Environmentally-relevant forms of the prion protein

Chronic Wasting Disease (CWD) in Sami Reindeer Herding: The Socio-Political Dimension of an Epizootic in an Indigenous Context

Chronic wasting disease (CWD) is the most transmissible of the prion diseases. In 2016, an unexpected case was found in Norway, the first in Europe. Since then, there have been 32 confirmed cases in Norway, Sweden, and Finland. This paper aims to examine the situation from a social and political perspective: considering the management of CWD in the Swedish part of Sápmi-the Sami ancestral land; identifying the place of the Sami people in the risk management-because of the threats to Sami reindeer herding that CWD presents; and understanding how the disease can modify the modalities of Indigenous reindeer husbandry, whether or not CWD is epizootic. Based on interviews with various stakeholders and by examining the social sciences literature, this paper shows that the health risk management is structured by a politico-scientific controversy about the recognition, or not, of atypical and classical CWD. The Sami herders are currently cooperating with the state authorities in the surveillance program to sample their herds. This involvement takes place in a situation where the balance of power between the Sami people and the state, or the European Union, is framed by its colonial context. This has consequences with respect to the definition of a common interest and to implementing sanitary measures. The particular features of reindeer herding are seen as a challenge to managing CWD risk, compared with European health standards. We argue that CWD will greatly modify the modalities of Indigenous reindeer herding, whether there are positive cases or not in the Sami reindeer. By implementing new health guidelines, the authorities will create a cascading effect in Sami land and its use. The CWD situation in Fennoscandia is full of uncertainty but may cause a major shift in the organization and the governance of Sápmi. In September 2020, the identification of a new CWD case in a wild reindeer in Norway started a new episode in the disease management in Fennoscandia. Our paper raises various questions linked to understanding this new step in this crisis which is not only epidemiological, but also socio-cultural and political.

Shedding and stability of CWD prion seeding activity in cervid feces

CWD is an emergent prion disease that now affects cervid species on three continents. CWD is efficiently spread in wild and captive populations, likely through both direct animal contact and environmental contamination. Here, by longitudinally assaying in feces of CWD-exposed white-tailed deer by RT-QuIC, we demonstrate fecal shedding of prion seeding activity months before onset of clinical symptoms and continuing throughout the disease course. We also examine the impact of simulated environmental conditions such as repeated freeze-thaw cycles and desiccation on fecal prion seeding activity. We found that while multiple (n = 7) freeze-thaw cycles substantially decreased fecal seeding activity, desiccation had little to no effect on seeding activity. Finally, we examined whether RT-QuIC testing of landscape fecal deposits could distinguish two premises with substantial known CWD prevalence from one in which no CWD-infected animals had been detected. In the above pilot study, this distinction was possible. We conclude that fecal shedding of CWD prions occurs over much of the disease course, that environmental factors influence prion seeding activity, and that it is feasible to detect fecal prion contamination using RT-QuIC.

Chronic wasting disease: an evolving prion disease of cervids

Chronic wasting disease (CWD) is a relatively new and burgeoning prion epidemic of deer, elk, reindeer, and moose, which are members of the cervid family. While the disease was first described in captive deer, its subsequent discovery in various species of free-ranging animals makes it the only currently recognized prion disorder of both wild and farmed animals. In addition to its expanding range of host species, CWD continues to spread from North America to new geographic areas, including South Korea, and most recently Norway, marking the first time this disease was detected in Europe. Its unparalleled efficiency of contagious transmission, combined with high densities of deer in certain areas, complicates strategies for controlling CWD, raising concerns about its potential for spread to new species. Because there is a high prevalence of CWD in deer and elk, which are commonly hunted and consumed by humans, and since prions from cattle with bovine spongiform encephalopathy have been transmitted to humans causing variant Creutzfeldt-Jakob disease, the possibility of zoonotic transmission of CWD is particularly concerning. Here we review the clinical and pathologic features of CWD and its disturbing epidemiology, and discuss features that affect its transmission, including genetic susceptibility, pathogenesis, and agent strain variability. Finally, we discuss evidence that speaks to the potential for zoonotic transmission of this emerging disease.

