Direct detection of soil-bound prions

A review of cleaning and disinfection guidelines and recommendations following an outbreak of classical scrapie

Classical scrapie is a prion disease of small ruminants, the infectious agent of which has been shown to be extremely persistent in the environment. Cleaning and disinfection (C&D) after a scrapie outbreak is currently recommended by many governments' veterinary advisors and implemented in most farms affected. Yet, the effectiveness of these procedures remains unclear. The aim of this study was to review existing literature and guidelines regarding farm C&D protocols following classical scrapie outbreaks and assess their effectiveness and the challenges that translation of policy and legislative requirements present at a practical level. A review of the literature was conducted to identify the on-farm C&D protocols used following outbreaks of scrapie, assess those materials with high risk for persistence of the scrapie agent on farms, and review the existing evidence of the effectiveness of recommended C&D protocols. An expert workshop was also organised in Great Britain (GB) to assess: the decision-making process used when implementing C&D protocols on GB farms, the experts' perceptions on the effectiveness of these protocols and changes needed, and their views on potential recommendations for policy and research. Outputs of the literature review revealed that the current recommended protocol for C&D [1 h treatment with sodium hypochlorite containing 20,000 ppm free chlorine or 2 M sodium hydroxide (NaOH)] is based on laboratory experiments. Only four field farm experiments have been conducted, indicating a lack of data on effectiveness of C&D protocols on farms by the re-occurrence of scrapie infection post re-stocking. Recommendations related to the control of outdoor environment, which are difficult and expensive to implement, vary between countries. The expert workshop concluded that there are no practical, cost-effective C&D alternatives to be considered at this time, with control therefore based on C&D only in combination with additional time restrictions on re-stocking and replacement with non-susceptible livestock or more genetically resistant types, where available. Participants agreed that C&D should still be completed on scrapie affected farms, as it is considered to be "good disease practice" and likely to reduce the levels of the prion protein. Participants felt that any additional protocols developed should not be "too prescriptive" (should not be written down in specific policies) because of significant variation in farm types, farm equipment and installations. Under this scenario, control of classical scrapie on farms should be designed with a level of C&D in combination with re-stocking temporal ban and replacement with livestock of limited susceptibility.

Bovine spongiform encephalopathy (BSE) cases born after the total feed ban

Sixty bovine spongiform encephalopathy (BSE) cases of Classical or unknown type (BARB‐60 cases) were born after the date of entry into force of the EU total feed ban on 1 January 2001. The European Commission has requested EFSA to provide a scientific opinion on the most likely origin(s) of these BARB‐60 cases; whether feeding with material contaminated with the BSE agent can be excluded as the origin of any of these cases and, if so, whether there is enough scientific evidence to conclude that such cases had a spontaneous origin. The source of infection cannot be ascertained at the individual level for any BSE case, including these BARB‐60 cases, so uncertainty remains high about the origin of disease in each of these animals, but when compared with other biologically plausible sources of infection (maternal, environmental, genetic, iatrogenic), feed‐borne exposure is the most likely. This exposure was apparently excluded for only one of these BARB‐60 cases. However, there is considerable uncertainty associated with the data collected through the field investigation of these cases, due to a time span of several years between the potential exposure of the animal and the confirmation of disease, recall difficulty, and the general paucity of documented objective evidence available in the farms at the time of the investigation. Thus, feeding with material contaminated with the BSE agent cannot be excluded as the origin of any of the BARB‐60 cases, nor is it possible to definitively attribute feed as the cause of any of the BARB‐60 cases. A case of disease is classified as spontaneous by a process of elimination, excluding all other definable possibilities; with regard to the BARB‐60 cases, it is not possible to conclude that any of them had a spontaneous origin.

The standard scrapie cell assay: development, utility and prospects

Prion diseases are a family of fatal neurodegenerative diseases that involve the misfolding of a host protein, PrP^C. Measuring prion infectivity is necessary for determining efficacy of a treatment or infectivity of a prion purification procedure; animal bioassays are, however, very expensive and time consuming. The Standard Scrapie Cell Assay (SSCA) provides an alternative approach. The SSCA facilitates quantitative in vitro analysis of prion strains, titres and biological properties. Given its robust nature and potential for high throughput, the SSCA has substantial utility for in vitro characterization of prions and can be deployed in a number of settings. Here we provide an overview on establishing the SSCA, its use in studies of disease dissemination and pathogenesis, potential pitfalls and a number of remaining challenges.

