Prions are secreted in milk from clinically normal scrapie-exposed sheep

Chronic wasting disease prions in mule deer interdigital glands

Chronic wasting disease (CWD) is a geographically expanding, fatal neurodegenerative disease in cervids. The disease can be transmitted directly (animal-animal) or indirectly via infectious prions shed into the environment. The precise mechanisms of indirect CWD transmission are unclear but known sources of the infectious prions that contaminate the environment include saliva, urine and feces. We have previously identified PrPC expression in deer interdigital glands, sac-like exocrine structures located between the digits of the hooves. In this study, we assayed for CWD prions within the interdigital glands of CWD infected deer to determine if they could serve as a source of prion shedding and potentially contribute to CWD transmission. Immunohistochemical analysis of interdigital glands from a CWD-infected female mule deer identified disease-associated PrPCWD within clusters of infiltrating leukocytes adjacent to sudoriferous and sebaceous glands, and within the acrosyringeal epidermis of a sudoriferous gland tubule. Proteinase K-resistant PrPCWD material was amplified by serial protein misfolding cyclic amplification (sPMCA) from soil retrieved from between the hoof digits of a clinically affected mule deer. Blinded testing of interdigital glands from 11 mule deer by real-time quake-induced conversion (RT-QuIC) accurately identified CWD-infected animals. The data described suggests that interdigital glands may play a role in the dissemination of CWD prions into the environment, warranting future investigation.

Combinatorial treatment of brain samples from sheep with scrapie using sodium percarbonate, sodium dodecyl sulfate, and proteinase K increases survival time in inoculated susceptible sheep

The agent of scrapie is resistant to most chemical and physical methods of inactivation. Prions bind to soils, metals, and various materials and persist in the environment confounding the control of prion diseases. Most methods of prion inactivation require severe conditions such as prolong exposure to sodium hypochlorite or autoclaving, which may not be suitable for field conditions. We evaluated the efficacy of a combinatorial approach to inactivation of US scrapie strain x124 under the mild conditions of treating scrapie-affected brain homogenate with sodium percarbonate (SPC), sodium dodecyl sulfate (SDS), or in combination followed by proteinase K (PK) digestion at room temperature. Western blot analysis of treated brain homogenate demonstrates partial reduction in PrP^Sc immunoreactivity. Genetically susceptible VRQ/ARQ Suffolk sheep were oronasally inoculated with 1 g of SPC (n = 1), SDS (n = 2), SDS + PK (n = 2), and SPC + SDS + PK (n = 4) treated brain homogenate. Sheep were assessed daily for clinical signs, euthanized at the development of clinical disease, and tissues were assessed for accumulation of PrP^Sc. Scrapie status in all sheep was determined by western blot, enzyme immunoassay, and immunohistochemistry. Mean incubation periods (IPs) for SPC (11.9 months, 0% survival), SDS (12.6 months, 0% survival), SDS + PK (14.0 months, 0% survival), and SPC + SDS + PK (12.5 months, 25% survival) were increased compared to positive control sheep (n = 2, 10.7 months, 0% survival) by 1.2, 1.9, 3.3, and 1.8 months, respectively. Treatment did not influence PrP^Sc accumulation and distribution at the clinical stage of disease. Differences in mean IPs and survival indicates partial but not complete reduction in scrapie infectivity.

Extracellular vesicles with diagnostic and therapeutic potential for prion diseases

Prion diseases (PrD) or transmissible spongiform encephalopathies (TSE) are invariably fatal and pathogenic neurodegenerative disorders caused by the self-propagated misfolding of cellular prion protein (PrP^C) to the neurotoxic pathogenic form (PrP^TSE) via a yet undefined but profoundly complex mechanism. Despite several decades of research on PrD, the basic understanding of where and how PrP^C is transformed to the misfolded, aggregation-prone and pathogenic PrP^TSE remains elusive. The primary clinical hallmarks of PrD include vacuolation-associated spongiform changes and PrP^TSE accumulation in neural tissue together with astrogliosis. The difficulty in unravelling the disease mechanisms has been related to the rare occurrence and long incubation period (over decades) followed by a very short clinical phase (few months). Additional challenge in unravelling the disease is implicated to the unique nature of the agent, its complexity and strain diversity, resulting in the heterogeneity of the clinical manifestations and potentially diverse disease mechanisms. Recent advances in tissue isolation and processing techniques have identified novel means of intercellular communication through extracellular vesicles (EVs) that contribute to PrP^TSE transmission in PrD. This review will comprehensively discuss PrP^TSE transmission and neurotoxicity, focusing on the role of EVs in disease progression, biomarker discovery and potential therapeutic agents for the treatment of PrD.