Prion formation, but not clearance, is supported by protein misfolding cyclic amplification

Prion diseases are fatal transmissible neurodegenerative disorders that affect animals including humans. The kinetics of prion infectivity and PrP(Sc) accumulation can differ between prion strains and within a single strain in different tissues. The net accumulation of PrP(Sc) in animals is controlled by the relationship between the rate of PrP(Sc) formation and clearance. Protein misfolding cyclic amplification (PMCA) is a powerful technique that faithfully recapitulates PrP(Sc) formation and prion infectivity in a cell-free system. PMCA has been used as a surrogate for animal bioassay and can model species barriers, host range, strain co-factors and strain interference. In this study we investigated if degradation of PrP(Sc) and/or prion infectivity occurs during PMCA. To accomplish this we performed PMCA under conditions that do not support PrP(Sc) formation and did not observe either a reduction in PrP(Sc) abundance or an extension of prion incubation period, compared to untreated control samples. These results indicate that prion clearance does not occur during PMCA. These data have significant implications for the interpretation of PMCA based experiments such as prion amplification rate, adaptation to new species and strain interference where production and clearance of prions can affect the outcome.

Longitudinal Detection of Prion Shedding in Saliva and Urine by Chronic Wasting Disease-Infected Deer by Real-Time Quaking-Induced Conversion

Chronic wasting disease (CWD) is an emergent, rapidly spreading prion disease of cervids. Shedding of infectious prions in saliva and urine is thought to be an important factor in CWD transmission. To help to elucidate this issue, we applied an in vitro amplification assay to determine the onset, duration, and magnitude of prion shedding in longitudinally collected saliva and urine samples from CWD-exposed white-tailed deer. We detected prion shedding as early as 3 months after CWD exposure and sustained shedding throughout the disease course. We estimated that the 50% lethal dose (LD50) for cervidized transgenic mice would be contained in 1 ml of infected deer saliva or 10 ml of urine. Given the average course of infection and daily production of these body fluids, an infected deer would shed thousands of prion infectious doses over the course of CWD infection. The direct and indirect environmental impacts of this magnitude of prion shedding on cervid and noncervid species are surely significant.

IMPORTANCE

Chronic wasting disease (CWD) is an emerging and uniformly fatal prion disease affecting free-ranging deer and elk and is now recognized in 22 U.S. states and 2 Canadian provinces. It is unique among prion diseases in that it is transmitted naturally through wild populations. A major hypothesis to explain CWD's florid spread is that prions are shed in excreta and transmitted via direct or indirect environmental contact. Here we use a rapid in vitro assay to show that infectious doses of CWD prions are in fact shed throughout the multiyear disease course in deer. This finding is an important advance in assessing the risks posed by shed CWD prions to animals as well as humans.

Can plants serve as a vector for prions causing chronic wasting disease?

Prions, the causative agent of chronic wasting disease (CWD) enter the environment through shedding of bodily fluids and carcass decay, posing a disease risk as a result of their environmental persistence. Plants have the ability to take up large organic particles, including whole proteins, and microbes. This study used wheat (Triticum aestivum L.) to investigate the uptake of infectious CWD prions into roots and their transport into aerial tissues. The roots of intact wheat plants were exposed to infectious prions (PrP(TSE)) for 24 h in three replicate studies with PrP(TSE) in protein extracts being detected by western blot, IDEXX and Bio-Rad diagnostic tests. Recombinant prion protein (PrP(C)) bound to roots, but was not detected in the stem or leaves. Protease-digested CWD prions (PrP(TSE)) in elk brain homogenate interacted with root tissue, but were not detected in the stem. This suggests wheat was unable to transport sufficient PrP(TSE) from the roots to the stem to be detectable by the methods employed. Undigested PrP(TSE) did not associate with roots. The present study suggests that if prions are transported from the roots to the stems it is at levels that are below those that are detectable by western blot, IDEXX or Bio-Rad diagnostic kits.

Quantitative assessment of prion infectivity in tissues and body fluids by real-time quaking-induced conversion

Prions are amyloid-forming proteins that cause transmissible spongiform encephalopathies through a process involving the templated conversion of the normal cellular prion protein (PrP(C)) to a pathogenic misfolded conformation. Templated conversion has been modelled in several in vitro assays, including serial protein misfolding amplification, amyloid seeding and real-time quaking-induced conversion (RT-QuIC). As RT-QuIC measures formation of amyloid fibrils in real-time, it can be used to estimate the rate of seeded conversion. Here, we used samples from deer infected with chronic wasting disease (CWD) in RT-QuIC to show that serial dilution of prion seed was linearly related to the rate of amyloid formation over a range of 10(-3) to 10(-8) µg. We then used an amyloid formation rate standard curve derived from a bioassayed reference sample (CWD+ brain homogenate) to estimate the prion seed concentration and infectivity in tissues, body fluids and excreta. Using these methods, we estimated that urine and saliva from CWD-infected deer both contained 1-5 LD50 per 10 ml. Thus, over the 1-2 year course of an infection, a substantial environmental reservoir of CWD prion contamination accumulates.

