Variable patterns of distribution of PrP(CWD) in the obex and cranial lymphoid tissues of Rocky Mountain elk (Cervus elaphus nelsoni) with subclinical chronic wasting disease

Comparative study of immunoassays, a microelectromechanical systems-based biosensor, and RT-QuIC for the diagnosis of chronic wasting disease in white-tailed deer

BACKGROUND

Chronic wasting disease (CWD) is a fatal transmissible spongiform encephalopathy in cervids. The disease is caused by a pathogenic prion, namely PrPSc. Currently, diagnosis of CWD relies on IHC detection of PrPSc in the obex or retropharyngeal lymph nodes (RPLN) or ELISA screening of obex and RPLN followed by IHC confirmation of positive results. In this study, we assessed the performance characteristics of two immunoassays: CWD Ag-ELISA and TeSeE ELISA, RT-QuIC, and MEMS biosensor via testing 30 CWD + and 30 CWD- white-tailed deer RPLN samples.

RESULTS

Both CWD Ag-ELISA and TeSeE ELISA correctly identified all CWD + and CWD- samples. A greater intra-assay coefficient of variation (CV) in S/P ratios was observed for the TeSeE ELISA (16.52%), compared to CWD Ag-ELISA (9.49%). However, the high CV did not affect the qualitative results of triplicate assays when the corresponding manufacturer's cutoff was used. The MEMS biosensor not only correctly identified all CWD + and CWD- RPLN samples, but also demonstrated a 100% detection rate for all CWD + samples at dilutions from 10- 0 to 10- 3. Evaluation of RT-QuIC indicated that the rate of false negative reactions decreased from 21.98% at 10- 2 dilution to 0% at 10- 4 and 10- 5 dilutions; and the rate of false positive reactions reduced from 56.42% at 10- 2 dilution to 8.89% and 2.22% at 10- 4 and 10- 5 dilutions, respectively. Based on a stringent threshold of 2 x the first 10 fluorescent readings of each well and a final cutoff of 2/3 positive reactions for each sample, RT-QuIC correctly identified all positive and negative samples at 10- 4 and 10- 5 dilutions. Both MEMS biosensor and RT-QuIC achieved 100% sensitivity and 100% specificity under the experimental conditions described in this study.

CONCLUSIONS

The two immunoassays (CWD Ag-ELISA and TeSeE ELISA) performed comparably on white-tailed deer RPLN samples. MEMS biosensor is a reliable portable tool for CWD diagnosis and RT-QuIC can be used for routine testing of CWD if appropriate testing parameters and interpretive criteria are applied.

Inter-laboratory comparison of real-time quaking-induced conversion (RT-QuIC) for the detection of chronic wasting disease prions in white-tailed deer retropharyngeal lymph nodes

The rapid geographic spread of chronic wasting disease (CWD) in white-tailed deer (WTD; Odocoileus virginianus) increases the need for the development and validation of new detection tests. Real-time quaking-induced conversion (RT-QuIC) has emerged as a sensitive tool for CWD prion detection, but federal approval in the United States has been challenged by practical constraints on validation and uncertainty surrounding RT-QuIC robustness between laboratories. To evaluate the effect of inter-laboratory variation on CWD prion detection using RT-QuIC, we conducted a multi-institution comparison on a shared anonymized sample set. We hypothesized that RT-QuIC can accurately and reliably detect the prions that cause CWD in postmortem samples from medial retropharyngeal lymph node (RPLN) tissue despite variation in laboratory protocols. Laboratories from 6 U.S. states (Michigan, Minnesota, Missouri, New York, Pennsylvania, Wisconsin) were enlisted to compare the use of RT-QuIC in determining CWD prion status (positive or negative) among 50 anonymized RPLNs of known prion status. Our sample set included animals of 3 codon 96 WTD genotypes known to affect CWD progression and detection (G96G, G96S, S96S). All 6 laboratories successfully identified the true disease status consistently for all 3 tested codon 96 genotypes. Our results indicate that RT-QuIC is a suitable test for the detection of CWD prions in RPLN tissues in several genotypes of WTD.

Ecology and Chronic Wasting Disease Epidemiology Shape Prion Protein Gene Variation in Rocky Mountain Elk (Cervus elaphus nelsoni)

As chronic wasting disease (CWD) continues to spread across North America, the relationship between CWD and host genetics has become of interest. In Rocky Mountain elk (Cervus elaphus nelsoni), one or two copies of a leucine allele at codon 132 of the prion protein gene (132L*) has been shown to prolong the incubation period of CWD. Our study examined the relationship between CWD epidemiology and codon 132 evolution in elk from Wyoming, USA, from 2011 to 2018. Using PCR and Sanger sequencing, we genotyped 997 elk and assessed the relationship between genotype and CWD prevalence estimated from surveillance data. Using logistic regression, we showed that each 1% increase in CWD prevalence is associated with a 9.6% increase in the odds that an elk would have at least one copy of leucine at codon 132. In some regions, however, 132L* variants were found in the absence of CWD, indicating that evolutionary and epidemiologic patterns can be heterogeneous across space and time. We also provide evidence that naturally occurring CWD is not rare in 132L* elk, which merits the study of shedding kinetics in 132L* elk and the influence of genotype on CWD strain diversity. The management implications of cervid adaptations to CWD are difficult to predict. Studies that investigate the degree to which evolutionary outcomes are shaped by host spatial structure can provide useful epidemiologic insight, which can in turn aid management by informing scale and extent of mitigation actions.

Longitudinal detection of prion shedding in nasal secretions of CWD-infected white-tailed deer

Chronic wasting disease (CWD) is an emergent prion disease spreading in cervid populations in North America, South Korea and Scandinavia. Rapid detection of CWD prions shed by live animals using minimally invasive methods remains an important need. Previous studies in deer, elk and hamsters have demonstrated prion replication in the nasal olfactory mucosa, yet the temporal profile of CWD prion shedding in nasal secretions has not been well characterized. Here we report nasal prion shedding in 18 deer orally exposed to low doses of CWD prions and monitored longitudinally by several parameters. Serially collected nasal swabs were assayed for CWD prion seeding activity using iron oxide magnetic extraction and real-time quaking-induced conversion (IOME RT-QuIC). These findings were correlated with the results from longitudinal tonsil biopsies, terminal tissues and PRNP genotype. We detected nasal prion shedding 3-16 months after the first positive tonsil biopsy in ten of the 18 deer; detectable shedding persisted thereafter in nine of the ten animals. Surprisingly, nasal swabs were negative in eight deer, even though all were CWD-infected as determined by tonsil biopsies and terminal tissue assays. Nasal shedding was detected more often in deer that were homozygous for glycine at codon 96, and those that were near or demonstrating symptoms of clinical disease shed earlier and more frequently, irrespective of prion exposure dose. The results of this study demonstrate nasal shedding of CWD prions that can be detected using minimally invasive nasal swab sampling and RT-QuIC analysis.

