brain tissues of sheep with WTD scrapie and x-124 scrapie have similar patterns of immunoreactivity that are distinct from sheep with No. 13-7 scrapie. Multiple lines of evidence suggest a phenotype switch when No. 13-7 scrapie prions are passaged through deer. This represents one example of interspecies transmission of prions resulting in the emergence or selection of new strain properties that could confound disease eradication and control efforts.

Short incubation periods of atypical H-type BSE in cattle with EK211 and KK211 prion protein genotypes after intracranial inoculation

In 2006, a case of atypical H-type BSE (H-BSE) was found to be associated with a germline mutation in the PRNP gene that resulted in a lysine substitution for glutamic acid at codon 211 (E211K). The E211K amino acid substitution in cattle is analogous to E200K in humans, which is associated with the development of genetic Creutzfeldt-Jakob disease (CJD). In the present study, we aimed to determine the effect of the EK211 prion protein genotype on incubation time in cattle inoculated with the agent of H-BSE; to characterize the molecular profile of H-BSE in KK211 and EK211 genotype cattle; and to assess the influence of serial passage on BSE strain. Eight cattle, representing three PRNP genotype groups (EE211, EK211, and KK211), were intracranially inoculated with the agent of H-BSE originating from either a case in a cow with the EE211 prion protein genotype or a case in a cow with E211K amino acid substitution. All inoculated animals developed clinical disease; post-mortem samples were collected, and prion disease was confirmed through enzyme immunoassay, anti-PrPSc immunohistochemistry, and western blot. Western blot molecular analysis revealed distinct patterns in a steer with KK211 H-BSE compared to EK211 and EE211 cattle. Incubation periods were significantly shorter in cattle with the EK211 and KK211 genotypes compared to the EE211 genotype. Inoculum type did not significantly influence the incubation period. This study demonstrates a shorter incubation period for H-BSE in cattle with the K211 genotype in both the homozygous and heterozygous forms.

Understanding Intra-Species and Inter-Species Prion Conversion and Zoonotic Potential Using Protein Misfolding Cyclic Amplification

Prion diseases are fatal neurodegenerative disorders that affect humans and animals, and can also be transmitted from animals to humans. A fundamental event in prion disease pathogenesis is the conversion of normal host prion protein (PrP^C) to a disease-associated misfolded form (PrP^Sc). Whether or not an animal prion disease can infect humans cannot be determined a priori. There is a consensus that classical bovine spongiform encephalopathy (C-type BSE) in cattle transmits to humans, and that classical sheep scrapie is of little or no risk to human health. However, the zoonotic potential of more recently identified animal prion diseases, such as atypical scrapie, H-type and L-type BSE and chronic wasting disease (CWD) in cervids, remains an open question. Important components of the zoonotic barrier are (i) physiological differences between humans and the animal in question, (ii) amino acid sequence differences of the animal and human PrP^C, and (iii) the animal prion strain, enciphered in the conformation of PrP^Sc. Historically, the direct inoculation of experimental animals has provided essential information on the transmissibility and compatibility of prion strains. More recently, cell-free molecular conversion assays have been used to examine the molecular compatibility on prion replication and zoonotic potential. One such assay is Protein Misfolding Cyclic Amplification (PMCA), in which a small amount of infected tissue homogenate, containing PrP^Sc, is added as a seed to an excess of normal tissue homogenate containing PrP^C, and prion conversion is accelerated by cycles of incubation and ultrasonication. PMCA has been used to measure the molecular feasibility of prion transmission in a range of scenarios using genotypically homologous and heterologous combinations of PrP^Sc seed and PrP^C substrate. Furthermore, this method can be used to speculate on the molecular profile of PrP^Sc that might arise from a zoonotic transmission. We discuss the experimental approaches that have been used to model both the intra- and inter-species molecular compatibility of prions, and the factors affecting PrP^c to PrP^Sc conversion and zoonotic potential. We conclude that cell-free prion protein conversion assays, especially PMCA, are useful, rapid and low-cost approaches for elucidating the mechanisms of prion propagation and assessing the risk of animal prions to humans.

Second passage of chronic wasting disease of mule deer to sheep by intracranial inoculation compared to classical scrapie

The origin of chronic wasting disease (CWD) in cervids is unclear. One hypothesis suggests that CWD originated from scrapie in sheep. We compared the disease phenotype of sheep-adapted CWD to classical scrapie in sheep. We inoculated sheep intracranially with brain homogenate from first-passage mule deer CWD in sheep (sCWD^md). The attack rate in second-passage sheep was 100% (12 of 12). Sheep had prominent lymphoid accumulations of PrP^Sc reminiscent of classical scrapie. The pattern and distribution of PrP^Sc in the brains of sheep with CWD^md was similar to scrapie strain 13-7 but different from scrapie strain x124. The western blot glycoprofiles of sCWD^md were indistinguishable from scrapie strain 13-7; however, independent of sheep genotype, glycoprofiles of sCWD^md were different than x124. When sheep genotypes were evaluated individually, there was considerable overlap in the glycoprofiles that precluded significant discrimination between sheep CWD and scrapie strains. Our data suggest that the phenotype of CWD in sheep is indistinguishable from some strains of scrapie in sheep. Given our results, current detection techniques would be unlikely to distinguish CWD in sheep from scrapie in sheep if cross-species transmission occurred naturally. It is unknown if sheep are naturally vulnerable to CWD; however, the susceptibility of sheep after intracranial inoculation and lymphoid accumulation indicates that the species barrier is not absolute.

Neuropathology of Animal Prion Diseases

Transmissible Spongiform Encephalopathies (TSEs) or prion diseases are a fatal group of infectious, inherited and spontaneous neurodegenerative diseases affecting human and animals. They are caused by the conversion of cellular prion protein (PrP^C) into a misfolded pathological isoform (PrP^Sc or prion- proteinaceous infectious particle) that self-propagates by conformational conversion of PrP^C. Yet by an unknown mechanism, PrP^C can fold into different PrP^Sc conformers that may result in different prion strains that display specific disease phenotype (incubation time, clinical signs and lesion profile). Although the pathways for neurodegeneration as well as the involvement of brain inflammation in these diseases are not well understood, the spongiform changes, neuronal loss, gliosis and accumulation of PrP^Sc are the characteristic neuropathological lesions. Scrapie affecting small ruminants was the first identified TSE and has been considered the archetype of prion diseases, though atypical and new animal prion diseases continue to emerge highlighting the importance to investigate the lesion profile in naturally affected animals. In this report, we review the neuropathology and the neuroinflammation of animal prion diseases in natural hosts from scrapie, going through the zoonotic bovine spongiform encephalopathy (BSE), the chronic wasting disease (CWD) to the newly identified camel prion disease (CPD).