Attachment of pathogenic prion protein to model oxide surfaces

Prions are the infectious agents in the class of fatal neurodegenerative diseases known as transmissible spongiform encephalopathies, which affect humans, deer, sheep, and cattle. Prion diseases of deer and sheep can be transmitted via environmental routes, and soil is has been implicated in the transmission of these diseases. Interaction with soil particles is expected to govern the transport, bioavailability and persistence of prions in soil environments. A mechanistic understanding of prion interaction with soil components is critical for understanding the behavior of these proteins in the environment. Here, we report results of a study to investigate the interactions of prions with model oxide surfaces (Al2O3, SiO2) using quartz crystal microbalance with dissipation monitoring and optical waveguide light mode spectroscopy. The efficiency of prion attachment to Al2O3 and SiO2 depended strongly on pH and ionic strength in a manner consistent with electrostatic forces dominating interaction with these oxides. The presence of the N-terminal portion of the protein appeared to promote attachment to Al2O3 under globally electrostatically repulsive conditions. We evaluated the utility of recombinant prion protein as a surrogate for prions in attachment experiments and found that its behavior differed markedly from that of the infectious agent. Our findings suggest that prions would tend to associate with positively charged mineral surfaces in soils (e.g., iron and aluminum oxides).

Estimating prion adsorption capacity of soil by BioAssay of Subtracted Infectivity from Complex Solutions (BASICS)

Prions, the infectious agent of scrapie, chronic wasting disease and other transmissible spongiform encephalopathies, are misfolded proteins that are highly stable and resistant to degradation. Prions are known to associate with clay and other soil components, enhancing their persistence and surprisingly, transmissibility. Currently, few detection and quantification methods exist for prions in soil, hindering an understanding of prion persistence and infectivity in the environment. Variability in apparent infectious titers of prions when bound to soil has complicated attempts to quantify the binding capacity of soil for prion infectivity. Here, we quantify the prion adsorption capacity of whole, sandy loam soil (SLS) typically found in CWD endemic areas in Colorado; and purified montmorillonite clay (Mte), previously shown to bind prions, by BioAssay of Subtracted Infectivity in Complex Solutions (BASICS). We incubated prion positive 10% brain homogenate from terminally sick mice infected with the Rocky Mountain Lab strain of mouse-adapted prions (RML) with 10% SLS or Mte. After 24 hours samples were centrifuged five minutes at 200×g and soil-free supernatant was intracerebrally inoculated into prion susceptible indicator mice. We used the number of days post inoculation to clinical disease to calculate the infectious titer remaining in the supernatant, which we subtracted from the starting titer to determine the infectious prion binding capacity of SLS and Mte. BASICS indicated SLS bound and removed ≥ 95% of infectivity. Mte bound and removed lethal doses (99.98%) of prions from inocula, effectively preventing disease in the mice. Our data reveal significant prion-binding capacity of soil and the utility of BASICS to estimate prion loads and investigate persistence and decomposition in the environment. Additionally, since Mte successfully rescued the mice from prion disease, Mte might be used for remediation and decontamination protocols.

Biodegradation of specified risk material and fate of scrapie prions in compost

Composting may be a viable alternative to rendering and land filling for the disposal of specified risk material (SRM) provided that infectious prion proteins (PrP(TSE)) are inactivated. This study investigated the degradation of SRM and the fate of scrapie prions (PrP(Sc)) over 28 days in laboratory-scale composters, with and without feathers in the compost matrices. Compost was mixed at day 14 to generate a second heating cycle, with temperatures exceeding 65°C in the first cycle and 50°C in the second cycle. Approximately 63% and 77% of SRM was degraded after the first and second cycles, respectively. Inclusion of feathers in the compost matrices did not alter compost properties during composting other than increasing (P < 0.05) total nitrogen and reducing (P < 0.05) the C/N ratio. However, addition of feathers enhanced (P < 0.05) SRM degradation by 10% upon completion of experiment. Scrapie brain homogenates were spiked into manure at the start of composting and extracted using sodium dodecyl sulphate followed by detection using Western blotting (WB). Prior to composting, PrP(Sc) was detectable in manure with 1-2 log(10) sensitivity, but was not observable after 14 or 28 days of composting. This may have been due to either biological degradation of PrP(Sc) or the formation of complexes with compost components that precluded its detection.