A field-deployable diagnostic assay for the visual detection of misfolded prions

Diagnostic tools for the detection of protein-misfolding diseases (i.e., proteopathies) are limited. Gold nanoparticles (AuNPs) facilitate sensitive diagnostic techniques via visual color change for the identification of a variety of targets. In parallel, recently developed quaking-induced conversion (QuIC) assays leverage protein-amplification and fluorescent signaling for the accurate detection of misfolded proteins. Here, we combine AuNP and QuIC technologies for the visual detection of amplified misfolded prion proteins from tissues of wild white-tailed deer infected with chronic wasting disease (CWD), a prion disease of cervids. Our newly developed assay, MN-QuIC, enables both naked-eye and light-absorbance measurements for detection of misfolded prions. MN-QuIC leverages basic laboratory equipment that is cost-effective and portable, thus facilitating real-time prion diagnostics across a variety of settings. In addition to laboratory-based tests, we deployed to a rural field-station in southeastern Minnesota and tested for CWD on site. We successfully demonstrated that MN-QuIC is functional in a non-traditional laboratory setting by performing a blinded analysis in the field and correctly identifying all CWD positive and CWD not-detected deer at the field site in 24 h, thus documenting the portability of the assay. White-tailed deer tissues used to validate MN-QuIC included medial retropharyngeal lymph nodes, parotid lymph nodes, and palatine tonsils. Importantly, all of the white-tailed deer (n = 63) were independently tested using ELISA, IHC, and/or RT-QuIC technologies and results secured with MN-QuIC were 95.7% and 100% consistent with these tests for positive and non-detected animals, respectively. We hypothesize that electrostatic forces help govern the AuNP/prion interactions and conclude that MN-QuIC has great potential for sensitive, field-deployable diagnostics for CWD, with future potential diagnostic applications for a variety of proteopathies.

Assessment of Real-Time Quaking-Induced Conversion (RT-QuIC) Assay, Immunohistochemistry and ELISA for Detection of Chronic Wasting Disease under Field Conditions in White-Tailed Deer: A Bayesian Approach

Chronic wasting disease (CWD) is a transmissible prion disease of the cervidae family. ELISA and IHC tests performed postmortem on the medial retropharyngeal lymph nodes (RPLN) or obex are considered diagnostic gold standards for prion detection. However, differences in CWD transmission, stage of infection, pathogenesis, and strain can limit performance. To overcome these uncertainties, we used Bayesian statistics to assess the accuracy of RT-QuIC, an increasingly used prion amplification assay, to diagnose CWD on tonsil (TLN), parotid (PLN) and submandibular lymph nodes (SMLN), and ELISA/IHC on RPLN of white-tailed deer (WTD) sampled from Minnesota. Dichotomous RT-QuIC and ELISA/IHC results from wild (n = 61) and captive (n = 46) WTD were analyzed with two-dependent-test, one-population models. RT-QuIC performed on TLN and SMLN of the wild WTD population had similar sensitivity (median range (MR): 92.2–95.1) to ELISA/IHC on RPLN (MR: 91.1–92.3). Slightly lower (4–7%) sensitivity estimates were obtained from farmed animal and PLN models. RT-QuIC specificity estimates were high (MR: 94.5–98.5%) and similar to ELISA/IHC estimates (MR: 95.7–97.6%) in all models. This study offers new insights on RT-QuIC and ELISA/IHC performance at the population level and under field conditions, an important step in CWD diagnosis and management.

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

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

Chronic wasting disease: a cervid prion infection looming to spillover

The spread of chronic wasting disease (CWD) during the last six decades has resulted in cervid populations of North America where CWD has become enzootic. This insidious disease has also been reported in wild and captive cervids from other continents, threatening ecosystems, livestock and public health. These CWD "hot zones" are particularly complex given the interplay between cervid PRNP genetics, the infection biology, the strain diversity of infectious prions and the long-term environmental persistence of infectivity, which hinder eradication efforts. Here, we review different aspects of CWD including transmission mechanisms, pathogenesis, epidemiology and assessment of interspecies infection. Further understanding of these aspects could help identify "control points" that could help reduce exposure for humans and livestock and decrease CWD spread between cervids.

Exposure Risk of Chronic Wasting Disease in Humans

The majority of human prion diseases are sporadic, but acquired disease can occur, as seen with variant Creutzfeldt–Jakob disease (vCJD) following consumption of bovine spongiform encephalopathy (BSE). With increasing rates of cervid chronic wasting disease (CWD), there is concern that a new form of human prion disease may arise. Currently, there is no evidence of transmission of CWD to humans, suggesting the presence of a strong species barrier; however, in vitro and in vivo studies on the zoonotic potential of CWD have yielded mixed results. The emergence of different CWD strains is also concerning, as different strains can have different abilities to cross species barriers. Given that venison consumption is common in areas where CWD rates are on the rise, increased rates of human exposure are inevitable. If CWD was to infect humans, it is unclear how it would present clinically; in vCJD, it was strain-typing of vCJD prions that proved the causal link to BSE. Therefore, the best way to screen for CWD in humans is to have thorough strain-typing of harvested cervids and human CJD cases so that we will be in a position to detect atypical strains or strain shifts within the human CJD population.

Experimental oral transmission of chronic wasting disease to sika deer (Cervus nippon)

Chronic wasting disease (CWD) affects a broad array of cervid species and continues to be detected in an expanding geographic range. Initially introduced into the Republic of Korea through the importation of CWD-infected elk (Cervus canadensis), additional cases of CWD were subsequently detected in farmed Korean elk and sika deer (Cervus nippon). Wild and farmed sika deer are found in many regions of Asia, North America, and Europe, although natural transmission to this species has not been detected outside of the Republic of Korea. In this study, the oral transmission of CWD to sika deer was investigated using material from CWD-affected elk. Pathological prion (PrP^CWD) immunoreactivity was detected in oropharyngeal lymphoid tissues of one sika deer at 3.9 months post-inoculation (mpi) and was more widely distributed in a second sika deer examined at 10.9 mpi. The remaining four sika deer progressed to clinical disease between 21 and 24 mpi. Analysis of PrP^CWD tissue distribution in clinical sika deer revealed widespread deposition in central and peripheral nervous systems, lymphoreticular tissues, and the gastrointestinal tract. Prion protein gene (PRNP) sequences of these sika deer were identical and consistent with those reported in natural sika deer populations. These findings demonstrate the efficient oral transmission of CWD from elk to sika deer.

Evaluation of Antemortem Diagnostic Techniques in Goats Naturally Infected With Scrapie

Scrapie is a naturally occurring transmissible spongiform encephalopathy (TSE) that affects sheep and goats. Sheep and goats can be infected with scrapie as lambs or kids via contact with the placenta or placental fluids, or from ingestion of prions shed in the environment and/or bodily fluids (e.g., saliva, urine, and feces). Like other TSEs, scrapie is generally not diagnosed before extensive and irreversible brain damage has occurred. Therefore, a reliable method to screen animals may facilitate diagnosis. Additionally, while natural scrapie in sheep has been widely described, naturally acquired goat scrapie is less well-characterized. The purpose of this study was to better understand natural goat scrapie in regard to disease phenotype (i.e., incubation period, clinical signs, neuroanatomical deposition patterns of PrP^Sc, and molecular profile as detected by Western blot) and to evaluate the efficacy of antemortem tests to detect scrapie-positive animals in a herd of goats. Briefly, 28 scrapie-exposed goats were removed from a farm depopulated due to previous diagnoses of scrapie on the premises and observed daily for 30 months. Over the course of the observation period, antemortem biopsies of recto-anal mucosa-associated lymphoid tissue (RAMALT) were taken and tested using immunohistochemistry and real-time quaking-induced conversion (RT-QuIC), and retinal thickness was measured in vivo using optical coherence tomography (OCT). Following the observation period, immunohistochemistry and Western blot were performed to assess neuroanatomical deposition patterns of PrP^Sc and molecular profile. Our results demonstrate that antemortem rectal biopsy was 77% effective in identifying goats naturally infected with scrapie and that a positive antemortem rectal biopsy was associated with the presence of clinical signs of neurologic disease and a positive dam status. We report that changes in retinal thickness are not detectable over the course of the observation period in goats naturally infected with scrapie. Finally, our results indicate that the accumulation of PrP^Sc in central nervous system (CNS) and non-CNS tissues is consistent with previous reports of scrapie in sheep and goats.