Detection of Chronic Wasting Disease Prions in Fetal Tissues of Free-Ranging White-Tailed Deer

The transmission of chronic wasting disease (CWD) has largely been attributed to contact with infectious prions shed in excretions (saliva, urine, feces, blood) by direct animal-to-animal exposure or indirect contact with the environment. Less-well studied has been the role that mother-to-offspring transmission may play in the facile transmission of CWD, and whether mother-to-offspring transmission before birth may contribute to the extensive spread of CWD. We thereby focused on a population of free-ranging white-tailed deer from West Virginia, USA, in which CWD has been detected. Fetal tissues, ranging from 113 to 158 days of gestation, were harvested from the uteri of CWD+ dams in the asymptomatic phase of infection. Using serial protein misfolding amplification (sPMCA), we detected evidence of prion seeds in 7 of 14 fetuses (50%) from 7 of 9 pregnancies (78%), with the earliest detection at 113 gestational days. This is the first report of CWD detection in free ranging white-tailed deer fetal tissues. Further investigation within cervid populations across North America will help define the role and impact of mother-to-offspring vertical transmission of CWD.

Animal prion diseases: A review of intraspecies transmission

Animal prion diseases are a group of neurodegenerative, transmissible, and fatal disorders that affect several animal species. The causative agent, prion, is a misfolded isoform of normal cellular prion protein, which is found in cells with higher concentration in the central nervous system. This review explored the sources of infection and different natural transmission routes of animal prion diseases in susceptible populations. Chronic wasting disease in cervids and scrapie in small ruminants are prion diseases capable of maintaining themselves in susceptible populations through horizontal and vertical transmission. The other prion animal diseases can only be transmitted through food contaminated with prions. Bovine spongiform encephalopathy (BSE) is the only animal prion disease considered zoonotic. However, due to its inability to transmit within a population, it could be controlled. The emergence of atypical cases of scrapie and BSE, even the recent report of prion disease in camels, demonstrates the importance of understanding the transmission routes of prion diseases to take measures to control them and to assess the risks to human and animal health.

BSE can propagate in sheep co-infected or pre-infected with scrapie

To understand the possible role of mixed-prion infections in disease presentation, the current study reports the co-infection of sheep with bovine spongiform encephalopathy (BSE) and scrapie. The bovine BSE agent was inoculated subcutaneously into sheep with ARQ/ARQ or VRQ/ARQ PRNP genotypes either at the same time as subcutaneous challenge with scrapie, or three months later. In addition, VRQ/VRQ sheep naturally infected with scrapie after being born into a scrapie-affected flock were challenged subcutaneously with BSE at eight or twenty one months-of-age. Sheep were analysed by incubation period/attack rate, and western blot of brain tissue determined the presence of BSE or scrapie-like PrP^Sc. Serial protein misfolding cyclic amplification (sPMCA) that can detect very low levels of BSE in the presence of an excess of scrapie agent was also applied to brain and lymphoreticular tissue. For VRQ/ARQ sheep challenged with mixed infections, scrapie-like incubation periods were produced, and no BSE agent was detected. However, whilst ARQ/ARQ sheep developed disease with BSE-like incubation periods, some animals had a dominant scrapie western blot phenotype in brain, but BSE was detected in these sheep by sPMCA. In addition, VRQ/VRQ animals challenged with BSE after natural exposure to scrapie had scrapie-like incubation periods and dominant scrapie PrP^Sc in brain, but one sheep had BSE detectable by sPMCA in the brain. Overall, the study demonstrates for the first time that for scrapie/BSE mixed infections, VRQ/ARQ sheep with experimental scrapie did not propagate BSE but VRQ/VRQ sheep with natural scrapie could propagate low levels of BSE, and whilst BSE readily propagated in ARQ/ARQ sheep it was not always the dominant PrP^Sc strain in brain tissue. Indeed, for several animals, a dominant scrapie biochemical phenotype in brain did not preclude the presence of BSE prion.

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

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

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.