Resistance of soil-bound prions to rumen digestion

Before prion uptake and infection can occur in the lower gastrointestinal system, ingested prions are subjected to anaerobic digestion in the rumen of cervids and bovids. The susceptibility of soil-bound prions to rumen digestion has not been evaluated previously. In this study, prions from infectious brain homogenates as well as prions bound to a range of soils and soil minerals were subjected to in vitro rumen digestion, and changes in PrP levels were measured via western blot. Binding to clay appeared to protect noninfectious hamster PrP^c from complete digestion, while both unbound and soil-bound infectious PrP^Sc proved highly resistant to rumen digestion. In addition, no change in intracerebral incubation period was observed following active rumen digestion of unbound hamster HY TME prions and HY TME prions bound to a silty clay loam soil. These results demonstrate that both unbound and soil-bound prions readily survive rumen digestion without a reduction in infectivity, further supporting the potential for soil-mediated transmission of chronic wasting disease (CWD) and scrapie in the environment.

Soil-mediated prion transmission: is local soil-type a key determinant of prion disease incidence?

Prion diseases, including chronic wasting disease (CWD) and scrapie, can be transmitted via indirect environmental routes. Animals habitually ingest soil, and results from laboratory experiments demonstrate prions can bind to a wide range of soils and soil minerals, retain the ability to replicate, and remain infectious, indicating soil could serve as a reservoir for natural prion transmission and a potential prion exposure route for humans. Preliminary epidemiological modeling suggests soil texture may influence the incidence of prion disease. These results are supported by experimental work demonstrating variance in prion interactions with soil, including variance in prion soil adsorption and soil-bound prion replication with respect to soil type. Thus, local soil type may be a key determinant of prion incidence. Further experimental and epidemiological work is required to fully elucidate the dynamics of soil-mediated prion transmission, an effort that should lead to effective disease management and mitigation strategies.

An enzymatic treatment of soil-bound prions effectively inhibits replication

Chronic wasting disease (CWD) and scrapie can be transmitted through indirect environmental routes, possibly via soil, and a practical decontamination strategy for prion-contaminated soil is currently unavailable. In the laboratory, an enzymatic treatment under environmentally relevant conditions (22°C, pH 7.4) can degrade soil-bound PrPSc below the limits of Western blot detection. We developed and used a quantitative serial protein misfolding cyclic amplification (PMCA) protocol to characterize the amplification efficiency of treated soil samples relative to controls of known infectious titer. Our results suggest large (10(4)- to >10(6)-fold) decreases in soil-bound prion infectivity following enzyme treatment, demonstrating that a mild enzymatic treatment could effectively reduce the risk of prion disease transmission via soil or other environmental surfaces.

Effects of solution chemistry and aging time on prion protein adsorption and replication of soil-bound prions

Prion interactions with soil may play an important role in the transmission of chronic wasting disease (CWD) and scrapie. Prions are known to bind to a wide range of soil surfaces, but the effects of adsorption solution chemistry and long-term soil binding on prion fate and transmission risk are unknown. We investigated HY TME prion protein (PrP(Sc)) adsorption to soil minerals in aqueous solutions of phosphate buffered saline (PBS), sodium chloride, calcium chloride, and deionized water using western blotting. The replication efficiency of bound prions following adsorption in these solutions was also evaluated by protein misfolding cyclic amplification (PMCA). Aging studies investigated PrP(Sc) desorption and replication efficiency up to one year following adsorption in PBS or DI water. Results indicate that adsorption solution chemistry can affect subsequent prion replication or desorption ability, especially after incubation periods of 30 d or longer. Observed effects were minor over the short-term (7 d or less). Results of long-term aging experiments demonstrate that unbound prions or prions bound to a diverse range of soil surfaces can readily replicate after one year. Our results suggest that while prion-soil interactions can vary with solution chemistry, prions bound to soil could remain a risk for transmitting prion diseases after months in the environment.