Prion protein glycans reduce intracerebral fibril formation and spongiosis in prion disease

Posttranslational modifications (PTMs) are common among proteins that aggregate in neurodegenerative disease, yet how PTMs impact the aggregate conformation and disease progression remains unclear. By engineering knockin mice expressing prion protein (PrP) lacking 2 N-linked glycans (Prnp180Q/196Q), we provide evidence that glycans reduce spongiform degeneration and hinder plaque formation in prion disease. Prnp180Q/196Q mice challenged with 2 subfibrillar, non-plaque-forming prion strains instead developed plaques highly enriched in ADAM10-cleaved PrP and heparan sulfate (HS). Intriguingly, a third strain composed of intact, glycophosphatidylinositol-anchored (GPI-anchored) PrP was relatively unchanged, forming diffuse, HS-deficient deposits in both the Prnp180Q/196Q and WT mice, underscoring the pivotal role of the GPI-anchor in driving the aggregate conformation and disease phenotype. Finally, knockin mice expressing triglycosylated PrP (Prnp187N) challenged with a plaque-forming prion strain showed a phenotype reversal, with a striking disease acceleration and switch from plaques to predominantly diffuse, subfibrillar deposits. Our findings suggest that the dominance of subfibrillar aggregates in prion disease is due to the replication of GPI-anchored prions, with fibrillar plaques forming from poorly glycosylated, GPI-anchorless prions that interact with extracellular HS. These studies provide insight into how PTMs impact PrP interactions with polyanionic cofactors, and highlight PTMs as a major force driving the prion disease phenotype.

Novel Strain of the Chronic Wasting Disease Agent Isolated From Experimentally Inoculated Elk With LL132 Prion Protein

Chronic wasting disease (CWD) is a fatal, progressive disease that affects cervid species, including Rocky mountain elk (Cervus elaphus nelsoni). There are 2 allelic variants in the elk prion protein gene: L132 (leucine) and M132 (methionine). Following experimental oral challenge with the CWD agent incubation periods are longest in LL132 elk, intermediate in ML132 elk, and shortest in MM132 elk. In order to ascertain whether such CWD-infected elk carry distinct prion strains, groups of Tg12 mice that express M132 elk prion protein were inoculated intracranially with brain homogenate from individual CWD-infected elk of various genotypes (LL132, LM132, or MM132). Brain samples were examined for microscopic changes and assessment of the biochemical properties of disease-associated prion protein (PrP^Sc). On first passage, mice challenged with LL132 elk inoculum had prolonged incubation periods and greater PrP^Sc fibril stability compared to mice challenged with MM132 or LM132 inoculum. On second passage, relative incubation periods, western blot profiles, and neuropathology were maintained. These results suggest that the CWD prion isolated from LL132 elk is a novel CWD strain and that M132 PrP^C is able to propagate some biophysical properties of the L132 PrP^Sc conformation.

Progression of chronic wasting disease in white-tailed deer analyzed by serial biopsy RT-QuIC and immunohistochemistry

Chronic wasting disease (CWD) continues to spread or be recognized in the United States, Canada, and Europe. CWD is diagnosed by demonstration of the causative misfolded prion protein (PrP^CWD) in either brain or lymphoid tissue using immunodetection methods, with immunohistochemistry (IHC) recognized as the gold standard. In recent years, in vitro amplification assays have been developed that can detect CWD prion seeding activity in tissues, excreta, and body fluids of affected cervids. These methods potentially offer earlier and more facile detection of CWD, both pre- and post-mortem. Here we provide a longitudinal profile of CWD infection progression, as assessed by both real-time quaking-induced conversion (RT-QuIC) and IHC on serial biopsies of mucosal lymphoid tissues of white-tailed deer orally exposed to low doses of CWD prions. We report that detection of CWD infection by RT-QuIC preceded that by IHC in both tonsil and recto-anal lymphoid tissue (RAMALT) in 14 of 19 deer (74%). Of the 322 biopsy samples collected in post-exposure longitudinal monitoring, positive RT-QuIC results were obtained for 146 samples, 91 of which (62%) were concurrently also IHC-positive. The lower frequency of IHC positivity was manifest most in the earlier post-exposure periods and in biopsies in which lymphoid follicles were not detected. For all deer in which RT-QuIC seeding activity was detected in a tonsil or RAMALT biopsy, PrP^CWD was subsequently or concurrently detected by IHC. Overall, this study (a) provides a longitudinal profile of CWD infection in deer after low yet infectious oral prion exposure; (b) illustrates the value of RT-QuIC for sensitive detection of CWD; and (c) demonstrates an ultimate high degree of correlation between RT-QuIC and IHC positivity as CWD infection progresses.

Identification of circulating microRNA signatures as potential biomarkers in the serum of elk infected with chronic wasting disease

Chronic wasting disease (CWD) is an emerging infectious prion disorder that is spreading rapidly in wild populations of cervids in North America. The risk of zoonotic transmission of CWD is as yet unclear but a high priority must be to minimize further spread of the disease. No simple diagnostic tests are available to detect CWD quickly or in live animals; therefore, easily accessible biomarkers may be useful in identifying infected animals. MicroRNAs (miRNAs) are a class of small, non-coding RNA molecules that circulate in blood and are promising biomarkers for several infectious diseases. In this study we used next-generation sequencing to characterize the serum miRNA profiles of 35 naturally infected elk that tested positive for CWD in addition to 35 elk that tested negative for CWD. A total of 21 miRNAs that are highly conserved amongst mammals were altered in abundance in sera, irrespective of hemolysis in the samples. A number of these miRNAs have previously been associated with prion diseases. Receiver operating characteristic (ROC) curve analysis was performed to evaluate the discriminative potential of these miRNAs as biomarkers for the diagnosis of CWD. We also determined that a subgroup of 6 of these miRNAs were consistently altered in abundance in serum from hamsters experimentally infected with scrapie. This suggests that common miRNA candidate biomarkers could be selected for prion diseases in multiple species. Furthermore, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses pointed to a strong correlation for 3 of these miRNAs, miR-148a-3p, miR-186-5p, miR-30e-3p, with prion disease.

Use of different RT-QuIC substrates for detecting CWD prions in the brain of Norwegian cervids

Chronic wasting disease (CWD) is a highly contagious prion disease affecting captive and free-ranging cervid populations. CWD has been detected in United States, Canada, South Korea and, most recently, in Europe (Norway, Finland and Sweden). Animals with CWD release infectious prions in the environment through saliva, urine and feces sustaining disease spreading between cervids but also potentially to other non-cervids ruminants (e.g. sheep, goats and cattle). In the light of these considerations and due to CWD unknown zoonotic potential, it is of utmost importance to follow specific surveillance programs useful to minimize disease spreading and transmission. The European community has already in place specific surveillance measures, but the traditional diagnostic tests performed on nervous or lymphoid tissues lack sensitivity. We have optimized a Real-Time Quaking-Induced Conversion (RT-QuIC) assay for detecting CWD prions with high sensitivity and specificity to try to overcome this problem. In this work, we show that bank vole prion protein (PrP) is an excellent substrate for RT-QuIC reactions, enabling the detection of trace-amounts of CWD prions, regardless of prion strain and cervid species. Beside supporting the traditional diagnostic tests, this technology could be exploited for detecting prions in peripheral tissues from live animals, possibly even at preclinical stages of the disease.

Cellular distribution of the prion protein in palatine tonsils of mule deer (Odocoileus hemionus) and Rocky Mountain elk (Cervus elaphus nelsoni)

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) that affects members of the Cervidae family, including deer (Odocoileus spp.), elk (Cervus Canadensis spp.), and moose (Alces alces spp.). While CWD is a neurodegenerative disease, lymphoid accumulation of the abnormal isoform of the prion protein (PrP^Sc) is detectable early in the course of infection. It has been shown that a large portion of the PrP^Sc lymphoid accumulation in infected mule deer takes place on the surface of follicular dendritic cells (FDCs). In mice, FDC expression of PrP^C has been shown to be essential for PrP^Sc accumulation. FDCs have been shown to normally express high levels of PrP^C in mice and humans but this has not been examined in natural hosts for CWD. We used double immunofluorescent labeling and confocal microscopy to determine the PrP^C expression characteristics of B and T lymphocytes as well as FDCs in palatine tonsils of CWD-negative mule deer and elk. We detected substantial PrP^C colocalization with all cellular phenotypic markers used in this study, not just with FDC phenotypic markers.