Biosafety of Prions

Prions are the infectious agents that cause devastating and untreatable disorders known as Transmissible Spongiform Encephalopathies (TSEs). The pathologic events and the infectious nature of these transmissible agents are not completely understood yet. Due to the difficulties in inactivating prions, working with them requires specific recommendations and precautions. Moreover, with the advent of innovative technologies, such as the Protein Misfolding Cyclic Amplification (PMCA) and the Real Time Quaking-Induced Conversion (RT-QuIC), prions could be amplified in vitro and the infectious features of the amplified products need to be carefully assessed.

Prenatal transmission of scrapie in sheep and goats: A case study for veterinary public health

Unsettled knowledge as to whether scrapie transmits prenatally in sheep and goats and transmits by semen and preimplantation embryos has a potential to compromise measures for controlling, preventing and eliminating the disease. The remedy may be analysis according to a systematic review, allowing comprehensive and accessible treatment of evidence and reasoning, clarifying the issue and specifying the uncertainties. Systematic reviews have clearly formulated questions, can identify relevant studies and appraise their quality and can summarise evidence and reasoning with an explicit methodology. The present venture lays a foundation for a possible systematic review and applies three lines of evidence and reasoning to two questions. The first question is whether scrapie transmits prenatally in sheep and goats. It leads to the second question, which concerns the sanitary safety of artificial breeding technologies, and is whether scrapie transmits in sheep and goats by means of semen and washed or unwashed in vivo derived embryos. The three lines of evidence derive from epidemiological, field and clinical studies, experimentation, and causal reasoning, where inferences are made from the body of scientific knowledge and an understanding of animal structure and function. Evidence from epidemiological studies allow a conclusion that scrapie transmits prenatally and that semen and embryos are presumptive hazards for the transmission of scrapie. Evidence from experimentation confirms that semen and washed or unwashed in vivo derived embryos are hazards for the transmission of scrapie. Evidence from causal reasoning, including experience from other prion diseases, shows that mechanisms exist for prenatal transmission and transmission by semen and embryos in both sheep and goats.

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.

Prions are affected by evolution at two levels

Prions, infectious proteins, can transmit diseases or be the basis of heritable traits (or both), mostly based on amyloid forms of the prion protein. A single protein sequence can be the basis for many prion strains/variants, with different biological properties based on different amyloid conformations, each rather stably propagating. Prions are unique in that evolution and selection work at both the level of the chromosomal gene encoding the protein, and on the prion itself selecting prion variants. Here, we summarize what is known about the evolution of prion proteins, both the genes and the prions themselves. We contrast the one known functional prion, [Het-s] of Podospora anserina, with the known disease prions, the yeast prions [PSI+] and [URE3] and the transmissible spongiform encephalopathies of mammals.

Ultrasensitive detection of PrP(Sc) in the cerebrospinal fluid and blood of macaques infected with bovine spongiform encephalopathy prion

Prion diseases are characterized by the prominent accumulation of the misfolded form of a normal cellular protein (PrP(Sc)) in the central nervous system. The pathological features and biochemical properties of PrP(Sc) in macaque monkeys infected with the bovine spongiform encephalopathy (BSE) prion have been found to be similar to those of human subjects with variant Creutzfeldt-Jakob disease (vCJD). Non-human primate models are thus ideally suited for performing valid diagnostic tests and determining the efficacy of potential therapeutic agents. In the current study, we developed a highly efficient method for in vitro amplification of cynomolgus macaque BSE PrP(Sc). This method involves amplifying PrP(Sc) by protein misfolding cyclic amplification (PMCA) using mouse brain homogenate as a PrP(C) substrate in the presence of sulfated dextran compounds. This method is capable of amplifying very small amounts of PrP(Sc) contained in the cerebrospinal fluid (CSF) and white blood cells (WBCs), as well as in the peripheral tissues of macaques that have been intracerebrally inoculated with the BSE prion. After clinical signs of the disease appeared in three macaques, we detected PrP(Sc) in the CSF by serial PMCA, and the CSF levels of PrP(Sc) tended to increase with disease progression. In addition, PrP(Sc) was detectable in WBCs at the clinical phases of the disease in two of the three macaques. Thus, our highly sensitive, novel method may be useful for furthering the understanding of the tissue distribution of PrP(Sc) in non-human primate models of CJD.