Replication efficiency of soil-bound prions varies with soil type

Prion sorption to soil is thought to play an important role in the transmission of scrapie and chronic wasting disease (CWD) via the environment. Sorption of PrP to soil and soil minerals is influenced by the strain and species of PrP(Sc) and by soil characteristics. However, the ability of soil-bound prions to convert PrP(c) to PrP(Sc) under these wide-ranging conditions remains poorly understood. We developed a semiquantitative protein misfolding cyclic amplification (PMCA) protocol to evaluate replication efficiency of soil-bound prions. Binding of the hyper (HY) strain of transmissible mink encephalopathy (TME) (hamster) prions to a silty clay loam soil yielded a greater-than-1-log decrease in PMCA replication efficiency with a corresponding 1.3-log reduction in titer. The increased binding of PrP(Sc) to soil over time corresponded with a decrease in PMCA replication efficiency. The PMCA efficiency of bound prions varied with soil type, where prions bound to clay and organic surfaces exhibited significantly lower replication efficiencies while prions bound to sand exhibited no apparent difference in replication efficiency compared to unbound controls. PMCA results from hamster and CWD agent-infected elk prions yielded similar findings. Given that PrP(Sc) adsorption affinity varies with soil type, the overall balance between prion adsorption affinity and replication efficiency for the dominant soil types of an area may be a significant determinant in the environmental transmission of prion diseases.

Fate of prions in soil: a review

Prions are the etiological agents of transmissible spongiform encephalopathies (TSSEs), a class of fatal neurodegenerative diseases affecting humans and other mammals. The pathogenic prion protein is a misfolded form of the host-encoded prion protein and represents the predominant, if not sole, component of the infectious agent. Environmental routes of TSE transmission areimplicated in epizootics of sheep scrapie and chronic wasting disease (CWD) of deer, elk, and moose. Soil represents a plausible environmental reservoir of scrapie and CWD agents, which can persist in the environment for years. Attachment to soil particles likely influences the persistence and infectivity of prions in the environment. Effective methods to inactivate TSE agents in soil are currently lacking, and the effects of natural degradation mechanisms on TSE infectivity are largely unknown. An improved understanding of the processes affecting the mobility, persistence, and bioaviailability of prions in soil is needed for the management of TSE-contaminated environments.

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).

Transport of the pathogenic prion protein through soils

Transmissible spongiform encephalopathies (TSEs) are progressive neurodegenerative diseases and include bovine spongiform encephalopathy of cattle, chronic wasting disease (CWD) of deer and elk, scrapie in sheep and goats, and Creutzfeldt-Jakob disease in humans. An abnormally folded form of the prion protein (designated PrP(TSE)) is typically associated with TSE infectivity and may constitute the major, if not sole, component of the infectious agent. Transmission of CWD and scrapie is mediated in part by an environmental reservoir of infectivity. Soil appears to be a plausible candidate for this reservoir. The transport of TSE agent through soil is expected to influence the accessibility of the pathogen to animals after deposition and must be understood to assess the risks associated with burial of infected carcasses. We report the results of saturated column experiments designed to evaluate PrP(TSE) transport through five soils with relatively high sand or silt contents and low organic carbon content. Protease-treated TSE-infected brain homogenate was used as a model for PrP(TSE) present in decomposing infected tissue. Synthetic rainwater was used as the eluent. All five soils retained PrP(TSE); no detectable PrP(TSE) was eluted over more than 40 pore volumes of flow. Lower bound apparent attachment coefficients were estimated for each soil. Our results suggest that TSE agent released from decomposing tissues to soils with low organic carbon content would remain near the site of initial deposition. In the case of infected carcasses deposited on the land surface, this may result in local sources of infectivity to other animals.

Enzymatic digestion of chronic wasting disease prions bound to soil

Chronic wasting disease (CWD) and sheep scrapie can be transmitted via indirect environmental routes, and it is known that soil can serve as a reservoir of prion infectivity. Given the strong interaction between the prion protein (PrP) and soil, we hypothesized that binding to soil enhances prion resistance to enzymatic digestion, thereby facilitating prion longevity in the environment and providing protection from host degradation. We characterized the performance of a commercially available subtilisin enzyme, Prionzyme, to degrade soil-bound and unbound CWD and HY TME PrP as a function of pH, temperature, and treatment time. The subtilisin enzyme effectively degraded PrP adsorbed to a wide range of soils and soil minerals below the limits of detection. Signal loss occurred rapidly at high pH (12.5) and within 7 days under conditions representative of the natural environment (pH 7.4, 22 degrees C). We observed no apparent difference in enzyme effectiveness between bound and unbound CWD PrP. Our results show that although adsorbed prions do retain relative resistance to enzymatic digestion compared with other brain homogenate proteins, they can be effectively degraded when bound to soil. Our results also suggest a topical application of a subtilisin enzyme solution may be an effective decontamination method to limit disease transmission via environmental "hot spots" of prion infectivity.