Detection of CWD in cervids by RT-QuIC assay of third eyelids

The diagnosis of chronic wasting disease (CWD) relies on demonstration of the disease-associated misfolded CWD prion protein (PrP^CWD) in brain or retropharyngeal lymph node tissue by immunodetection methods, e.g. ELISA and immunohistochemistry (IHC). The success of these methods relies on a quality sample of tissues, which requires both anatomical knowledge and considerable dissection to collect. As the prevalence of CWD continues to increase globally, the development of fast and cost-effective methods to detect the disease is vital to facilitate CWD detection and surveillance. To address these issues, we have evaluated third eyelids from CWD-infected deer and elk using real-time quaking induced conversion (RT-QuIC). We identified prion seeding activity in third eyelids in 24 of 25 (96%) CWD-infected white-tailed deer (Odocoileus virginianus). We detected RT-QuIC positivity in the third eyelid as early as 1 month after experimental CWD exposure. In addition, we identified prion seeding activity in third eyelids of 18 of 25 (72%) naturally exposed asymptomatic CWD-positive rocky mountain elk (Cervus canadensis nelson). We compared CWD detection by RT-QuIC and IHC in third eyelid, retropharyngeal lymph node, and brain in 10 deer in early symptomatic stage of disease. IHC detected PrP^CWD deposition in third eyelid lymphoid follicles in 5 of 10 deer (50%) whereas third eyelids of all 10 animals were positive by RT-QuIC. This difference reflected in part a lower requirement for lymphoid follicle presence for seeding activity detection by RT-QuIC. In conclusion, RT-QuIC analysis of the third eyelid, an easily accessed tissue, has potential to advance CWD detection and testing compliance.

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.

Pathologic and biochemical characterization of PrPSc from elk with PRNP polymorphisms at codon 132 after experimental infection with the chronic wasting disease agent

BACKGROUND

The Rocky Mountain elk (Cervus elaphus nelsoni) prion protein gene (PRNP) is polymorphic at codon 132, with leucine (L132) and methionine (M132) allelic variants present in the population. In elk experimentally inoculated with the chronic wasting disease (CWD) agent, different incubation periods are associated with PRNP genotype: LL132 elk survive the longest, LM132 elk are intermediate, and MM132 elk the shortest. The purpose of this study was to investigate potential mechanisms underlying variations in incubation period in elk of different prion protein genotypes. Elk calves of three PRNP genotypes (n = 2 MM132, n = 2 LM132, n = 4 LL132) were orally inoculated with brain homogenate from elk clinically affected with CWD.

RESULTS

Elk with longer incubation periods accumulated relatively less PrP^Sc in the brain than elk with shorter incubation periods. PrP^Sc accumulation in LM132 and MM132 elk was primarily neuropil-associated while glial-associated immunoreactivity was prominent in LL132 elk. The fibril stability of PrP^Sc from MM132 and LM132 elk were similar to each other and less stable than that from LL132 elk. Real-time quaking induced conversion assays (RT-QuIC) revealed differences in the ability of PrP^Sc seed from elk of different genotypes to convert recombinant 132 M or 132 L substrate.

CONCLUSIONS

This study provides further evidence of the importance of PRNP genotype in the pathogenesis of CWD of elk. The longer incubation periods observed in LL132 elk are associated with PrP^Sc that is more stable and relatively less abundant at the time of clinical disease. The biochemical properties of PrP^Sc from MM132 and LM132 elk are similar to each other and different to PrP^Sc from LL132 elk. The shorter incubation periods in MM132 compared to LM132 elk may be the result of genotype-dependent differences in the efficiency of propagation of PrP^Sc moieties present in the inoculum. A better understanding of the mechanisms by which the polymorphisms at codon 132 in elk PRNP influence disease pathogenesis will help to improve control of CWD in captive and free-ranging elk populations.

Scientific opinion on chronic wasting disease (II)

The European Commission asked EFSA for a scientific opinion on chronic wasting disease in two parts. Part one, on surveillance, animal health risk-based measures and public health risks, was published in January 2017. This opinion (part two) addresses the remaining Terms of Reference, namely, 'are the conclusions and recommendations in the EFSA opinion of June 2004 on diagnostic methods for chronic wasting disease still valid? If not, an update should be provided', and 'update the conclusions of the 2010 EFSA opinion on the results of the European Union survey on chronic wasting disease in cervids, as regards its occurrence in the cervid population in the European Union'. Data on the performance of authorised rapid tests in North America are not comprehensive, and are more limited than those available for the tests approved for statutory transmissible spongiform encephalopathies surveillance applications in cattle and sheep. There are no data directly comparing available rapid test performances in cervids. The experience in Norway shows that the Bio-Rad TeSeE™ SAP test, immunohistochemistry and western blotting have detected reindeer, moose and red deer cases. It was shown that testing both brainstem and lymphoid tissue from each animal increases the surveillance sensitivity. Shortcomings in the previous EU survey limited the reliability of inferences that could be made about the potential disease occurrence in Europe. Subsequently, testing activity in Europe was low, until the detection of the disease in Norway, triggering substantial testing efforts in that country. Available data neither support nor refute the conclusion that chronic wasting disease does not occur widely in the EU and do not preclude the possibility that the disease was present in Europe before the survey was conducted. It appears plausible that chronic wasting disease could have become established in Norway more than a decade ago.

Ante-mortem detection of chronic wasting disease in recto-anal mucosa-associated lymphoid tissues from elk (Cervus elaphus nelsoni) using real-time quaking-induced conversion (RT-QuIC) assay: A blinded collaborative study

Prion diseases are transmissible spongiform encephalopathies (TSEs) characterized by fatal, progressive neurologic diseases with prolonged incubation periods and an accumulation of infectious misfolded prion proteins. Antemortem diagnosis is often difficult due to a long asymptomatic incubation period, differences in the pathogenesis of different prions, and the presence of very low levels of infectious prion in easily accessible samples. Chronic wasting disease (CWD) is a TSE affecting both wild and captive populations of cervids, including mule deer, white-tailed deer, elk, moose, muntjac, and most recently, wild reindeer. This study represents a well-controlled evaluation of a newly developed real-time quaking-induced conversion (RT-QuIC) assay as a potential CWD diagnostic screening test using rectal biopsy sections from a depopulated elk herd. We evaluated 69 blinded samples of recto-anal mucosa-associated lymphoid tissue (RAMALT) obtained from USDA Veterinary Services. The results were later un-blinded and statistically compared to immunohistochemical (IHC) results from the USDA National Veterinary Services Laboratories (NVSL) for RAMALT, obex, and medial retropharyngeal lymph node (MRPLN). Comparison of RAMALT RT-QuIC assay results with the IHC results of RAMALT revealed 92% relative sensitivity (95% confidence limits: 61.52-99.8%) and 95% relative specificity (95% confidence limits: 85.13-99%). Collectively, our results show a potential utility of the RT-QuIC assay to advance the development of a rapid, sensitive, and specific prion diagnostic assay for CWD prions.