PRNP Polymorphisms in Eight Local Goat Populations/Breeds from Central and Southern Italy

In goats, as in sheep, genotypes of the prion protein gene (PRNP) can influence animals' susceptibility to scrapie. Since the polymorphic codons in sheep are well known, a genetic selection plan has been implemented in Europe, in order to reduce the prevalence of susceptible genotypes to scrapie. In Italy, no breeding plan for scrapie resistance in goats has been adopted, yet. Likewise, according to the most recent modification of Regulation EU 999/2001 (Regulation EU 772/2020) of the European Commission (EU), based on all the available experimental and in field data, K222, D146 and S146 polymorphisms could be used as scrapie resistance alleles in genetic management both in scrapie outbreaks and in disease prevention. In order to collect data on the variability of PRNP, the present study aimed to analyze the sequence of the PRNP gene in eight Italian local goat populations/breeds reared in central and southern Italy (Bianca Monticellana, Capestrina, Facciuta della Valnerina, Fulva del Lazio, Garganica, Grigia Ciociara, Grigia Molisana, and Teramana), some of which were investigated for the first time; moreover, two cosmopolitan breeds (Alpine and Saanen) were included. Blood samples were collected from 219 goats. Genomic DNA was extracted from whole blood. DNA was used as template in PCR amplification of the entire PRNP open reading frame (ORF). Purified amplicons have been sequenced and aligned to Capra hircus PRNP. Particularly, the alleles carrying the resistance-related 222 K polymorphism occurred in all populations with a frequency between 2.5% and 12.5%. An additional resistance allele carrying the S146 variant was observed with a frequency of 3.7% only in the Alpine breed. For three of the estimated alleles, we could not establish if the found double polymorphisms in heterozygosis were in phase, due to technical limitations. In this context, in addition to selective culling in scrapie outbreaks according to the European regulation in force, in the future, selection plans could be adopted to deal with scrapie and to control its diffusion, meanwhile paying attention to preserve a high variability of PRNP.

Bovine adapted transmissible mink encephalopathy is similar to L-BSE after passage through sheep with the VRQ/VRQ genotype but not VRQ/ARQ

Background

Transmissible mink encephalopathy (TME) is a fatal neurologic disease of farmed mink. Evidence indicates that TME and L-BSE are similar and may be linked in some outbreaks of TME. We previously transmitted bovine adapted TME (bTME) to sheep. The present study compared ovine passaged bTME (o-bTME) to C-BSE and L-BSE in transgenic mice expressing wild type bovine prion protein (TgBovXV). To directly compare the transmission efficiency of all prion strains in this study, we considered the attack rates and mean incubation periods. Additional methods for strain comparison were utilized including lesion profiles, fibril stability, and western blotting.

Results

Sheep donor genotype elicited variable disease phenotypes in bovinized mice. Inoculum derived from a sheep with the VRQ/VRQ genotype (o-bTME_VV) resulted in an attack rate, incubation period, western blot profile, and neuropathology most similar to bTME and L-BSE. Conversely, donor material from a sheep with the VRQ/ARQ genotype (o-bTME_AV) elicited a phenotype distinct from o-bTME_VV, bTME and L-BSE. The TSE with the highest transmission efficiency in bovinized mice was L-BSE. The tendency to efficiently transmit to TgBovXV mice decreased in the order bTME, C-BSE, o-bTME_VV, and o-bTME_AV. The transmission efficiency of L-BSE was approximately 1.3 times higher than o-bTME_VV and 3.2 times higher than o-bTME_AV.

Conclusions

Our findings provide insight on how sheep host genotype modulates strain genesis and influences interspecies transmission characteristics. Given that the transmission efficiencies of L-BSE and bTME are higher than C-BSE, coupled with previous reports of L-BSE transmission to mice expressing the human prion protein, continued monitoring for atypical BSE is advisable in order to prevent occurrences of interspecies transmission that may affect humans or other species.

Four types of scrapie in goats differentiated from each other and bovine spongiform encephalopathy by biochemical methods

Scrapie in goats has been known since 1942, the archetype of prion diseases in which only prion protein (PrP) in misfolded state (PrPSc) acts as infectious agent with fatal consequence. Emergence of bovine spongiform encephalopathy (BSE) with its zoonotic behaviour and detection in goats enhanced fears that its source was located in small ruminants. However, in goats knowledge on prion strain typing is limited. A European-wide study is presented concerning the biochemical phenotypes of the protease resistant fraction of PrPSc (PrPres) in over thirty brain isolates from transmissible spongiform encephalopathy (TSE) affected goats collected in seven countries. Three different scrapie forms were found: classical scrapie (CS), Nor98/atypical scrapie and one case of CH1641 scrapie. In addition, CS was found in two variants-CS-1 and CS-2 (mainly Italy)-which differed in proteolytic resistance of the PrPres N-terminus. Suitable PrPres markers for discriminating CH1641 from BSE (C-type) appeared to be glycoprofile pattern, presence of two triplets instead of one, and structural (in)stability of its core amino acid region. None of the samples exhibited BSE like features. BSE and these four scrapie types, of which CS-2 is new, can be recognized in goats with combinations of a set of nine biochemical parameters.

Prion protein polymorphisms associated with reduced CWD susceptibility limit peripheral PrPCWD deposition in orally infected white-tailed deer

Background

Chronic wasting disease (CWD) is a prion disease affecting members of the Cervidae family. PrP^C primary structures play a key role in CWD susceptibility resulting in extended incubation periods and regulating the propagation of CWD strains. We analyzed the distribution of abnormal prion protein (PrP^CWD) aggregates in brain and peripheral organs from orally inoculated white-tailed deer expressing four different PRNP genotypes: Q95G96/Q95G96 (wt/wt), S96/wt, H95/wt and H95/S96 to determine if there are substantial differences in the deposition pattern of PrP^CWD between different PRNP genotypes.

Results

Although we detected differences in certain brain areas, globally, the different genotypes showed similar PrP^CWD deposition patterns in the brain. However, we found that clinically affected deer expressing H95 PrP^C, despite having the longest survival periods, presented less PrP^CWD immunoreactivity in particular peripheral organs. In addition, no PrP^CWD was detected in skeletal muscle of any of the deer.

Conclusions

Our data suggest that expression of H95-PrP^C limits peripheral accumulation of PrP^CWD as detected by immunohistochemistry. Conversely, infected S96/wt and wt/wt deer presented with similar PrP^CWD peripheral distribution at terminal stage of disease, suggesting that the S96-PrP^C allele, although delaying CWD progression, does not completely limit the peripheral accumulation of the infectious agent.

Low-volume goat milk transmission of classical scrapie to lambs and goat kids

The risk of classical scrapie transmission in small ruminants is highest during the neonatal period with the placenta recognized as a significant source of infection. Milk has also been identified as a source of scrapie with sheep-to-sheep transmission occurring after neonatal consumption of as little as 1-2 liters of milk; concurrent mastitis due to small ruminant lentivirus (SRLV) infection may be associated with increased scrapie transmission via milk in sheep. In contrast, goat-to-sheep transmission has been documented only after prolonged consumption of >30 liters of milk. The goal of the current study was to assess transmission of scrapie to goat kids and lambs following low volume, short duration consumption of milk from infected goats. Milk from two does (female goats) with pre-clinical scrapie was fed to four goat kids (≤4.5 L each) and four lambs (~3.7 L each) beginning ~24 hours after birth. Scrapie transmission was detected in three sheep as early as 18 months post inoculation; transmission was also detected in two goats but not until postmortem analyses at 33 months post inoculation. Each milk donor goat also had naturally-acquired infection with SRLV. Different degrees of lymphohistiocytic inflammation and PrPSc accumulation were observed in mammary gland tissues of the donors, which appeared to associate with transmission of scrapie via milk. Thus, similar to the risks of milk transmission of scrapie from sheep, even limited exposure to milk from goats can pose significant risk for scrapie transmission to both goat kids and lambs.

Expression of selected genes isolated from whole blood, liver and obex in lambs with experimental classical scrapie and healthy controls, showing a systemic innate immune response at the clinical end-stage

BACKGROUND

Incubation period, disease progression, pathology and clinical presentation of classical scrapie in sheep are highly dependent on PRNP genotype, time and route of inoculation and prion strain. Our experimental model with pre-colostrum inoculation of homozygous VRQ lambs has shown to be an effective model with extensive PrPSc dissemination in lymphatic tissue and a short incubation period with severe clinical disease. Serum protein analysis has shown an elevation of acute phase proteins in the clinical stages of this experimental model, and here, we investigate changes in gene expression in whole blood, liver and brain.