Prion protein adsorption to soil in a competitive matrix is slow and reduced

It is likely that the soil environment serves as a stable reservoir of infectious CWD and scrapie prions as well as a potential reservoir of BSE. Prion adsorption to soil could play an important role in prion mobility, proteolysis, and infectivity. We modified previously published methods to quantify adsorbed prions via direct detection and studied prion adsorption to soil and soil minerals as a function of time through 60 days. Prion-infected brain homogenate was used as a complex, relevant prion source. We determined that maximum PrP adsorption requires days or weeks, depending on the soil or mineral, and is 2-5 orders of magnitude lower than previous studies using purified PrP(Sc) or recPrP. Because PrP adsorption to soil is slow and reduced in tissue homogenate, the possibility of prion transport in soil environments cannot be excluded and requires further investigation. Our results indicate that binding to soil may protect prions from degradation, consistent with prions' longevity in the environment. Adsorption of PrP to sterilized soil did not differ significantly from adsorption to unsterilized soil, which suggests that active biological processes do not significantly affect prion adsorption or degradation in the soil environment.

Influence of prion strain on prion protein adsorption to soil in a competitive matrix

It is likely that the soil environment serves as a stable reservoir of infectious chronic wasting disease (CWD) and scrapie prions, as well as a potential reservoir of bovine spongiform encephalopathy (BSE, or "mad cow" disease). Prion adsorption to soil may play an important role in prion mobility, proteolysis, and infectivity. Differences in PrP environmental fate are possible due to the strain- and species-dependent structure of PrP(Sc). Kinetic and isothermal studies of PrP adsorption to sand and two whole soils were conducted using HY and DY TME-infected hamster, uninfected hamster, and CWD-infected elk brain homogenates as competitive PrP sources. The role of the N-terminus in PrP adsorption was also investigated. We report strain and species differences in PrP adsorption to soil over time and as a function of aqueous concentration, indicating that the fate of prions in the environment may vary with the prion strain and species infected. Our data also provide evidence that the N-terminal region of PrP enhances adsorption to clay but may hinder adsorption to sand. PrP adsorption was maximal at an intermediate aqueous concentration, most likely due to the competitive brain homogenate matrix in which it enters the soil environment.

Transport of the pathogenic prion protein through landfill materials

Transmissible spongiform encephalopathies (TSEs, prion diseases) are a class of fatal neurodegenerative diseases affecting a variety of mammalian species including humans. A misfolded form of the prion protein (PrP(TSE)) is the major, if not sole, component of the infectious agent RecentTSE outbreaks in domesticated and wild animal populations have created the need for safe and effective disposal of large quantities of potentially infected materials. Here, we report results of a study to evaluate the potential for transport of PrP(TSE) derived from carcasses and associated wastes in municipal solid waste (MSW) landfills. Column experiments were conducted to evaluate PrP(TSE) transport in quartz sand, two fine-textured burial soils currently used in landfill practice, a green waste residual material (a potential burial material), and fresh and aged MSW. PrP(TSE) was retained by quartz sand and the fine-textured burial soils, with no detectable PrP(TSE) eluted over more than 40 pore volumes. In contrast, PrP(TSE) was more mobile in MSW and green waste residual. Transport parameters were estimated from the experimental data and used to model PrP(TSE) migration in a MSW landfill. To the extent that the PrP(TSE) used mimics that released from decomposing carcasses and the column experiments adequately simulate prion transport through burial soils, burial of CWD-infected materials at MSW landfills could provide secure containment of PrP(TSE) provided reasonable burial strategies (e.g., encasement in fine-grained soil) are used.

Prions in the environment: occurrence, fate and mitigation

Scrapie and CWD are horizontally transmissible, and the environment likely serves as a stable reservoir of infectious prions, facilitating a sustained incidence of CWD in free-ranging cervid populations and complicating efforts to eliminate disease in captive herds. Prions will enter the environment through mortalities and/or shedding from live hosts. Unfortunately, a sensitive detection method to identify prion contamination in environmental samples has not yet been developed. An environmentally-relevant prion model must be used in experimental studies. Changes in PrP(Sc) structure upon environmental exposure may be as significant as changes in PrP(Sc) quantity, since the structure can directly affect infectivity and disease pathology. Prions strongly bind to soil and remain infectious. Conformational changes upon adsorption, competitive sorption and potential for desorption and transport all warrant further investigation. Mitigation of contaminated carcasses or soil might be accomplished with enzyme treatments or composting in lieu of incineration.