Evidence in sheep for pre-natal transmission of scrapie to lambs from infected mothers

Natural scrapie transmission from infected ewes to their lambs is thought to occur by the oral route around the time of birth. However the hypothesis that scrapie transmission can also occur before birth (in utero) is not currently favoured by most researchers. As scrapie is an opportunistic infection with multiple infection routes likely to be functional in sheep, definitive evidence for or against transmission from ewe to her developing fetus has been difficult to achieve. In addition the very early literature on maternal transmission of scrapie in sheep was compromised by lack of knowledge of the role of the PRNP (prion protein) gene in control of susceptibility to scrapie. In this study we experimentally infected pregnant ewes of known PRNP genotype with a distinctive scrapie strain (SSBP/1) and looked for evidence of transmission of SSBP/1 to the offspring. The sheep were from the NPU Cheviot flock, which has endemic natural scrapie from which SSBP/1 can be differentiated on the basis of histology, genetics of disease incidence and strain typing bioassay in mice. We used embryo transfer techniques to allow sheep fetuses of scrapie-susceptible PRNP genotypes to develop in a range of scrapie-resistant and susceptible recipient mothers and challenged the recipients with SSBP/1. Scrapie clinical disease, caused by both natural scrapie and SSBP/1, occurred in the progeny but evidence (including mouse strain typing) of SSBP/1 infection was found only in lambs born to fully susceptible recipient mothers. Progeny were not protected from transmission of natural scrapie or SSBP/1 by washing of embryos to International Embryo Transfer Society standards or by caesarean derivation and complete separation from their birth mothers. Our results strongly suggest that pre-natal (in utero) transmission of scrapie may have occurred in these sheep.

Small-molecule theranostic probes: a promising future in neurodegenerative diseases

Prion diseases are fatal neurodegenerative illnesses, which include Creutzfeldt-Jakob disease in humans and scrapie, chronic wasting disease, and bovine spongiform encephalopathy in animals. They are caused by unconventional infectious agents consisting primarily of misfolded, aggregated, β -sheet-rich isoforms, denoted prions, of the physiological cellular prion protein (PrP(C)). Many lines of evidence suggest that prions (PrP(Sc)) act both as a template for this conversion and as a neurotoxic agent causing neuronal dysfunction and cell death. As such, PrP(Sc) may be considered as both a neuropathological hallmark of the disease and a therapeutic target. Several diagnostic imaging probes have been developed to monitor cerebral amyloid lesions in patients with neurodegenerative disorders (such as Alzheimer's disease, Parkinson's disease, and prion disease). Examples of these probes are Congo red, thioflavin T, and their derivatives. We synthesized a series of styryl derivatives, denoted theranostics, and studied their therapeutic and/or diagnostic potentials. Here we review the salient traits of these small molecules that are able to detect and modulate aggregated forms of several proteins involved in protein misfolding diseases. We then highlight the importance of further studies for their practical implications in therapy and diagnostics.

Evidence of effective scrapie transmission via colostrum and milk in sheep

Background

Evidence for scrapie transmission from VRQ/VRQ ewes to lambs via milk was first reported in 2008 but in that study there were concerns that lateral transmission may have contributed to the high transmission rate observed since five control lambs housed with the milk recipients also became infected. This report provides further information obtained from two follow-up studies, one where milk recipients were housed separately after milk consumption to confirm the validity of the high scrapie transmission rate via milk and the second to assess any difference in infectivity from colostrum and subsequent milk. Protein misfolding cyclic amplification (PMCA) was also used to detect prion protein in milk samples as a comparison with the infectivity data and extended to milk samples from ewes without a VRQ allele.

Results

Seven pairs of lambs fed colostrum and milk individually from seven scrapie-affected sheep (pre-clinical or clinical) presented with disease-associated prion protein, PrP^d, in rectal lymphoid tissue at 4–5 months of age. Five further pairs of lambs fed either colostrum or subsequent milk from five pre-clinical scrapie-affected sheep equally presented with PrP^d in lymphoid tissue by 9 months of age. Nine sheep were lost due to intercurrent diseases but all remaining milk or colostrum recipients, including those in the original study with the lateral transmission controls, developed clinical signs of scrapie from 19 months of age and scrapie was confirmed by brain examination. Unexposed control sheep totalling 19 across all three studies showed no evidence of infection.

Scrapie PrP was amplified repeatedly by PMCA in all tested milk samples from scrapie-affected VRQ/VRQ sheep, and in one scrapie-affected ARQ/ARQ sheep. By contrast, milk samples from five VRQ/VRQ and 11 ARQ/ARQ scrapie-free sheep did not have detectable scrapie PrP on repeated tests.