RESULTS

The animals in the scrapie group showed severe signs of illness 22 weeks post inoculation necessitating euthanasia at 23 weeks post inoculation. This severe clinical presentation was accompanied by changes in expression of several genes. The following genes were differentially expressed in whole blood: TLR2, TLR4, C3, IL1B, LF and SAA, in liver tissue, the following genes differentially expressed: TNF-α, SAA, HP, CP, AAT, TTR and TF, and in the brain tissue, the following genes were differentially expressed: HP, CP, ALB and TTR.

CONCLUSIONS

We report a strong and evident transcriptional innate immune response in the terminal stage of classical scrapie in these animals. The PRNP genotype and time of inoculation are believed to contribute to the clinical presentation, including the extensive dissemination of PrPSc throughout the lymphatic tissue.

Review: Update on Classical and Atypical Scrapie in Sheep and Goats

Scrapie is a naturally occurring transmissible spongiform encephalopathy (TSE) or prion disease of sheep and goats. Scrapie is a protein misfolding disease where the normal prion protein (PrP^C) misfolds into a pathogenic form (PrP^Sc) that is highly resistant to enzymatic breakdown within the cell and accumulates, eventually leading to neurodegeneration. The amino acid sequence of the prion protein and tissue distribution of PrP^Sc within affected hosts have a major role in determining susceptibility to and potential environmental contamination with the scrapie agent. Many countries have genotype-based eradication programs that emphasize using rams that express arginine at codon 171 in the prion protein, which is associated with resistance to the classical scrapie agent. In classical scrapie, accumulation of PrP^Sc within lymphoid and other tissues facilitates environmental contamination and spread of the disease within flocks. A major distinction can be made between classical scrapie strains that are readily spread within populations of susceptible sheep and goats and atypical (Nor-98) scrapie that has unique molecular and phenotype characteristics and is thought to occur spontaneously in older sheep or goats. This review provides an overview of classical and atypical scrapie with consideration of potential transmission of classical scrapie to other mammalian hosts.

Genetic resistance to transmissible spongiform encephalopathies (TSE) in goats

Breeding programmes to promote resistance to classical scrapie, similar to those for sheep in existing transmissible spongiform encephalopathies (TSE) regulations, have not been established in goats. The European Commission requested a scientific opinion from EFSA on the current knowledge of genetic resistance to TSE in goats. An evaluation tool, which considers both the weight of evidence and strength of resistance to classical scrapie of alleles in the goat PRNP gene, was developed and applied to nine selected alleles of interest. Using the tool, the quality and certainty of the field and experimental data are considered robust enough to conclude that the K222, D146 and S146 alleles both confer genetic resistance against classical scrapie strains known to occur naturally in the EU goat population, with which they have been challenged both experimentally and under field conditions. The weight of evidence for K222 is greater than that currently available for the D146 and S146 alleles and for the ARR allele in sheep in 2001. Breeding for resistance can be an effective tool for controlling classical scrapie in goats and it could be an option available to member states, both at herd and population levels. There is insufficient evidence to assess the impact of K222, D146 and S146 alleles on susceptibility to atypical scrapie and bovine spongiform encephalopathy (BSE), or on health and production traits. These alleles are heterogeneously distributed across the EU Member States and goat breeds, but often at low frequencies (< 10%). Given these low frequencies, high selection pressure may have an adverse effect on genetic diversity so any breeding for resistance programmes should be developed at Member States, rather than EU level and their impact monitored, with particular attention to the potential for any negative impact in rare or small population breeds.

Codon 141 polymorphisms of the ovine prion protein gene affect the phenotype of classical scrapie transmitted from goats to sheep

Background

A study to investigate transmission of classical scrapie via goat milk was carried out in sheep: firstly, lambs were challenged orally with goat scrapie brain homogenate to confirm transmission of scrapie from goats to sheep. In the second study phase, milk from scrapie-infected goats was fed to lambs. Lambs were selected according to their prion protein gene (PRNP) genotype, which was either VRQ/VRQ or ARQ/ARQ, with or without additional polymorphisms at codon 141 (FF₁₄₁, LF₁₄₁ or LL₁₄₁) of the ovine PRNP. This report describes the clinical, pathological and molecular phenotype of goat scrapie in those sheep that progressed to clinical end-stage.

Results

Ten sheep (six VRQ/VRQ and four ARQ/ARQ, of which three FF₁₄₁ and one LL₁₄₁) challenged with one of two scrapie brain homogenates, and six pairs of sheep (ARQ, of which five LL₁₄₁ and seven LF₁₄₁) fed milk from six different goats, developed clinical disease, which was characterised by a pruritic (all VRQ/VRQ and LL₁₄₁ sheep) or a non-pruritic form (all LF₁₄₁ and FF₁₄₁ sheep). Immunohistochemical (IHC) examination revealed that the pattern of intra- and extracellular accumulation of disease-associated prion protein in the brain was also dependent on PRNP polymorphisms at codon 141, which was similar in VRQ and LL₁₄₁ sheep but different from LF₁₄₁ and FF₁₄₁ sheep. The influence of codon 141 was also seen in discriminatory Western blot (WB), with LF₁₄₁ and FF₁₄₁ sheep showing a bovine spongiform encephalopathy-like profile (diminished reactivity with P4 antibody) on brain tissue. However, discriminatory WB in lymphoid tissues, and IHC pattern and profile both in lymphoid and brain tissue was consistent with classical scrapie in all sheep.

Conclusions

This study provided further evidence that the clinical presentation and the pathological and molecular phenotypes of scrapie in sheep are influenced by PRNP polymorphisms, particularly at codon 141. Differences in the truncation of disease-associated prion protein between LL₁₄₁ sheep and those carrying the F₁₄₁ allele may be responsible for these observations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1036-1) contains supplementary material, which is available to authorized users.

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.

Evidence of scrapie transmission to sheep via goat milk

Background

Previous studies confirmed that classical scrapie can be transmitted via milk in sheep. The current study aimed to investigate whether scrapie can also be transmitted via goat milk using in vivo (new-born lambs fed milk from scrapie-affected goats due to the unavailability of goat kids from guaranteed scrapie-free herds) and in vitro methods (serial protein misfolding cyclic amplification [sPMCA] on milk samples).

Results

In an initial pilot study, new-born lambs of two different prion protein gene (PRNP) genotypes (six VRQ/VRQ and five ARQ/ARQ) were orally challenged with 5 g brain homogenate from two scrapie-affected goats to determine susceptibility of sheep to goat scrapie. All sheep challenged with goat scrapie brain became infected based on the immunohistochemical detection of disease-associated PrP (PrP^sc) in lymphoid tissue, with an ARQ/ARQ sheep being the first to succumb. Subsequent feeding of milk to eight pairs of new-born ARQ/ARQ lambs, with each pair receiving milk from a different scrapie-affected goat, resulted in scrapie in the six pairs that received the largest volume of milk (38–87 litres per lamb), whereas two pairs fed 8–9 litres per lamb, and an environmental control group raised on sheep milk from healthy ewes, did not show evidence of infection when culled at up to 1882 days of age. Infection in those 12 milk recipients occurred regardless of the clinical status, PrP^sc distribution, caprine arthritis-encephalitis virus infection status and PRNP polymorphisms at codon 142 (II or IM) of the donor goats, but survival time was influenced by PRNP polymorphisms at codon 141. Serial PMCA applied to a total of 32 milk samples (four each from the eight donor goats collected throughout lactation) detected PrP^sc in one sample each from two goats.