Pathogen-mediated selection in free-ranging elk populations infected by chronic wasting disease

Pathogens can exert a large influence on the evolution of hosts via selection for alleles or genotypes that moderate pathogen virulence. Inconsistent interactions between parasites and the host genome, such as those resulting from genetic linkages and environmental stochasticity, have largely prevented observation of this process in wildlife species. We examined the prion protein gene (PRNP) in North American elk (Cervus elaphus nelsoni) populations that have been infected with chronic wasting disease (CWD), a contagious, fatal prion disease, and compared allele frequency to populations with no history of exposure to CWD. The PRNP in elk is highly conserved and a single polymorphism at codon 132 can markedly extend CWD latency when the minor leucine allele (132L) is present. We determined population exposure to CWD, genotyped 1,018 elk from five populations, and developed a hierarchical Bayesian model to examine the relationship between CWD prevalence and PRNP 132L allele frequency. Populations infected with CWD for at least 30-50 y exhibited 132L allele frequencies that were on average twice as great (range = 0.23-0.29) as those from uninfected populations (range = 0.04-0.17). Despite numerous differences between the elk populations in this study, the consistency of increase in 132L allele frequency suggests pathogen-mediated selection has occurred due to CWD. Although prior modeling work predicted that selection will continue, the potential for fitness costs of the 132L allele or new prion protein strains to arise suggest that it is prudent to assume balancing selection may prevent fixation of the 132L allele in populations with CWD.

Scrapie, CWD, and Transmissible Mink Encephalopathy

Transmissible spongiform encephalopathies (TSEs), or prions, are neurodegenerative diseases that affect a variety of animal species, including humans. Cruetzfeldt-Jakob disease (CJD) in humans, sheep and goat scrapie, chronic wasting disease (CWD) of cervids, and transmissible mink encephalopathy (TME) of mink are classified as TSEs. According to the "protein-only" hypothesis (Prusiner, 1982),¹ prions are devoid of nucleic acids and consist of assemblies of misfolded host-encoded normal protein, the prion protein (PrP^C). Prion propagation is thought to occur by a templating mechanism during which PrP^C is recruited, converted to a disease-associated isoform (PrP^D), and assembled onto the growing amyloid fibril. This fibular assembly is infectious, with ability to initiate disease processes similar to other pathogenic agents. Evidence indicates that scrapie, CWD, and TME disease processes follow this rule.

Evolution of Diagnostic Tests for Chronic Wasting Disease, a Naturally Occurring Prion Disease of Cervids

Since chronic wasting disease (CWD) was first identified nearly 50 years ago in a captive mule deer herd in the Rocky Mountains of the United States, it has slowly spread across North America through the natural and anthropogenic movement of cervids and their carcasses. As the endemic areas have expanded, so has the need for rapid, sensitive, and cost effective diagnostic tests—especially those which take advantage of samples collected antemortem. Over the past two decades, strategies have evolved from the recognition of microscopic spongiform pathology and associated immunohistochemical staining of the misfolded prion protein to enzyme-linked immunoassays capable of detecting the abnormal prion conformer in postmortem samples. In a history that parallels the diagnosis of more conventional infectious agents, both qualitative and real-time amplification assays have recently been developed to detect minute quantities of misfolded prions in a range of biological and environmental samples. With these more sensitive and semi-quantitative approaches has come a greater understanding of the pathogenesis and epidemiology of this disease in the native host. Because the molecular pathogenesis of prion protein misfolding is broadly analogous to the misfolding of other pathogenic proteins, including Aβ and α-synuclein, efforts are currently underway to apply these in vitro amplification techniques towards the diagnosis of Alzheimer’s disease, Parkinson’s disease, and other proteinopathies. Chronic wasting disease—once a rare disease of Colorado mule deer—now represents one of the most prevalent prion diseases, and should serve as a model for the continued development and implementation of novel diagnostic strategies for protein misfolding disorders in the natural host.

Primary transmission of chronic wasting disease versus scrapie prions from small ruminants to transgenic mice expressing ovine or cervid prion protein

Development of mice expressing either ovine (Tg338) or cervid (TgElk) prion protein (PrP) have aided in characterization of scrapie and chronic wasting disease (CWD), respectively. Experimental inoculation of sheep with CWD prions has demonstrated the potential for interspecies transmission but, infection with CWD versus classical scrapie prions may be difficult to differentiate using validated diagnostic platforms. In this study, mouse bioassay in Tg338 and TgElk was utilized to evaluate transmission of CWD versus scrapie prions from small ruminants. Mice (≥5 per homogenate) were inoculated with brain homogenates from clinically affected sheep or goats with naturally acquired classical scrapie, white-tailed deer with naturally acquired CWD (WTD-CWD) or sheep with experimentally acquired CWD derived from elk (sheep-passaged-CWD). Survival time (time to clinical disease) and attack rates (brain accumulation of protease resistant PrP, PrPres) were determined. Inoculation with classical scrapie prions resulted in clinical disease and 100 % attack rates in Tg338, but no clinical disease at endpoint (>300 days post-inoculation, p.i.) and low attack rates (6.8 %) in TgElk. Inoculation with WTD-CWD prions yielded no clinical disease or brain PrPres accumulation in Tg338 at endpoint (>500 days p.i.), but rapid onset of clinical disease (~121 days p.i.) and 100 % attack rate in TgElk. Sheep-passaged-CWD resulted in transmission to both mouse lines with 100 % attack rates at endpoint in Tg338 and an attack rate of ~73 % in TgElk with some culled due to clinical disease. These primary transmission observations demonstrate the potential of bioassay in Tg338 and TgElk to help differentiate possible infection with CWD versus classical scrapie prions in sheep and goats.

Detection of PrPCWD in Postmortem Rectal Lymphoid Tissues in Rocky Mountain Elk (Cervus Elaphus Nelsoni) Infected with Chronic Wasting Disease

Preclinical diagnostic tests for transmissible spongiform encephalopathies have been described for mule deer ( Odocoileus hemionus), using biopsy tissues of palatine tonsil, and for sheep, using lymphoid tissues from palatine tonsil, third eyelid, and rectal mucosa. The utility of examining the rectal mucosal lymphoid tissues to detect chronic wasting disease (CWD) was investigated in Rocky Mountain elk ( Cervus elaphus nelsoni), a species for which there is not a live-animal diagnostic test. Postmortem rectal mucosal sections were examined from 308 elk from two privately owned herds that were depopulated. The results of the postmortem rectal mucosal sections were compared to immunohistochemical staining of the brainstem, retropharyngeal lymph nodes, and palatine tonsil. Seven elk were found positive using the brainstem (dorsal motor nucleus of the vagus nerve), retropharyngeal lymph nodes, and palatine tonsil. Six of these elk were also found positive using postmortem rectal mucosal sections. The remaining 301 elk in which CWD-associated abnormal isoform of the prion protein (PrPCWD) was not detected in the brainstem and cranial lymphoid tissues were also found to be free of PrPCWD when postmortem rectal mucosal sections were examined. The use of rectal mucosal lymphoid tissues may be suitable for a live-animal diagnostic test as part of an integrated management strategy to limit CWD in elk.

Seeded Amplification of Chronic Wasting Disease Prions in Nasal Brushings and Recto-anal Mucosa-Associated Lymphoid Tissues from Elk by Real-Time Quaking-Induced Conversion

Chronic wasting disease (CWD), a transmissible spongiform encephalopathy of cervids, was first documented nearly 50 years ago in Colorado and Wyoming and has since been detected across North America and the Republic of Korea. The expansion of this disease makes the development of sensitive diagnostic assays and antemortem sampling techniques crucial for the mitigation of its spread; this is especially true in cases of relocation/reintroduction or prevalence studies of large or protected herds, where depopulation may be contraindicated. This study evaluated the sensitivity of the real-time quaking-induced conversion (RT-QuIC) assay of recto-anal mucosa-associated lymphoid tissue (RAMALT) biopsy specimens and nasal brushings collected antemortem. These findings were compared to results of immunohistochemistry (IHC) analysis of ante- and postmortem samples. RAMALT samples were collected from populations of farmed and free-ranging Rocky Mountain elk (Cervus elaphus nelsoni;n= 323), and nasal brush samples were collected from a subpopulation of these animals (n= 205). We hypothesized that the sensitivity of RT-QuIC would be comparable to that of IHC analysis of RAMALT and would correspond to that of IHC analysis of postmortem tissues. We found RAMALT sensitivity (77.3%) to be highly correlative between RT-QuIC and IHC analysis. Sensitivity was lower when testing nasal brushings (34%), though both RAMALT and nasal brush test sensitivities were dependent on both thePRNPgenotype and disease progression determined by the obex score. These data suggest that RT-QuIC, like IHC analysis, is a relatively sensitive assay for detection of CWD prions in RAMALT biopsy specimens and, with further investigation, has potential for large-scale and rapid automated testing of antemortem samples for CWD.