Conclusions

Feeding of milk from scrapie-affected sheep results in a high transmission rate in VRQ/VRQ sheep and both colostrum and milk transmit scrapie. Detection of scrapie prion protein in individual milk samples from scrapie-affected ewes confirms PMCA as a valuable in vitro test.

Differentiating ovine BSE from CH1641 scrapie by serial protein misfolding cyclic amplification

Whilst ovine BSE displays distinct pathological characteristics to ovine CH1641-like scrapie upon passage in rodents, they have very similar molecular phenotypes. As such, the in vitro differentiation of these strains in routine surveillance programmes presents a significant diagnostic challenge. In this study, using serial protein-misfolding cyclic amplification (sPMCA), ovine BSE was readily amplified in vitro in brain substrates from sheep with V₁₃₆R₁₅₄Q₁₇₁/V₁₃₆R₁₅₄Q₁₇₁ or AHQ/AHQ PRNP genotypes. In contrast, the CH1641 strain was refractory to such amplification. This method allowed for complete and unequivocal differentiation of experimental BSE from CH1641 prion strains within an ovine host.

Ultrasensitive detection of scrapie prion protein derived from ARQ and AHQ homozygote sheep by interspecies in vitro amplification

Prions, infectious agents causing TSEs, are composed primarily of the pathogenic form (PrP(Sc)) of the PrP(C). The susceptibility of sheep to scrapie is determined by polymorphisms in the coding region of the PRNP, mainly at codons 136, 154, and 171. The efficiency of in vitro amplification of sheep PrP(Sc) seems to be linked also to the PrP genotype. PrP(Sc) derived from sheep with V(136)R(154)Q(171)-associated genotypes can be amplified efficiently by PMCA in the presence of additional polyanion such as poly A, but there are no reports that cite ultrasensitive detection of PrP(Sc) derived from sheep of other PrP genotypes. We report here that sheep PrP(Sc) derived from ARQ and AHQ homozygotes was amplified efficiently by serial PMCA using mouse brain homogenate as PrP(C) substrate. ARQ/ARQ PrP(Sc) was detected in infected brain homogenates diluted up to 10(-10) after five rounds of amplification, and AHQ/AHQ PrP(Sc) was detected in samples diluted up to 10(-8) after four rounds of amplification. On the other hand, amplification of PrP(Sc) from VRQ/ARQ sheep seemed to be less efficient under the experimental conditions used. The interspecies PMCA developed in this study may be useful in the detailed analysis of PrP(Sc) distribution in classical scrapie-infected ARQ and AHQ homozygote sheep.

In vitro amplification of ovine prions from scrapie-infected sheep from Great Britain reveals distinct patterns of propagation

Background

Protein misfolding cyclic amplification (PMCA) is a method that facilitates the detection of prions from many sources of transmissible spongiform encephalopathy (TSE). Sheep scrapie represents a unique diversity of prion disease agents in a range of susceptible PRNP genotypes. In this study PMCA was assessed on a range of Great Britain (GB) sheep scrapie isolates to determine the applicability to veterinary diagnosis of ovine TSE.

Results

PrP^Sc amplification by protein misfolding cyclic amplification (PMCA) was assessed as a diagnostic tool for field cases of scrapie. The technique was initially applied to thirty-seven isolates of scrapie from diverse geographical locations around GB, and involved sheep of various breeds and PRNP genotypes. All samples were amplified in either VRQ and/or ARQ PrP^C substrate. For PrP^Sc from sheep with at least one VRQ allele, all samples amplified efficiently in VRQ PrP^C but only PrP^Sc from ARH/VRQ sheep amplified in both substrates. PrP^Sc from ARQ/ARQ sheep displayed two amplification patterns, one that amplified in both substrates and one that only amplified in ARQ PrP^C. These amplification patterns were consistent for a further 14/15 flock/farm mates of these sheep. Furthermore experimental scrapie strains SSBP1, Dawson, CH1641 and MRI were analysed. SSBP1 and Dawson (from VRQ/VRQ sheep) amplified in VRQ but not ARQ substrate. MRI scrapie (from ARQ/ARQ sheep) nor CH1641 did not amplify in ARQ or VRQ substrate; these strains required an enhanced PMCA method incorporating polyadenylic acid (poly(A)) to achieve amplification.

Conclusions

PrP^sc from 52 classical scrapie GB field isolates amplified in VRQ or ARQ or both substrates and supports the use of PMCA as a rapid assay for the detection of a wide range of ovine classical scrapie infections involving multiple PRNP genotypes and scrapie strains.