Conclusions

The scrapie agent was present in the milk from infected goats and was able to transmit to susceptible species even at early preclinical stage of infection, when PrP^sc was undetectable in the brain of the donor goats. Serial PMCA as a PrP^sc detection method to assess the risk of scrapie transmission via milk in goats proved inefficient compared to the bioassay.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0807-4) contains supplementary material, which is available to authorized users.

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.

Stability of murine scrapie strain 87V after passage in sheep and comparison with the CH1641 ovine strain

Breed- and prion protein (PRNP) genotype-related disease phenotype variability has been observed in sheep infected with the 87V murine scrapie strain. Therefore, the stability of this strain was tested by inoculating sheep-derived 87V brain material back into VM mice. As some sheep-adapted 87V disease phenotypes were reminiscent of CH1641 scrapie, transgenic mice (Tg338) expressing ovine prion protein (PrP) were inoculated with the same sheep-derived 87V sources and with CH1641. Although at first passage in VM mice the sheep-derived 87V sources showed some divergence from the murine 87V control, all the characteristics of murine 87V infection were recovered at second passage from all sheep sources. These included 100 % attack rates and indistinguishable survival times, lesion profiles, immunohistochemical features of disease-associated PrP accumulation in the brain and PrP biochemical properties. All sheep-derived 87V sources, as well as CH1641, were transmitted to Tg338 mice with identical clinical, pathological, immunohistochemical and biochemical features. While this might potentially indicate that sheep-adapted 87V and CH1641 are the same strain, profound divergences were evident, as murine 87V was unable to infect Tg338 mice but was lethal for VM mice, while the reverse was true for CH1641. These combined data suggest that: (i) murine 87V is stable and retains its properties after passage in sheep; (ii) it can be isolated from sheep showing a CH1641-like or a more conventional scrapie phenotype; and (iii) sheep-adapted 87V scrapie, with conventional or CH1641-like phenotype, is biologically distinct from experimental CH1641 scrapie, despite the fact that they behave identically in a single transgenic mouse line.

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.

Minimum Effective Dose of Cattle and Sheep BSE for Oral Sheep Infection

The minimum dose required to cause infection of Romney and Suffolk sheep of the ARQ/ARQ or ARQ/ARR prion protein gene genotypes following oral inoculation with Romney or Suffolk a sheep Bovine spongiform encephalopathy (BSE)-derived or cattle BSE-derived agent was investigated using doses ranging from 0.0005g to 5g. ARQ/ARQ sheep which were methionine (M) / threonine (T) heterozygous or T/T homozygous at codon 112 of the Prnp gene, dosed ARQ/ARR sheep and undosed controls did not show any evidence of infection. Within groups of susceptible sheep, the minimum effective oral dose of BSE was found to be 0.05g, with higher attack rates following inoculation with the 5g dose. Surprisingly, this study found no effect of dose on survival time suggesting a possible lack of homogeneity within the inoculum. All clinical BSE cases showed PrP^d accumulation in brain; however, following cattle BSE inoculation, LRS involvement within Romney recipients was found to be significantly lower than within the Suffolk sheep inoculated group which is in agreement with previous reports.

The transmissible spongiform encephalopathies of livestock

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

Prion Type-Dependent Deposition of PRNP Allelic Products in Heterozygous Sheep

Susceptibility or resistance to prion infection in humans and animals depends on single prion protein (PrP) amino acid substitutions in the host, but the agent's modulating role has not been well investigated. Compared to disease incubation times in wild-type homozygous ARQ/ARQ (where each triplet represents the amino acids at codons 136, 154, and 171, respectively) sheep, scrapie susceptibility is reduced to near resistance in ARR/ARR animals while it is strongly enhanced in VRQ/VRQ carriers. Heterozygous ARR/VRQ animals exhibit delayed incubation periods. In bovine spongiform encephalopathy (BSE) infection, the polymorphism effect is quite different although the ARR allotype remains the least susceptible. In this study, PrP allotype composition in protease-resistant prion protein (PrP(res)) from brain of heterozygous ARR/VRQ scrapie-infected sheep was compared with that of BSE-infected sheep with a similar genotype. A triplex Western blotting technique was used to estimate the two allotype PrP fractions in PrP(res) material from BSE-infected ARR/VRQ sheep. PrP(res) in BSE contained equimolar amounts of VRQ- and ARR-PrP, which contrasts with the excess (>95%) VRQ-PrP fraction found in PrP in scrapie. This is evidence that transmissible spongiform encephalopathy (TSE) agent properties alone, perhaps structural aspects of prions (such as PrP amino acid sequence variants and PrP conformational state), determine the polymorphic dependence of the PrP(res) accumulation process in prion formation as well as the disease-associated phenotypic expressions in the host.

IMPORTANCE

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative and transmissible diseases caused by prions. Amino acid sequence variants of the prion protein (PrP) determine transmissibility in the hosts, as has been shown for classical scrapie in sheep. Each individual produces a separate PrP molecule from its two PrP gene copies. Heterozygous scrapie-infected sheep that produce two PrP variants associated with opposite scrapie susceptibilities (136V-PrP variant, high; 171R-PrP variant, very low) contain in their prion material over 95% of the 136V PrP variant. However, when these sheep are infected with prions from cattle (bovine spongiform encephalopathy [BSE]), both PrP variants occur in equal ratios. This shows that the infecting prion type determines the accumulating PrP variant ratio in the heterozygous host. While the host's PrP is considered a determining factor, these results emphasize that prion structure plays a role during host infection and that PrP variant involvement in prions of heterozygous carriers is a critical field for understanding prion formation.

Comparative Susceptibility of Sheep of Different Origins, Breeds and PRNP Genotypes to Challenge with Bovine Spongiform Encephalopathy and Scrapie

Sheep are natural hosts of the prion disease, scrapie. They are also susceptible to experimental challenge with various scrapie strains and with bovine spongiform encephalopathy (BSE), which affects cattle and has been accidentally transmitted to a range of other species, including man. Incidence and incubation period of clinical disease in sheep following inoculation is controlled by the PRNP gene, which has different alleles defined on the basis of polymorphisms, particularly at codons 136, 154 and 171, although other codons are associated with survival time, and the exact responses of the sheep may be influenced by other breed-related differences. Here we report the results of a long term single study of experimental scrapie and BSE susceptibility of sheep of Cheviot, Poll Dorset and Suffolk breeds, originating from New Zealand and of a wide range of susceptible and resistant PRNP genotypes. Responses were compared with those of sheep from a closed Cheviot flock of UK origin (Roslin Cheviot flock). The unusually long observation period (6-8 years for most, but up to 12 years for others) allows us to draw robust conclusions about rates of survival of animals previously regarded as resistant to infection, particularly PRNP heterozygotes, and is the most comprehensive such study reported to date. BSE inoculation by an intracerebral route produced disease in all genotype groups with differing incubation periods, although M112T and L141F polymorphisms seemed to give some protection. Scrapie isolate SSBP/1, which has the shortest incubation period in sheep with at least one VRQ PRNP allele, also produced disease following sub-cutaneous inoculation in ARQ/ARQ animals of New Zealand origin, but ARQ/ARQ sheep from the Roslin flock survived the challenge. Our results demonstrate that the links between PRNP genotype and clinical prion disease in sheep are much less secure than previously thought, and may break down when, for example, a different breed of sheep is moved into a new flock.