Progressive accumulation of the abnormal conformer of the prion protein and spongiform encephalopathy in the obex of nonsymptomatic and symptomatic Rocky Mountain elk (Cervus elaphus nelsoni) with chronic wasting disease

The purpose of our study was to describe the progressive accumulation of the abnormal conformer of the prion protein (PrP(CWD)) and spongiform degeneration in a single section of brain stem in Rocky Mountain elk (Cervus elaphus nelsoni) with chronic wasting disease (CWD). A section of obex from 85 CWD-positive elk was scored using the presence and abundance of PrP(CWD) immunoreactivity and spongiform degeneration in 10 nuclear regions and the presence and abundance of PrP(CWD) in 10 axonal tracts, the subependymal area of the fourth ventricle, and the thin subpial astrocytic layer (glial limitans). Data was placed in a formula to generate an overall obex score. Data suggests that PrP(CWD) immunoreactivity and spongiform degeneration has a unique and relatively consistent pattern of progression throughout a section of obex. This scoring technique utilizing a single section of obex may prove useful in future work for estimating the presence and abundance of PrP(CWD) in peripheral tissues and the nervous system in elk with CWD.

Prion amplification and hierarchical Bayesian modeling refine detection of prion infection

Prions are unique infectious agents that replicate without a genome and cause neurodegenerative diseases that include chronic wasting disease (CWD) of cervids. Immunohistochemistry (IHC) is currently considered the gold standard for diagnosis of a prion infection but may be insensitive to early or sub-clinical CWD that are important to understanding CWD transmission and ecology. We assessed the potential of serial protein misfolding cyclic amplification (sPMCA) to improve detection of CWD prior to the onset of clinical signs. We analyzed tissue samples from free-ranging Rocky Mountain elk (Cervus elaphus nelsoni) and used hierarchical Bayesian analysis to estimate the specificity and sensitivity of IHC and sPMCA conditional on simultaneously estimated disease states. Sensitivity estimates were higher for sPMCA (99.51%, credible interval (CI) 97.15–100%) than IHC of obex (brain stem, 76.56%, CI 57.00–91.46%) or retropharyngeal lymph node (90.06%, CI 74.13–98.70%) tissues, or both (98.99%, CI 90.01–100%). Our hierarchical Bayesian model predicts the prevalence of prion infection in this elk population to be 18.90% (CI 15.50–32.72%), compared to previous estimates of 12.90%. Our data reveal a previously unidentified sub-clinical prion-positive portion of the elk population that could represent silent carriers capable of significantly impacting CWD ecology.

Chronic wasting disease of cervids: current knowledge and future perspectives

A naturally occurring transmissible spongiform encephalopathy (TSE) of mule deer was first reported in Colorado and Wyoming in 1967 and has since spread to other members of the cervid family in 22 states, 2 Canadian provinces, and the Republic of Korea. Chronic wasting disease (CWD), caused by exposure to an abnormally folded isoform of the cellular prion protein, is characterized by progressive neurological disease in susceptible natural and experimental hosts and is ultimately fatal. CWD is thought to be transmitted horizontally in excreta and through contaminated environments, features common to scrapie of sheep, though rare among TSEs. Evolving detection methods have revealed multiple strains of CWD and with continued development may lead to an effective antemortem test. Managing the spread of CWD, through the development of a vaccine or environmental cleanup strategies, is an active area of interest. As such, CWD represents a unique challenge in the study of prion diseases.

Prion-seeding activity in cerebrospinal fluid of deer with chronic wasting disease

Transmissible spongiform encephalopathies (TSEs), or prion diseases, are a uniformly fatal family of neurodegenerative diseases in mammals that includes chronic wasting disease (CWD) of cervids. The early and ante-mortem identification of TSE-infected individuals using conventional western blotting or immunohistochemistry (IHC) has proven difficult, as the levels of infectious prions in readily obtainable samples, including blood and bodily fluids, are typically beyond the limits of detection. The development of amplification-based seeding assays has been instrumental in the detection of low levels of infectious prions in clinical samples. In the present study, we evaluated the cerebrospinal fluid (CSF) of CWD-exposed (n=44) and naïve (n=4) deer (n=48 total) for CWD prions (PrP^d) using two amplification assays: serial protein misfolding cyclic amplification with polytetrafluoroethylene beads (sPMCAb) and real-time quaking induced conversion (RT-QuIC) employing a truncated Syrian hamster recombinant protein substrate. Samples were evaluated blindly in parallel with appropriate positive and negative controls. Results from amplification assays were compared to one another and to obex immunohistochemistry, and were correlated to available clinical histories including CWD inoculum source (e.g. saliva, blood), genotype, survival period, and duration of clinical signs. We found that both sPMCAb and RT-QuIC were capable of amplifying CWD prions from cervid CSF, and results correlated well with one another. Prion seeding activity in either assay was observed in approximately 50% of deer with PrP^d detected by IHC in the obex region of the brain. Important predictors of amplification included duration of clinical signs and time of first tonsil biopsy positive results, and ultimately the levels of PrP^d identified in the obex by IHC. Based on our findings, we expect that both sPMCAb and RT-QuIC may prove to be useful detection assays for the detection of prions in CSF.

A case of chronic wasting disease in a captive red deer (Cervus elaphus)

A 22-month-old, female red deer (Cervus elaphus) was submitted to the University of Minnesota Veterinary Diagnostic Laboratory for necropsy and chronic wasting disease (CWD) testing. The deer was found positive for the abnormal prion protein in the obex and the retropharyngeal lymph node by immunohistochemical staining. Microscopic lesions of spongiform encephalopathy and immunohistochemical staining patterns and intensity were similar to those in CWD-positive elk and experimentally infected red deer.

Prion protein: structural features and related toxicity

Transmissible spongiform encephalopathies, or prion diseases, is a group of infectious neurodegenerative disorders. The conformational conversion from cellular form (PrP(C)) to disease-causing isoform (PrP(Sc)) is considered to be the most important and remarkable event in these diseases, while accumulation of PrP(Sc) is thought to be the main reason for cell death, inflammation and spongiform degeneration observed in infected individuals. Although these rare but unique neurodegenerative disorders have attracted much attention, there are still many questions that remain to be answered. Knowledge of the scrapie agent structures and the toxic species may have significance for understanding the causes of the diseases, and could be helpful for rational design of novel therapeutic and diagnostic methods. In this review, we summarized the available experimental evidence concerning the relationship among the structural features, aggregation status of misfolded PrP and related neurotoxicity in the course of prion diseases development. In particular, most data supports the idea that the smaller oligomeric PrP(Sc) aggregates, rather than the mature amyloid fibers, exhibit the highest toxicity to the host.