Rapid transepithelial transport of prions following inhalation

Prion infection and pathogenesis are dependent on the agent crossing an epithelial barrier to gain access to the recipient nervous system. Several routes of infection have been identified, but the mechanism(s) and timing of in vivo prion transport across an epithelium have not been determined. The hamster model of nasal cavity infection was used to determine the temporal and spatial parameters of prion-infected brain homogenate uptake following inhalation and to test the hypothesis that prions cross the nasal mucosa via M cells. A small drop of infected or uninfected brain homogenate was placed below each nostril, where it was immediately inhaled into the nasal cavity. Regularly spaced tissue sections through the entire extent of the nasal cavity were processed immunohistochemically to identify brain homogenate and the disease-associated isoform of the prion protein (PrP(d)). Infected or uninfected brain homogenate was identified adhering to M cells, passing between cells of the nasal mucosa, and within lymphatic vessels of the nasal cavity at all time points examined. PrP(d) was identified within a limited number of M cells 15 to 180 min following inoculation, but not in the adjacent nasal mucosa-associated lymphoid tissue (NALT). While these results support M cell transport of prions, larger amounts of infected brain homogenate were transported paracellularly across the respiratory, olfactory, and follicle-associated epithelia of the nasal cavity. These results indicate that prions can immediately cross the nasal mucosa via multiple routes and quickly enter lymphatics, where they can spread systemically via lymph draining the nasal cavity.

Salivary prions in sheep and deer

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

Temperature influences the interaction of ruminant PrP (TSE) with soil

Ovine scrapie and cervid chronic wasting disease can be transmitted in the absence of animal-to-animal contact, and environmental reservoirs of infectivity have been implicated in their spread and persistence. Investigating environmental factors that influence the interaction of disease-associated PrP with soils is imperative to understanding what is likely to be the complex role of soil in disease transmission. Here, we describe the effects of soil temperature on the binding/desorption and persistence of both ovine scrapie- and bovine BSE-PrP (TSE) . Binding of PrP (TSE) to a sandy loam soil at temperatures of 4°C, 8-12°C and 25-30°C demonstrated that an increase in temperature resulted in (1) a decrease in the amount of PrP (TSE) recovered after 24 h of interaction with soil, (2) an increase in the amount of N-terminal cleavage of the prion protein over 11 d and (3) a decrease in the persistence of PrP (TSE) on soil over an 18 mo period.

Differential immunoreactivity of goat derived scrapie following in vitro misfolding versus mouse bioassay

The protein misfolding cyclic amplification (PMCA) assay allows for detection of prion protein misfolding activity in tissues and fluids from sheep with scrapie where it was previously undetected by conventional western blot and immunohistochemistry assays. Studies of goats with scrapie have yet to take advantage of PMCA, which could aid in discerning the risk of transmission between goats and goats to sheep. The aim of the current study was to adapt PMCA for evaluation of scrapie derived from goats. Diluted brain homogenate from scrapie-infected goats (i.e., the scrapie seed, PrP(Sc)) was subjected to PMCA using normal brain homogenate from ovinized transgenic mice (tg338) as the source of normal cellular prion protein (the substrate, PrP(C)). The assay end-point was detection of the proteinase K-resistant misfolded prion protein core (PrP(res)) by western blot. Protein misfolding activity was consistently observed in caprine brain homogenate diluted 10,000-fold after 5 PMCA rounds. Epitope mapping by western blot analyses demonstrated that PrP(res) post-PMCA was readily detected with an N-terminus anti-PrP monoclonal antibody (P4), similar to scrapie inoculum from goats. This was in contrast to limited detection of PrP(res) with P4 following mouse bioassay. The inverse was observed with a monoclonal antibody to the C-terminus (F99/97.6.1). Thus, brain homogenate prepared from uninoculated tg338 served as an appropriate substrate for serial PMCA of PrP(Sc) derived from goats. These observations suggest that concurrent PMCA and bioassay with tg338 could improve characterization of goat derived scrapie.