Dynamics of the natural transmission of bovine spongiform encephalopathy within an intensively managed sheep flock

Sheep are susceptible to the bovine spongiform encephalopathy (BSE) agent and in the UK they may have been exposed to BSE via contaminated meat and bone meal. An experimental sheep flock was established to determine whether ovine BSE could be naturally transmitted under conditions of intensive husbandry. The flock consisted of 113 sheep of different breeds and susceptible PRNP genotypes orally dosed with BSE, 159 sheep subsequently born to them and 125 unchallenged sentinel controls. BSE was confirmed in 104 (92%) orally dosed sheep and natural transmission was recorded for 14 of 79 (18%) lambs born to BSE infected dams, with rates varying according to PRNP genotype. The likelihood of natural BSE transmission was linked to stage of incubation period of the dam: the attack rate for lambs born within 100 days of the death of BSE infected dams was significantly higher (9/22, 41%) than for the rest (5/57, 9%). Within the group of ewes lambing close to death, those rearing infected progeny (n = 8, for 9/12 infected lambs) showed a significantly greater involvement of lymphoid tissues than those rearing non-infected offspring (n = 8, for 0/10 infected lambs). Horizontal transmission to the progeny of non-infected mothers was recorded only once (1/205, 0.5%). This low rate of lateral transmission was attributed, at least partly, to an almost complete absence of infected placentas. We conclude that, although BSE can be naturally transmitted through dam-lamb close contact, the infection in this study flock would not have persisted due to low-efficiency maternal and lateral transmissions.

Diagnostic pathology in microbial diseases of sheep or goats

Post-mortem examination is a key step in the diagnostic process of infectious diseases in sheep and goats. Diagnostic pathology deals with identification and study of lesions, at the same time providing also significant clues regarding pathogenesis of the diseases. This article reviews the salient pathological findings associated with the most significant infectious diseases of sheep and goats present in countries where small ruminants are a relevant agricultural industry. Lesions are reviewed according to the different organ systems where they occur. Emphasis has been given in the description of the salient lesional patterns than can be identified in each organ and which can be of help in the differential diagnosis of the lesions caused by bacteria, viruses, fungi or prions. Finally, a review of the usefulness of ancillary tests that may be used on various tissue samples for performing an aetiological diagnosis, is included; the application of various techniques, from immunohistochemistry to molecular biology-based tests, is described.

A prion reduction filter does not completely remove endogenous prion infectivity from sheep blood

BACKGROUND

Variant Creutzfeldt-Jakob disease (vCJD) is a transmissible spongiform encephalopathy affecting humans, acquired initially through infection with bovine spongiform encephalopathy (BSE). A small number of vCJD cases have been acquired through the transfusion of blood from asymptomatic donors who subsequently developed vCJD. Filter devices that selectively bind the infectious agent associated with prion disease have been developed for removal of infection from blood. This study independently assessed one such filter, the P-CAPT filter, for efficacy in removing infectivity associated with the BSE agent in sheep blood. The sheep BSE model has previously been used to evaluate the distribution of infectivity in clinically relevant blood components. This is the first study to assess the ability of the P-CAPT filter to remove endogenous infectivity associated with blood components prepared from a large animal model.

STUDY DESIGN AND METHODS

Paired units of leukoreduced red blood cells (LR-RBCs) were prepared from donors at the clinical stage of infection and confirmed as having BSE. One cohort of recipients was transfused with LR-RBCs alone, whereas a parallel cohort received LR and P-CAPT-filtered RBCs (LR-RBCs-P-CAPT).

RESULTS

Of 14 recipients, two have been confirmed as having BSE. These sheep had received LR-RBCs and LR-RBCs-P-CAPT from the same donor.

CONCLUSIONS

The results indicate that, after leukoreduction and P-CAPT filtration, there can still be sufficient residual infectivity in sheep RBCs to transmit infection when transfused into a susceptible recipient.

The placenta shed from goats with classical scrapie is infectious to goat kids and lambs

The placenta of domestic sheep plays a key role in horizontal transmission of classical scrapie. Domestic goats are frequently raised with sheep and are susceptible to classical scrapie, yet potential routes of transmission from goats to sheep are not fully defined. Sparse accumulation of disease-associated prion protein in cotyledons casts doubt about the role of the goat's placenta. Thus, relevant to mixed-herd management and scrapie-eradication efforts worldwide, we determined if the goat's placenta contains prions orally infectious to goat kids and lambs. A pooled cotyledon homogenate, prepared from the shed placenta of a goat with naturally acquired classical scrapie disease, was used to orally inoculate scrapie-naïve prion genotype-matched goat kids and scrapie-susceptible lambs raised separately in a scrapie-free environment. Transmission was detected in all four goats and in two of four sheep, which importantly identifies the goat's placenta as a risk for horizontal transmission to sheep and other goats.

Susceptibility of European red deer (Cervus elaphus elaphus) to alimentary challenge with bovine spongiform encephalopathy

European red deer (Cervus elaphus elaphus) are susceptible to the agent of bovine spongiform encephalopathy, one of the transmissible spongiform encephalopathies, when challenged intracerebrally but their susceptibility to alimentary challenge, the presumed natural route of transmission, is unknown. To determine this, eighteen deer were challenged via stomach tube with a large dose of the bovine spongiform encephalopathy agent and clinical signs, gross and histological lesions, presence and distribution of abnormal prion protein and the attack rate recorded. Only a single animal developed clinical disease, and this was acute with both neurological and respiratory signs, at 1726 days post challenge although there was significant (27.6%) weight loss in the preceding 141 days. The clinically affected animal had histological lesions of vacuolation in the neuronal perikaryon and neuropil, typical of transmissible spongiform encephalopathies. Abnormal prion protein, the diagnostic marker of transmissible encephalopathies, was primarily restricted to the central and peripheral nervous systems although a very small amount was present in tingible body macrophages in the lymphoid patches of the caecum and colon. Serial protein misfolding cyclical amplification, an in vitro ultra-sensitive diagnostic technique, was positive for neurological tissue from the single clinically diseased deer. All other alimentary challenged deer failed to develop clinical disease and were negative for all other investigations. These findings show that transmission of bovine spongiform encephalopathy to European red deer via the alimentary route is possible but the transmission rate is low. Additionally, when deer carcases are subjected to the same regulations that ruminants in Europe with respect to the removal of specified offal from the human food chain, the zoonotic risk of bovine spongiform encephalopathy, the cause of variant Creutzfeldt-Jakob disease, from consumption of venison is probably very low.