Oxidation of methionine 216 in sheep and elk prion protein is highly dependent upon the amino acid at position 218 but is not important for prion propagation

We employed a sensitive mass spectrometry-based method to deconstruct, confirm, and quantitate the prions present in elk naturally infected with chronic wasting disease and sheep naturally infected with scrapie. We used this approach to study the oxidation of a methionine at position 216 (Met216), because this oxidation (MetSO216) has been implicated in prion formation. Three polymorphisms (Ile218, Val218, and Thr218) of sheep recombinant prion protein were prepared. Our analysis showed the novel result that the proportion of MetSO216 was highly dependent upon the amino acid residue at position 218 (I > V > T), indicating that Ile218 in sheep and elk prion protein (PrP) renders the Met216 intrinsically more susceptible to oxidation than the Val218 or Thr218 analogue. We were able to quantitate the prions in the attomole range. The presence of prions was verified by the detection of two confirmatory peptides: GENFTETDIK (sheep and elk) and ESQAYYQR (sheep) or ESEAYYQR (elk). This approach required much smaller amounts of tissue (600 μg) than traditional methods of detection (enzyme-linked immunosorbent assay, Western blot, and immunohistochemical analysis) (60 mg). In sheep and elk, a normal cellular prion protein containing MetSO216 is not actively recruited and converted to prions, although we observed that this Met216 is intrinsically more susceptible to oxidation.

Diagnostic accuracy of rectal mucosa biopsy testing for chronic wasting disease within white-tailed deer (Odocoileus virginianus) herds in North America: effects of age, sex, polymorphism at PRNP codon 96, and disease progression

An effective live animal diagnostic test is needed to assist in the control of chronic wasting disease (CWD), which has spread through captive and wild herds of white-tailed deer (Odocoileus virginianus) in Canada and the United States. In the present study, the diagnostic accuracy of rectal mucosa biopsy sample testing was determined in white-tailed deer from 4 CWD-infected captive herds. Specifically, the current study compared the immunohistochemical detection of disease-associated prion protein in postmortem rectal mucosa biopsy samples to the CWD status of each deer as determined by immunodiagnostic evaluations of the brainstem at the obex, the medial retropharyngeal lymph node, and the palatine tonsil. The effects of age, sex, genotype, and disease progression were also evaluated. Diagnostic sensitivity on rectal biopsy samples for CWD in white-tailed deer ranged from 63% to 100%; the pooled estimate of sensitivity was 68% with 95% confidence limits (95% CLs) of 49% and 82%. However, diagnostic sensitivity was dependent on genotype at prion protein gene (PRNP) codon 96 and on disease progression as assessed by obex grade. Diagnostic sensitivity was 76% (95% CLs: 49%, 91%) for 96GG deer but only 42% (95% CLs: 13%, 79%) for 96GS deer. Furthermore, diagnostic sensitivity was only 36% for deer in the earliest stage of disease (obex grade 0) but was 100% for deer in the last 2 stages of preclinical disease (obex grades 3 and 4). The overall diagnostic specificity was 99.8%. Selective use of antemortem rectal biopsy sample testing would provide valuable information during disease investigations of CWD-suspect deer herds.

Disease-associated prion protein in neural and lymphoid tissues of mink (Mustela vison) inoculated with transmissible mink encephalopathy

Transmissible spongiform encephalopathies (TSEs) are diagnosed by immunodetection of disease-associated prion protein (PrP(d)). The distribution of PrP(d) within the body varies with the time-course of infection and between species, during interspecies transmission, as well as with prion strain. Mink are susceptible to a form of TSE known as transmissible mink encephalopathy (TME), presumed to arise due to consumption of feed contaminated with a single prion strain of ruminant origin. After extended passage of TME isolates in hamsters, two strains emerge, HY and DY, each of which is associated with unique structural isoforms of PrP(TME) and of which only the HY strain is associated with accumulation of PrP(TME) in lymphoid tissues. Information on the structural nature and lymphoid accumulation of PrP(TME) in mink is limited. In this study, 13 mink were challenged by intracerebral inoculation using late passage TME inoculum, after which brain and lymphoid tissues were collected at preclinical and clinical time points. The distribution and molecular nature of PrP(TME) was investigated by techniques including blotting of paraffin wax-embedded tissue and epitope mapping by western blotting. PrP(TME) was detected readily in the brain and retropharyngeal lymph node during preclinical infection, with delayed progression of accumulation within other lymphoid tissues. For comparison, three mink were inoculated by the oral route and examined during clinical disease. Accumulation of PrP(TME) in these mink was greater and more widespread, including follicles of rectoanal mucosa-associated lymphoid tissue. Western blot analyses revealed that PrP(TME) accumulating in the brain of mink is structurally most similar to that accumulating in the brain of hamsters infected with the DY strain. Collectively, the results of extended passage in mink are consistent with the presence of only a single strain of TME, the DY strain, capable of inducing accumulation of PrP(TME) in the lymphoid tissues of mink but not in hamsters. Thus, mink are a relevant animal model for further study of this unique strain, which ultimately may have been introduced through consumption of a TSE of ruminant origin.

Transmissibility of caprine scrapie in ovine transgenic mice

Background

The United States control program for classical ovine scrapie is based in part on the finding that infection is typically spread through exposure to shed placentas from infected ewes. Transmission from goats to sheep is less well described. A suitable rodent model for examining the effect of caprine scrapie isolates in the ovine host will be useful in the ovine scrapie eradication effort. In this study, we describe the incubation time, brain lesion profile, glycoform pattern and PrP^Sc distribution patterns in a well characterized transgenic mouse line (Tg338) expressing the ovine VRQ prion allele, following inoculation with brain from scrapie infected goats.

Results

First passage incubation times of caprine tissue in Tg338 ovinized mice varied widely but second passage intervals were shorter and consistent. Vacuolation profiles, glycoform patterns and paraffin-embedded tissue blots from terminally ill second passage mice derived from sheep or goat inocula were similar. Proteinase K digestion products of murine tissue were slightly smaller than the original ruminant inocula, a finding consistent with passage of several ovine strains in previous reports.

Conclusions

These findings demonstrate that Tg338 mice propagate prions of caprine origin and provide a suitable baseline for examination of samples identified in the expanded US caprine scrapie surveillance program.

Sensitivity of protein misfolding cyclic amplification versus immunohistochemistry in ante-mortem detection of chronic wasting disease

As the only prion disease affecting free-ranging animals, ante-mortem identification of affected cervids has become paramount in understanding chronic wasting disease (CWD) pathogenesis, prevalence and control of horizontal or vertical transmission. To seek maximal sensitivity in ante-mortem detection of CWD infection, this study used paired tonsil biopsy samples collected at various time points from 48 CWD-exposed cervids to compare blinded serial protein misfolding cyclic amplification (sPMCA) with the assay long considered the 'gold standard' for CWD detection, immunohistochemistry (IHC). sPMCA-negative controls (34 % of the samples evaluated) included tissues from mock-inoculated animals and unspiked negative controls, all of which tested negative throughout the course of the study. It was found that sPMCA on tonsil biopsies detected CWD infection significantly earlier (2.78 months, 95 % confidence interval 2.40-3.15) than conventional IHC. Interestingly, a correlation was observed between early detection by sPMCA and host PRNP genotype. These findings demonstrate that in vitro-amplification assays provide enhanced sensitivity and advanced detection of CWD infection in the peripheral tissues of cervids, with a potential role for spike or substrate genotype in sPMCA amplification efficiency.