New generation QuIC assays for prion seeding activity

The ability of abnormal TSE-associated forms of PrP to seed the formation of amyloid fibrils from recombinant PrP(Sen) has served as the basis for several relatively rapid and highly sensitive tests for prion diseases. These tests include rPrP-PMCA (rPMCA), standard quaking-induced conversion (S-QuIC), amyloid seeding assay (ASA), real-time QuIC (RT-QuIC) and enhanced QuIC (eQuIC). Here, we summarize recent improvements in the RT-QuIC-based assays that enhance the practicality, sensitivity and quantitative attributes of assays QuIC and promote the detection of prion seeding activity in dilute, inhibitor-laden fluids such as blood plasma.

Accelerated shedding of prions following damage to the olfactory epithelium

In this study, we investigated the role of damage to the nasal mucosa in the shedding of prions into nasal samples as a pathway for prion transmission. Here, we demonstrate that prions can replicate to high levels in the olfactory sensory epithelium (OSE) in hamsters and that induction of apoptosis in olfactory receptor neurons (ORNs) in the OSE resulted in sloughing off of the OSE from nasal turbinates into the lumen of the nasal airway. In the absence of nasotoxic treatment, olfactory marker protein (OMP), which is specific for ORNs, was not detected in nasal lavage samples. However, after nasotoxic treatment that leads to apoptosis of ORNs, both OMP and prion proteins were present in nasal lavage samples. The cellular debris that was released from the OSE into the lumen of the nasal airway was positive for both OMP and the disease-specific isoform of the prion protein, PrP(Sc). By using the real-time quaking-induced conversion assay to quantify prions, a 100- to 1,000-fold increase in prion seeding activity was observed in nasal lavage samples following nasotoxic treatment. Since neurons replicate prions to higher levels than other cell types and ORNs are the most environmentally exposed neurons, we propose that an increase in ORN apoptosis or damage to the nasal mucosa in a host with a preexisting prion infection of the OSE could lead to a substantial increase in the release of prion infectivity into nasal samples. This mechanism of prion shedding from the olfactory mucosa could contribute to prion transmission.

The oral secretion of infectious scrapie prions occurs in preclinical sheep with a range of PRNP genotypes

Preclinical sheep with the highly scrapie-susceptible VRQ/VRQ PRNP genotype secrete prions from the oral cavity. In order to further understand the significance of orally available prions, buccal swabs were taken from sheep with a range of PRNP genotypes and analyzed by serial protein misfolding cyclic amplification (sPMCA). Prions were detected in buccal swabs from scrapie-exposed sheep of genotypes linked to high (VRQ/VRQ and ARQ/VRQ) and low (ARR/VRQ and AHQ/VRQ) lymphoreticular system involvement in scrapie pathogenesis. For both groups, the level of prion detection was significantly higher than that for scrapie-resistant ARR/ARR sheep which were kept in the same farm environment and acted as sentinel controls for prions derived from the environment which might contaminate the oral cavity. In addition, sheep with no exposure to the scrapie agent did not contain any measurable prions within the oral cavity. Furthermore, prions were detected in sheep over a wide age range representing various stages of preclinical disease. These data demonstrate that orally available scrapie prions may be a common feature in sheep incubating scrapie, regardless of the PRNP genotype and any associated high-level accumulation of PrP(Sc) within lymphoreticular tissues. PrP(Sc) was present in buccal swabs from a large proportion of sheep with PRNP genotypes associated with relatively low disease penetrance, indicating that subclinical scrapie infection is likely to be a common occurrence. The significance of positive sPMCA reactions was confirmed by the transmission of infectivity in buccal swab extracts to Tg338 mice, illustrating the likely importance of orally available prions in the horizontal transmission of scrapie.

Detection of prions in the faeces of sheep naturally infected with classical scrapie

Classical scrapie is a naturally transmitted prion disease of sheep and goats. Contaminated environments may contribute to the spread of disease and evidence from animal models has implicated urine, blood, saliva, placenta and faeces as possible sources of the infection. Here we sought to determine whether sheep naturally infected with classical scrapie shed prions in their faeces. We used serial protein misfolding cyclic amplification (sPMCA) along with two extraction methods to examine faeces from sheep during both the clinical and preclinical phases of the disease and showed amplification of PrP^Sc in 7 of 15 and 14 of 14 sheep respectively. However PrP^Sc was not amplified from the faeces of 25 sheep not exposed to scrapie. These data represent the first demonstration of prion shedding in faeces from a naturally infected host and thus a likely source of prion contamination in the environment.