Influence of breed and genotype on the onset and distribution of infectivity and disease-associated prion protein in sheep following oral infection with the bovine spongiform encephalopathy agent

The onset and distribution of infectivity and disease-specific prion protein (PrP(d)) accumulation was studied in Romney and Suffolk sheep of the ARQ/ARQ, ARQ/ARR and ARR/ARR prion protein gene (Prnp) genotypes (where A stands for alanine, R for arginine and Q for glutamine at codons 136, 154 and 171 of PrP), following experimental oral infection with cattle-derived bovine spongiform encephalopathy (BSE) agent. Groups of sheep were killed at regular intervals and a wide range of tissues taken for mouse bioassay or immunohistochemistry (IHC), or both. Bioassay results for infectivity were mostly coincident with those of PrP(d) detection by IHC both in terms of tissues and time post infection. Neither PrP(d) nor infectivity was detected in any tissues of BSE-dosed ARQ/ARR or ARR/ARR sheep or of undosed controls. Moreover, four ARQ/ARQ Suffolk sheep, which were methionine (M)/threonine heterozygous at codon 112 of the Prnp gene, did not show any biological or immunohistochemical evidence of infection, while those homozygous for methionine (MARQ/MARQ) did. In MARQ/MARQ sheep of both breeds, initial PrP(d) accumulation was identified in lymphoreticular system (LRS) tissues followed by the central nervous system (CNS) and enteric nervous system (ENS) and finally by the autonomic nervous system and peripheral nervous system and other organs. Detection of infectivity closely mimicked this sequence. No PrP(d) was observed in the ENS prior to its accumulation in the CNS, suggesting that ENS involvement occurred simultaneously to that of, or followed centrifugal spread from, the CNS. The distribution of PrP(d) within the ENS further suggested a progressive spread from the ileal plexus to other ENS segments via neuronal connections of the gut wall. Differences between the two breeds were noted in terms of involvement of LRS and ENS tissues, with Romney sheep showing a more delayed and less consistent PrP(d) accumulation than Suffolk sheep in such tissues. Whether this accounted for the slight delay (∼5 months) in the appearance of clinical signs in Romney sheep is debatable since by the last scheduled kill before animals reached clinical end point, both breeds showed widespread accumulation and similar magnitudes of PrP(d) accumulation in the brain.

Genotype-dependent molecular evolution of sheep bovine spongiform encephalopathy (BSE) prions in vitro affects their zoonotic potential

Prion diseases are rare fatal neurological conditions of humans and animals, one of which (variant Creutzfeldt-Jakob disease) is known to be a zoonotic form of the cattle disease bovine spongiform encephalopathy (BSE). What makes one animal prion disease zoonotic and others not is poorly understood, but it appears to involve compatibility between the prion strain and the host prion protein sequence. Concerns have been raised that the United Kingdom sheep flock may have been exposed to BSE early in the cattle BSE epidemic and that serial BSE transmission in sheep might have resulted in adaptation of the agent, which may have come to phenotypically resemble scrapie while maintaining its pathogenicity for humans. We have modeled this scenario in vitro. Extrapolation from our results suggests that if BSE were to infect sheep in the field it may, with time and in some sheep genotypes, become scrapie-like at the molecular level. However, the results also suggest that if BSE in sheep were to come to resemble scrapie it would lose its ability to affect humans.

Incidence of infection in Prnp ARR/ARR sheep following experimental inoculation with or natural exposure to classical scrapie

The prion protein gene (Prnp) is highly influential in determining risk and susceptibility of sheep exposed to classical scrapie. Sheep homozygous for alanine at codon 136 and arginine at codons 154 and 171 (ARR/ARR) of the Prnp gene are historically considered to be highly resistant to classical scrapie, although they form a significant fraction of cases of atypical scrapie. To date, experimental transmission of prions to ARR/ARR sheep has only been achieved with the BSE agent and mostly by the intracerebral route. We summarise here the results of six separate studies, in which 95 sheep of the ARR/ARR genotype were naturally exposed to (n = 18) or experimentally challenged with (n = 77) natural or experimental sources of classical scrapie by the oral, intra-intestinal, subcutaneous or intracerebral routes and allowed to survive for periods of up to 94 months post-infection. Only the intracerebral route resulted in disease and/or amplification of disease associated PrP (PrP^d), and only in two of 19 sheep that survived for longer than 36 months. Discriminatory immunohistochemistry and Western blot confirmed the scrapie, non-BSE signature of PrP^d in those two sheep. However, the neuropathological phenotype was different from any other scrapie (classical or atypical) or BSE source previously reported in sheep of any Prnp genotype. These studies confirm the widely held view that ARR/ARR sheep are highly resistant to classical scrapie infection, at least within their commercial lifespan. Moreover, within the constraints of the present studies (only two infected sheep), these results do not support the suggestion that atypical scrapie or BSE are generated by adaptation or mutation of classical scrapie in sheep of resistant ARR/ARR genotype.

Enhanced virulence of sheep-passaged bovine spongiform encephalopathy agent is revealed by decreased polymorphism barriers in prion protein conversion studies

Bovine spongiform encephalopathy (BSE) can be efficiently transmitted to small ruminants (sheep and goats) with certain prion protein (PrP) genotypes. Polymorphisms in PrP of both the host and donor influence the transmission efficiency of transmissible spongiform encephalopathies (TSEs) in general. These polymorphisms in PrP also modulate the PrP conversion underlying TSE agent replication. Here we demonstrate that single-round protein misfolding cyclic amplification (PMCA) can be used to assess species and polymorphism barriers at the molecular level. We assessed those within and between the ovine and bovine species in vitro using a variety of natural scrapie and experimentally generated cross-species BSE agents. These BSE agents include ovBSE-ARQ isolates (BSE derived from sheep having the ARQ/ARQ PrP genotype), and two unique BSE-derived variants: BSE passaged in VRQ/VRQ sheep and a cow BSE agent isolate generated by back-transmission of ovBSE-ARQ into its original host. PMCA allowed us to quantitatively determine PrP conversion profiles that correlated with known in vivo transmissibility and susceptibility in the two ruminant species in which strain-specific molecular signatures, like its molecular weight after protease digestion, were maintained. Furthermore, both BSE agent isolates from ARQ and VRQ sheep demonstrated a surprising transmission profile in which efficient transmissions to both sheep and bovine variants was combined. Finally, all data support the notion that ARQ-derived sheep BSE points to a significant increase in virulence compared to all other tested scrapie- and BSE-derived variants reflected by the increased conversion efficiencies of previously inefficient convertible PrP variants (including the so-called "resistant" sheep ARR variant).

IMPORTANCE

Prion diseases such as scrapie in sheep and goats, BSE in cattle, and Creutzfeldt-Jakob disease (CJD) in humans are fatal neurodegenerative diseases caused by prions. BSE is known to be transmissible to a variety of hosts, including sheep and humans. Based on the typical BSE agent strain signatures and epidemiological data, the occurrence of a novel variant of CJD in humans was linked to BSE occurrence in the United Kingdom. Measures, including genetic selection of sheep toward less susceptible PrP genotypes, have been implemented to lower the risk of BSE transmission into sheep, since the disease could potentially spread into a natural reservoir. In this study, we demonstrated using molecular PrP conversion studies that when BSE is first transmitted through sheep, the host range is modified significantly and the PrP converting potency increased, allowing the ovine BSE to transmit more efficiently than cow BSE into supposedly less susceptible hosts.