Experimental interspecies transmission studies of the transmissible spongiform encephalopathies to cattle: comparison to bovine spongiform encephalopathy in cattle

Prion diseases or transmissible spongiform encephalopathies (TSEs) of animals include scrapie of sheep and goats; transmissible mink encephalopathy (TME); chronic wasting disease (CWD) of deer, elk and moose; and bovine spongiform encephalopathy (BSE) of cattle. The emergence of BSE and its spread to human beings in the form of variant Creutzfeldt-Jakob disease (vCJD) resulted in interest in susceptibility of cattle to CWD, TME and scrapie. Experimental cross-species transmission of TSE agents provides valuable information for potential host ranges of known TSEs. Some interspecies transmission studies have been conducted by inoculating disease-causing prions intracerebrally (IC) rather than orally; the latter is generally effective in intraspecies transmission studies and is considered a natural route by which animals acquire TSEs. The "species barrier" concept for TSEs resulted from unsuccessful interspecies oral transmission attempts. Oral inoculation of prions mimics the natural disease pathogenesis route whereas IC inoculation is rather artificial; however, it is very efficient since it requires smaller dosage of inoculum, and typically results in higher attack rates and reduces incubation time compared to oral transmission. A species resistant to a TSE by IC inoculation would have negligible potential for successful oral transmission. To date, results indicate that cattle are susceptible to IC inoculation of scrapie, TME, and CWD but it is only when inoculated with TME do they develop spongiform lesions or clinical disease similar to BSE. Importantly, cattle are resistant to oral transmission of scrapie or CWD; susceptibility of cattle to oral transmission of TME is not yet determined.

Classical scrapie prions in ovine blood are associated with B lymphocytes and platelet-rich plasma

Background

Classical scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep and goats characterized by cellular accumulation of abnormal isoforms of prion protein (PrP^Sc) in the central nervous system and the follicles of peripheral lymphoid tissues. Previous studies have shown that the whole blood and buffy coat blood fraction of scrapie infected sheep harbor prion infectivity. Although PrP^Sc has been detected in peripheral blood mononuclear cells (PBMCs), plasma, and more recently within a subpopulation of B lymphocytes, the infectivity status of these cells and plasma in sheep remains unknown. Therefore, the objective of this study was to determine whether circulating PBMCs, B lymphocytes and platelets from classical scrapie infected sheep harbor prion infectivity using a sheep bioassay.

Results

Serial rectal mucosal biopsy and immunohistochemistry were used to detect preclinical infection in lambs transfused with whole blood or blood cell fractions from preclinical or clinical scrapie infected sheep. PrP^Sc immunolabeling was detected in antemortem rectal and postmortem lymphoid tissues from recipient lambs receiving PBMCs (15/15), CD72⁺ B lymphocytes (3/3), CD21⁺ B lymphocytes (3/3) or platelet-rich plasma (2/3) fractions. As expected, whole blood (11/13) and buffy coat (5/5) recipients showed positive PrP^Sc labeling in lymphoid follicles. However, at 549 days post-transfusion, PrP^Sc was not detected in rectal or other lymphoid tissues in three sheep receiving platelet-poor plasma fraction.

Conclusions

Prion infectivity was detected in circulating PBMCs, CD72⁺ pan B lymphocytes, the CD21⁺ subpopulation of B lymphocytes and platelet-rich plasma of classical scrapie infected sheep using a sheep bioassay. Combining platelets with B lymphocytes might enhance PrP^Sc detection levels in blood samples.

Has prion protein important physiological function?

Despite the great effort aimed at uncovering the physiological function of cellular prion protein, its role remains unclear. The highly conserved amino acid sequence of PrP indicates its important function, but normally developing PrP knockout mice and cattle were prepared. Here we propose hypothesis that prion protein has no function or a redundant one and more importantly, that the conserved amino acid sequence of mammalian PrPs is not the result of their important function, but rather due to cytotoxicity of most mutations occurring in the PrP molecule. It is possible that the majority of mutations in PrP dramatically destabilizes the PrP(C) structure and causes a pathological change in conformation, so that natural selection favours individuals with non-mutated PrP.

Chronic wasting disease

Chronic wasting disease (CWD) is a prion disease of free-ranging and farmed ungulates (deer, elk, and moose) in North America and South Korea. First described by the late E.S. Williams and colleagues in northern Colorado and southern Wyoming in the 1970s, CWD has increased tremendously both in numerical and geographical distribution, reaching prevalence rates as high as 50% in free-ranging and >90% in captive deer herds in certain areas of USA and Canada. CWD is certainly the most contagious prion infection, with significant horizontal transmission of infectious prions by, e.g., urine, feces, and saliva. Dissemination and persistence of infectivity in the environment combined with the appearance in wild-living and migrating animals make CWD presently uncontrollable, and pose extreme challenges to wild-life disease management. Whereas CWD is extremely transmissible among cervids, its trans-species transmission seems to be restricted, although the possible involvement of rodent and carnivore species in environmental transmission has not been fully evaluated. Whether or not CWD has zoonotic potential as had Bovine spongiform encephalopathy (BSE) has yet to be answered. Of note, variant Creutzfeldt-Jakob disease (vCJD) was only detected because clinical presentation and age of patients were significantly different from classical CJD. Along with further understanding of the molecular biology and pathology of CWD, its transmissibility and species restrictions and development of methods for preclinical diagnosis and intervention will be crucial for effective containment of this highly contagious prion disease.

Association analysis of PRNP gene region with chronic wasting disease in Rocky Mountain elk

Background

Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE) of cervids including white-tailed (Odocoileus virginianus) and mule deer (Odocoileus hemionus), Rocky Mountain elk (Cervus elaphus nelsoni), and moose (Alces alces). A leucine variant at position 132 (132L) in prion protein of Rocky Mountain elk confers a long incubation time with CWD, but not complete resistance. However, variants in regulatory regions outside the open reading frame of PRNP have been associated with varying degrees of susceptibility to prion disease in other species, and some variants have been observed in similar regions of Rocky Mountain elk PRNP. Thus, additional genetic variants might provide increased protection, either alone or in combination with 132L.

Findings

This study provided genomic sequence of all exons for PRNP of Rocky Mountain elk. Many functional sites in and around the PRNP gene region were sequenced, and this report approximately doubled (to 75) the number of known variants in this region. A haplotype-tagging approach was used to reduce the number of genetic variants required to survey this variation in the PRNP gene region of 559 Rocky Mountain elk. Eight haplotypes were observed with frequencies over 1.0%, and one haplotype was present at 71.2% frequency, reflecting limited genetic diversity in the PRNP gene region.

Conclusions

The presence of 132L cut odds of CWD by more than half (Odds Ratio = 0.43; P = 0.0031), which was similar to a previous report. However after accounting for 132L, no association with CWD was found for any additional variants in the PRNP region (P > 0.05).

Detection of the abnormal isoform of the prion protein associated with chronic wasting disease in the optic pathways of the brain and retina of Rocky Mountain elk (Cervus elaphus nelsoni)

Eyes and nuclei of the visual pathways in the brain were examined in 30 Rocky Mountain elk (Cervus elaphus nelsoni) representing 3 genotypes of the prion protein gene PRNP (codon 132: MM, ML, or LL). Tissues were examined for the presence of the abnormal isoform of the prion protein associated with chronic wasting disease (PrP(CWD)). Nuclei and axonal tracts from a single section of brain stem at the level of the dorsal motor nucleus of the vagus nerve were scored for intensity and distribution of PrP(CWD) immunoreactivity and degree of spongiform degeneration. This obex scoring ranged from 0 (elk with no PrP(CWD) in the brain stem) to 10 (representing elk in terminal stage of disease). PrP(CWD) was detected in the retina of 16 of 18 (89%) elk with an obex score of > 7. PrP(CWD) was not detected in the retina of the 3 chronic wasting disease-negative elk and 9 elk with an obex score of < 6. PrP(CWD) was found in the nuclei of the visual pathways in the brain before it was found in the retina. Within the retina, PrP(CWD) was first found in the inner plexiform layer, followed by the outer plexiform layer. Intracytoplasmic accumulation of PrP(CWD) was found in a few neurons in the ganglion cell layer in the PRNP 132ML elk but was a prominent feature in the PRNP 132LL elk. Small aggregates of PrP(CWD) were present on the inner surface of the outer limiting membrane in PRNP 132LL elk but not in PRNP 132MM or 132ML elk. This study demonstrates PrP(CWD) accumulation in nuclei of the visual pathways of the brain, followed by PrP(CWD) in the retina.