Extraneural manifestations of prion infection in GPI-anchorless transgenic mice

Earlier studies indicated that transgenic (tg) mice engineered to express prion protein (PrP) lacking the glycophosphatidylinositol (GPI⁻/⁻) membrane anchor formed abnormal proteinase-resistant prion (PrPsc) amyloid deposits in their brains and hearts when infected with the RML strain of murine scrapie. In contrast, RML scrapie infection of normal mice with a GPI-anchored PrP did not deposit amyloid with PrPsc in the brain or the heart. Here we report that scrapie-infected GPI⁻/⁻ PrP tg mice also deposit PrP and transmissible infectious material in the gut, kidneys, and islets of Langerhans. Similar to previously reported amyloid deposits in the brain and heart, amyloid deposits were found in the gut; however, no amyloid deposited in the islets. By high-resolution electron microscopy, we show PrP is located primarily in α cells and also β cells. Islets contain abundant insulin and there is no abnormality in glucose metabolism in infected GPI⁻/⁻ PrP tg mice.

The interaction of ruminant PrP(Sc) with soils is influenced by prion source and soil type

The persistence of prions within the environment is implicated in the horizontal transmission of ovine scrapie and cervid chronic wasting disease. Description of the interaction of prion strains derived from their natural hosts with a range of soil types is imperative in understanding how prions persist in the environment and, therefore, the characteristics of prion transmission. Here, we demonstrate that all detectable ovine scrapie and bovine BSE PrP(Sc) bind to a range of soil types within 24 h. This highly efficient binding of prions to soils is characterized by truncation of desorbed PrP(Sc) in a soil-dependent manner, with clay-rich soils resulting in N-terminal truncation of the PrP(Sc) and sand-rich soils yielding full length PrP(Sc) species. PrP(Sc) did not migrate through soil columns during incubation for up to 18 months, and for all combinations of soil and prion types, a decrease in recoverable PrP(Sc) was seen over time. Persistence of PrP(Sc) within soil and their interaction with soil particles of distinct sizes was dictated by both the soil type and the source of the prion, with ovine scrapie being apparently more persistent in some soils than cattle BSE. These data indicate that natural ruminant prion strains are stable in the soil environment for at least 18 months and that PrP(Sc)-soil interaction is dictated by both the soil properties and the strain/host species of PrP(Sc).

Prion transmission: prion excretion and occurrence in the environment

Prion diseases range from being highly infectious, for example scrapie and CWD, which show facile transmission between susceptible individuals, to showing negligible horizontal transmission, such as BSE and CJD, which are spread via food or iatrogenically, respectively. Scrapie and CWD display considerable in vivo dissemination, with PrP(Sc) and infectivity being found in a range of peripheral tissues. This in vivo dissemination appears to facilitate the recently reported excretion of prion through multiple routes such as from skin, feces, urine, milk, nasal secretions, saliva and placenta. Furthermore, excreted scrapie and CWD agent is detected within environmental samples such as water and on the surfaces of inanimate objects. The cycle of "uptake of prion from the environment--widespread in vivo prion dissemination--prion excretion--prion persistence in the environment" is likely to explain the facile transmission and maintenance of these diseases within wild and farmed populations over many years.

Environmental sources of scrapie prions

Ovine scrapie and cervine chronic wasting disease show considerable horizontal transmission. Here we report that a scrapie-affected sheep farm has a widespread environmental contamination with prions. Prions were amplified by protein-misfolding cyclic amplification (sPMCA) from seven of nine environmental swab samples taken, including those from metal, plastic, and wooden surfaces. Sheep had been removed from the areas from which the swabs were taken up to 20 days prior to sampling, indicating that prions persist for at least that long. These data implicate inanimate objects as environmental reservoirs for prion infectivity that are likely to contribute to facile disease transmission.

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

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

In vitro amplification of prions from milk in the detection of subclinical infections

Prions can be amplified by serial protein misfolding cyclic amplification (sPMCA) from the milk of a high proportion of apparently healthy, scrapie exposed sheep with PRNP genotypes not previously associated with high disease penetrance. These data strongly suggest the widespread presence of subclinical scrapie infections within scrapie-exposed flocks containing sheep with a range of susceptible PRNP genotypes. These data also lead to the hypothesis that similar subclinical disease states may be common for other animal and human prion diseases. Furthermore, the application of sPMCA to milk provides a method to detect such subclinical disease. Here, we describe the high level amplification of bovine spongiform encephalopathy (BSE) prions from both ovine and bovine origin, a methodology that will facilitate the detection of any prions secreted within bovine and ovine milk during subclinical and clinical BSE disease.