Epigenetic dominance of prion conformers

Although they share certain biological properties with nucleic acid based infectious agents, prions, the causative agents of invariably fatal, transmissible neurodegenerative disorders such as bovine spongiform encephalopathy, sheep scrapie, and human Creutzfeldt Jakob disease, propagate by conformational templating of host encoded proteins. Once thought to be unique to these diseases, this mechanism is now recognized as a ubiquitous means of information transfer in biological systems, including other protein misfolding disorders such as those causing Alzheimer's and Parkinson's diseases. To address the poorly understood mechanism by which host prion protein (PrP) primary structures interact with distinct prion conformations to influence pathogenesis, we produced transgenic (Tg) mice expressing different sheep scrapie susceptibility alleles, varying only at a single amino acid at PrP residue 136. Tg mice expressing ovine PrP with alanine (A) at (OvPrP-A136) infected with SSBP/1 scrapie prions propagated a relatively stable (S) prion conformation, which accumulated as punctate aggregates in the brain, and produced prolonged incubation times. In contrast, Tg mice expressing OvPrP with valine (V) at 136 (OvPrP-V136) infected with the same prions developed disease rapidly, and the converted prion was comprised of an unstable (U), diffusely distributed conformer. Infected Tg mice co-expressing both alleles manifested properties consistent with the U conformer, suggesting a dominant effect resulting from exclusive conversion of OvPrP-V136 but not OvPrP-A136. Surprisingly, however, studies with monoclonal antibody (mAb) PRC5, which discriminates OvPrP-A136 from OvPrP-V136, revealed substantial conversion of OvPrP-A136. Moreover, the resulting OvPrP-A136 prion acquired the characteristics of the U conformer. These results, substantiated by in vitro analyses, indicated that co-expression of OvPrP-V136 altered the conversion potential of OvPrP-A136 from the S to the otherwise unfavorable U conformer. This epigenetic mechanism thus expands the range of selectable conformations that can be adopted by PrP, and therefore the variety of options for strain propagation.

Prions are infectious proteins, originally discovered as the cause of a group of transmissible, fatal mammalian neurodegenerative diseases. Propagation results from conversion of the host-encoded cellular form of the prion protein to a self-propagating disease-associated conformation. It is believed that the self-propagating pathogenic form exists in a variety of subtly different conformations that encipher prion strain information. Here we explored the mechanism by which prion protein primary structural variants, differing at only a single amino acid residue, interact with prion strain conformations to control disease phenotype. We show that under conditions of co-expression, a susceptible prion protein variant influences the ability of an otherwise resistant variant to propagate an otherwise unfavorable prion strain. While this phenomenon is analogous to the expression of genetically-determined phenotypes, our results support a mechanism whereby dominant and recessive prion traits are epigenetically controlled by means of protein-mediated conformational templating.

Influence of polymorphisms in the prion protein gene on the pathogenesis and neuropathological phenotype of sheep scrapie after oral infection

The prion protein gene (Prnp) plays a crucial role in the susceptibility of sheep to scrapie in terms of attack rate and/or incubation period. However, the influence of Prnp on the pathogenesis of the disease, specifically the involvement of tissues of the lymphoreticular system (LRS), pathways of neuroinvasion and neuropathological phenotypes, remains controversial. This study reports the onset and progression of disease-associated prion protein (PrP(d)) accumulation in the LRS and nervous tissues of sheep of six different Prnp genotypes infected by oral administration of the same mixed scrapie brain homogenate. Sheep homozygous for glutamine (Q) at codon 171 of PrP, with either valine (V) or alanine (A) at codon 136 (i.e. VRQ/VRQ, VRQ/ARQ and ARQ/ARQ), showed early and consistent PrP(d) accumulation in LRS tissues of the pharynx and gut. In contrast, LRS involvement was minimal, inconsistent and occurred late in the incubation period in sheep heterozygous for arginine (R) at codon 171 (i.e. VRQ/ARR and ARQ/ARR). Despite this difference, all five groups were susceptible to infection and developed clinical disease, albeit with significantly different incubation periods (shortest in VRQ/VRQ and longest in ARQ/ARR sheep). The remaining group of ARR/ARR homozygous sheep did not show evidence of infection at the end of the experiment or at previous predetermined time points. As for LRS tissues, the sites of initial PrP(d) accumulation in the brain were determined immunohistochemically. These were the same in all susceptible sheep (except for ARR/ARR sheep), regardless of their Prnp genotype which, together with an early and consistent accumulation of PrP(d) in circumventricular organs and a late or inconsistent involvement of the enteric and autonomic nervous system and of the spinal cord, suggests neuroinvasion occurring via the blood. The neuropathological phenotype (PrP(d) profile in the central nervous system) of clinically affected sheep was similar in the three V136 carrier groups, but showed some differences in the two A136 homozygous groups, suggesting a codon 136-driven selection of different strains from the mixture contained in the inoculum. ARQ/ARR sheep showed an irregular distribution of brain PrP(d), contrasting with the more widespread distribution of the other four groups. The results indicate that (1) ARQ/ARR sheep are more susceptible to oral scrapie infection than would be predicted from incidence figures in natural disease, (2) amplification and accumulation of PrP(d) in LRS tissues is host genotype dependent, but does not necessarily have a marked effect on the outcome of the infection and (3) the neuropathological phenotype of scrapie is related to the host genotype, but possibly in combination with the infecting source.

Identical pathogenesis and neuropathological phenotype of scrapie in valine, arginine, glutamine/valine, arginine, glutamine sheep infected experimentally by the oral and conjunctival routes

The pathogenesis of scrapie in sheep after natural or oral exposure to the infectious agent generally involves the early accumulation of disease-associated prion protein (PrP(d)) in the lymphoreticular system (LRS). This phase is followed by neuroinvasion, for which two routes, ascending neural and haematogenous, have been postulated. The present study reports the use of immunohistochemistry to track the tissue progression of PrP(d) deposition in sheep of a single, highly scrapie-susceptible PrP genotype administered by the oral or conjunctival routes. Regardless of the route of infection, the earliest detection of PrP(d) was in gut- and pharynx-associated LRS tissues. Subsequently, the brain became PrP(d) positive simultaneously with other LRS tissues, but before the spinal cord and peripheral nervous tissues of the enteric, parasympathetic and sympathetic systems. The sites of initial PrP(d) accumulation in the brain were the dorsal motor nucleus of the vagus and the hypothalamus and their related circumventricular organs (the area postrema and the median eminence, respectively). These were the same for both routes of infection. Rapid progression to clinical disease was observed in sheep infected orally or conjunctivally, with definite signs of scrapie recorded at around 6 and 8 months after infection, respectively. Longer incubation periods in sheep infected by the conjunctival route were probably due to them receiving a lower dose than those infected orally. Irrespective of the route of infection, clinically affected sheep showed the same pathological phenotype (PrP(d) profile) and PrP(d) distribution throughout the brain. The identical peripheral and central pathogenesis observed in sheep of both groups suggests early dissemination of the infectious agent in the bloodstream and a common neuroinvasion pathway. The late involvement of the enteric and autonomic nervous system supports a haematogenous route of infection to the brain.

In vitro prion protein conversion suggests risk of bighorn sheep (Ovis canadensis) to transmissible spongiform encephalopathies

Background

Transmissible spongiform encephalopathies (TSEs) affect both domestic sheep (scrapie) and captive and free-ranging cervids (chronic wasting disease; CWD). The geographical range of bighorn sheep (Ovis canadensis; BHS) overlaps with states or provinces that have contained scrapie-positive sheep or goats and areas with present epizootics of CWD in cervids. No TSEs have been documented in BHS, but the susceptibility of this species to TSEs remains unknown.

Results

We acquired a library of BHS tissues and found no evidence of preexisting TSEs in these animals. The prion protein gene (Prnp) in all BHS in our library was identical to scrapie-susceptible domestic sheep (A¹³⁶R¹⁵⁴Q¹⁷¹ genotype). Using an in vitro prion protein conversion assay, which has been previously used to assess TSE species barriers and, in our study appears to recollect known species barriers in mice, we assessed the potential transmissibility of TSEs to BHS. As expected based upon Prnp genotype, we observed BHS prion protein conversion by classical scrapie agent and evidence for a species barrier between transmissible mink encephalopathy (TME) and BHS. Interestingly, our data suggest that the species barrier of BHS to white-tailed deer or wapiti CWD agents is likely low. We also used protein misfolding cyclic amplification to confirm that CWD, but not TME, can template prion protein misfolding in A¹³⁶R¹⁵⁴Q¹⁷¹ genotype sheep.