Prion protein interaction with soil humic substances: environmental implications

Transmissible spongiform encephalopathies (TSE) are fatal neurodegenerative disorders caused by prions. Animal TSE include scrapie in sheep and goats, and chronic wasting disease (CWD) in cervids. Effective management of scrapie in many parts of the world, and of CWD in North American deer population is complicated by the persistence of prions in the environment. After shedding from diseased animals, prions persist in soil, withstanding biotic and abiotic degradation. As soil is a complex, multi-component system of both mineral and organic components, it is important to understand which soil compounds may interact with prions and thus contribute to disease transmission. Several studies have investigated the role of different soil minerals in prion adsorption and infectivity; we focused our attention on the interaction of soil organic components, the humic substances (HS), with recombinant prion protein (recPrP) material. We evaluated the kinetics of recPrP adsorption, providing a structural and biochemical characterization of chemical adducts using different experimental approaches. Here we show that HS act as potent anti-prion agents in prion infected neuronal cells and in the amyloid seeding assays: HS adsorb both recPrP and prions, thus sequestering them from the prion replication process. We interpreted our findings as highly relevant from an environmental point of view, as the adsorption of prions in HS may affect their availability and consequently hinder the environmental transmission of prion diseases in ruminants.

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.

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.

A simple, versatile and sensitive cell-based assay for prions from various species

Detection and quantification of prion infectivity is a crucial step for various fundamental and applied aspects of prion research. Identification of cell lines highly sensitive to prion infection led to the development of cell-based titration procedures aiming at replacing animal bioassays, usually performed in mice or hamsters. However, most of these cell lines are only permissive to mouse-adapted prions strains and do not allow titration of prions from other species. In this study, we show that epithelial RK13, a cell line permissive to mouse and bank vole prion strains and to natural prion agents from sheep and cervids, enables a robust and sensitive detection of mouse and ovine-derived prions. Importantly, the cell culture work is strongly reduced as the RK13 cell assay procedure designed here does not require subcultivation of the inoculated cultures. We also show that prions effectively bind to culture plastic vessel and are quantitatively detected by the cell assay. The possibility to easily quantify a wider range of prions, including rodent experimental strains but also natural agents from sheep and cervids, should prompt the spread of cell assays for routine prion titration and lead to valuable information in fundamental and applied studies.

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.

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.

Prions and manganese: A maddening beast

The prion protein is well known because of its association with prion diseases. These diseases, which include variant CJD, are unusual because they are neurodegenerative diseases that can be transferred between individuals experimentally. The prion protein is also widely known as a copper binding protein. The binding of copper to the prion protein is possibly necessary for its normal cellular function. The prion protein has also been suggested to bind other metals, and among these, manganese. Despite over ten years of research on manganese and prion disease, this interaction has often been dismissed or at best seen as a poor cousin to the involvement of copper. However, recent data has shown that manganese could stabilise prions in the environment and that chelation therapy specifically aimed at manganese can extend the life of animals with prion disease. This article reviews the evidence for a link between prions and manganese.

Sensitive detection of scrapie prion protein in soil

Prion diseases are fatal neurodegenerative disorders that are caused by infectious agents known as prions. Prions are composed primarily of the pathogenic prion protein isoform, PrP(Sc). Because significant levels of infectivity have been detected in excrement from animals infected with scrapie and chronic wasting disease, studies on the dynamics of PrP(Sc) levels in contaminated soil are needed to assess the possible horizontal transmission of prion diseases. Using protein misfolding cyclic amplification, we developed a sensitive detection method for scrapie PrP(Sc) that is mixed with soil. Our detection method has the advantage of not requiring extraction of PrP(Sc) from soil and could provide a sensitivity 1000 to 10,000 times higher than that obtained with an extraction-based method. In addition, we found that PrP(Sc) levels in experimentally contaminated agricultural soils declined to different extents over the course of a 6-month incubation period. Our method appears to be a very useful technique for monitoring PrP(Sc) levels in soil.

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.

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.

Fate of prions in soil: interactions of RecPrP with organic matter of soil aggregates as revealed by LTA-PAS

The contribution of soil organic matter (OM) to the adsorption of a recombinant prion protein (RecPrP) was studied in microcosm systems (soil aggregates from two different soils) before and after OM removal by low temperature ashing (LTA). The LTA technique allows a controlled removal of OM layer by layer, like a peeling of an onion skin, with minimal disturbance of the mineral matrix. Soil aggregates were selected as a representative model of the "in situ" conditions. Adsorption from batch vs percolation experiments were compared, and the aggregates were characterized by photoacustic Fourier-transform IR spectroscopy (PAS-FTIR). High affinity (H-type) adsorption isotherms were found with complete removal of RecPrP from solution for protein/soil ratios up to 1:62.5. OM removal from aggregates decreased the adsorbed RecPrP in amounts corresponding to 330-1000 microg mg(-1) of soil organic carbon (OC) indicating that native OM has specific adsorption capacity comparable and/or superior to the mineral matrix. The coupled LTA-PAS-FTIR approach demonstrated that, albeit OM composition was homogeneous throughout the aggregates, its presence in the most external surfaces of the aggregates affects the diffusion dynamics of RecPrP within the aggregates during percolation.