Conclusions

Our results indicate the in vitro conversion assay used in our study does mimic the species barrier of mice to the TSE agents that we tested. Based on Prnp genotype and results from conversion assays, BHS are likely to be susceptible to infection by classical scrapie. Despite mismatches in amino acids thought to modulate prion protein conversion, our data indicate that A¹³⁶R¹⁵⁴Q¹⁷¹ genotype sheep prion protein is misfolded by CWD agent, suggesting that these animals could be susceptible to CWD. Further investigation of TSE transmissibility to BHS, including animal studies, is warranted. The lack of reported TSEs in BHS may be attributable to other host factors or a lack of TSE surveillance in this species.

Prion protein gene variability in Spanish goats. Inference through susceptibility to classical scrapie strains and pathogenic distribution of peripheral PrP(sc.)

Classical scrapie is a neurological disorder of the central nervous system (CNS) characterized by the accumulation of an abnormal, partially protease resistant prion protein (PrP(sc)) in the CNS and in some peripheral tissues in domestic small ruminants. Whereas the pathological changes and genetic susceptibility of ovine scrapie are well known, caprine scrapie has been less well studied. We report here a pathological study of 13 scrapie-affected goats diagnosed in Spain during the last 9 years. We used immunohistochemical and biochemical techniques to discriminate between classical and atypical scrapie and bovine spongiform encephalopathy (BSE). All the animals displayed PrP(sc) distribution patterns and western blot characteristics compatible with classical scrapie. In addition, we determined the complete open reading frame sequence of the PRNP in these scrapie-affected animals. The polymorphisms observed were compared with those of the herd mates (n = 665) and with the frequencies of healthy herds (n = 581) of native Spanish goats (Retinta, Pirenaica and Moncaina) and other worldwide breeds reared in Spain (Saanen, Alpine and crossbreed). In total, sixteen polymorphic sites were identified, including the known amino acid substitutions at codons G37V, G127S, M137I, I142M, H143R, R151H, R154H, R211Q, Q222K, G232W, and P240S, and new polymorphisms at codons G74D, M112T, R139S, L141F and Q215R. In addition, the known 42, 138 and 179 silent mutations were detected, and one new one is reported at codon 122. The genetic differences observed in the population studied have been attributed to breed and most of the novel polymorphic codons show frequencies lower than 5%. This work provides the first basis of polymorphic distribution of PRNP in native and worldwide goat breeds reared in Spain.

Pathogenesis of scrapie in ARQ/ARQ sheep after subcutaneous infection: effect of lymphadenectomy and immune cell subset changes in relation to prion protein accumulation

It is well established that the infectious agent of scrapie can replicate in the lymphoreticular system (LRS). However, the effects of removal of LRS target tissues on the pathogenesis of the infection and the accumulation of disease-associated prion protein (PrP(d)) in LRS tissues on specific immune cell subsets are poorly understood aspects. To address these questions 16 ARQ/ARQ sheep were subcutaneously inoculated in the drainage area of the prefemoral lymph node with brain homogenate derived from Suffolk sheep naturally infected with scrapie. Fourteen sheep were then subjected to either early (14-17 days post-inoculation [dpi]) or late (175-201 dpi) lymphadenectomy and culled at preclinical or clinical stages of infection. Neither late nor even early lymphadenectomy prevented infection or had any effect on the accumulation of PrP(d) in the LRS or CNS suggesting a rapid organic dissemination of the infectious agent after inoculation. Lymph nodes from eight scrapie inoculated sheep selected on the basis of the amount of PrP(d) in their LRS tissues (negative, low or high) were examined for six different immune cell markers. The PrP(d) negative lymph nodes from two sheep with no evidence of scrapie infection showed lower numbers of cluster of determination (CD) 21 positive cells than PrP(d) positive nodes, irrespective of their location (hind leg or head). However, quantitative differences in the expression of this marker were not detected when comparing lymph nodes with low and high levels of PrP(d) accumulation, suggesting that proliferation of CD21 positive cells is related to scrapie infection, but not directly linked to the magnitude of PrP(d) accumulation. An additional observation of the study was that sheep that were methionin-threonine at codon 112 of the prion protein gene showed lower attack rates than methionine homozygotes (67% and 100%, respectively) and also generally lower levels of PrP(d) accumulation in the LRS and brain and increased survival times, suggesting an influence of such polymorphism in the susceptibility to scrapie.

The interpretation of disease phenotypes to identify TSE strains following murine bioassay: characterisation of classical scrapie

Mouse bioassay can be readily employed for strain typing of naturally occurring transmissible spongiform encephalopathy cases. Classical scrapie strains have been characterised historically based on the established methodology of assessing incubation period of disease and the distribution of disease-specific vacuolation across the brain following strain stabilisation in a given mouse line. More recent research has shown that additional methods could be used to characterise strains and thereby expand the definition of strain “phenotype”. Here we present the phenotypic characteristics of classical scrapie strains isolated from 24 UK ovine field cases through the wild-type mouse bioassay. PrP^Sc immunohistochemistry (IHC), paraffin embedded tissue blots (PET-blot) and Western blotting approaches were used to determine the neuroanatomical distribution and molecular profile of PrP^Sc associated with each strain, in conjunction with traditional methodologies. Results revealed three strains isolated through each mouse line, including a previously unidentified strain. Moreover IHC and PET-blot methodologies were effective in characterising the strain-associated types and neuroanatomical locations of PrP^Sc. The use of Western blotting as a parameter to define classical scrapie strains was limited. These data provide a comprehensive description of classical scrapie strain phenotypes on isolation through the mouse bioassay that can provide a reference for further scrapie strain identification.

Infectious titres of sheep scrapie and bovine spongiform encephalopathy agents cannot be accurately predicted from quantitative laboratory test results

It is widely accepted that abnormal forms of the prion protein (PrP) are the best surrogate marker for the infectious agent of prion diseases and, in practice, the detection of such disease-associated (PrP(d)) and/or protease-resistant (PrP(res)) forms of PrP is the cornerstone of diagnosis and surveillance of the transmissible spongiform encephalopathies (TSEs). Nevertheless, some studies question the consistent association between infectivity and abnormal PrP detection. To address this discrepancy, 11 brain samples of sheep affected with natural scrapie or experimental bovine spongiform encephalopathy were selected on the basis of the magnitude and predominant types of PrP(d) accumulation, as shown by immunohistochemical (IHC) examination; contra-lateral hemi-brain samples were inoculated at three different dilutions into transgenic mice overexpressing ovine PrP and were also subjected to quantitative analysis by three biochemical tests (BCTs). Six samples gave 'low' infectious titres (10⁶·⁵ to 10⁶·⁷ LD₅₀ g⁻¹) and five gave 'high titres' (10⁸·¹ to ≥ 10⁸·⁷ LD₅₀ g⁻¹) and, with the exception of the Western blot analysis, those two groups tended to correspond with samples with lower PrP(d)/PrP(res) results by IHC/BCTs. However, no statistical association could be confirmed due to high individual sample variability. It is concluded that although detection of abnormal forms of PrP by laboratory methods remains useful to confirm TSE infection, infectivity titres cannot be predicted from quantitative test results, at least for the TSE sources and host PRNP genotypes used in this study. Furthermore, the near inverse correlation between infectious titres and Western blot results (high protease pre-treatment) argues for a dissociation between infectivity and PrP(res).