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

Emerging contaminants: A One Health perspective

Environmental pollution is escalating due to rapid global development that often prioritizes human needs over planetary health. Despite global efforts to mitigate legacy pollutants, the continuous introduction of new substances remains a major threat to both people and the planet. In response, global initiatives are focusing on risk assessment and regulation of emerging contaminants, as demonstrated by the ongoing efforts to establish the UN's Intergovernmental Science-Policy Panel on Chemicals, Waste, and Pollution Prevention. This review identifies the sources and impacts of emerging contaminants on planetary health, emphasizing the importance of adopting a One Health approach. Strategies for monitoring and addressing these pollutants are discussed, underscoring the need for robust and socially equitable environmental policies at both regional and international levels. Urgent actions are needed to transition toward sustainable pollution management practices to safeguard our planet for future generations.

Sensitive detection of chronic wasting disease prions recovered from environmentally relevant surfaces

Chronic wasting disease (CWD) has been identified in 30 states in the United States, four provinces in Canada, and recently emerged in Scandinavia. The association of CWD prions with environmental materials such as soil, plants, and surfaces may enhance the persistence of CWD prion infectivity in the environment exacerbating disease transmission. Identifying and quantifying CWD prions in the environment is significant for prion monitoring and disease transmission control. A systematic method for CWD prion quantification from associated environmental materials, however, does not exist. In this study, we developed an innovative method for extracting prions from swabs and recovering CWD prions swabbed from different types of surfaces including glass, stainless steel, and wood. We found that samples dried on swabs were unfavorable for prion extraction, with the greatest prion recovery from wet swabs. Using this swabbing technique, the recovery of CWD prions dried to glass or stainless steel was approximately 30% in most cases, whereas that from wood was undetectable by conventional prion immunodetection techniques. Real-time quake-induced conversion (RT-QuIC) analysis of these same samples resulted in an increase of the detection limit of CWD prions from stainless steel by 4 orders of magnitude. More importantly, the RT-QuIC detection of CWD prions recovered from stainless steel surfaces using this method was similar to the original CWD prion load applied to the surface. This combined surface swabbing and RT-QuIC detection method provides an ultrasensitive means for prion detection across many settings and applications.

Environmental and host factors that contribute to prion strain evolution

Prions are novel pathogens that are composed entirely of PrPSc, the self-templating conformation of the host prion protein, PrPC. Prion strains are operationally defined as a heritable phenotype of disease that are encoded by strain-specific conformations of PrPSc. The factors that influence the relative distribution of strains in a population are only beginning to be understood. For prions with an infectious etiology, environmental factors, such as strain-specific binding to surfaces and resistance to weathering, can influence which strains are available for transmission to a naïve host. Strain-specific differences in efficiency of infection by natural routes of infection can also select for prion strains. The host amino acid sequence of PrPC has the greatest effect on dictating the repertoire of prion strains. The relative abundance of PrPC, post-translational modifications of PrPC and cellular co-factors involved in prion conversion can also provide conditions that favor the prevalence of a subset of prion strains. Additionally, prion strains can interfere with each other, influencing the emergence of a dominant strain. Overall, both environmental and host factors may influence the repertoire and distribution of strains within a population.

Alteration of Prion Strain Emergence by Nonhost Factors

The prion strain, surface type, and matrix containing PrP^Sc can influence PrP^Sc surface adsorption. The cumulative effect of these factors can result in strain- and soil-specific differences in prion bioavailability. Environmental weathering processes can result in decreases in PrP^Sc conversion efficiency and infectivity. Little is known about how incomplete inactivation of surface-bound PrP^Sc affects transmission and prion strain emergence. Here, we show that strain interference occurs with soil-bound prions and that altering the ratios of prion strains by strain-specific inactivation can affect strain emergence. Additionally, we identify a novel mechanism of inhibition of prion conversion by environmental treatment-induced changes at the soil-protein interface altering strain emergence. These novel findings suggest that environmental factors can influence strain emergence of surface-bound prions.

Prions can persist in the environment for extended periods of time after adsorption to surfaces, including soils, feeding troughs, or fences. Prion strain- and soil-specific differences in prion adsorption, infectivity, and response to inactivation may be involved in strain maintenance or emergence of new strains in a population. Extensive proteinase K (PK) digestion of Hyper (HY) and Drowsy (DY) PrP^Sc resulted in a greater reduction in the level of DY PrP^Sc than of HY PrP^Sc. Use of the PK-digested material in protein misfolding cyclic amplification strain interference (PMCAsi) resulted in earlier emergence of HY PrP^Sc than of undigested controls. This result established that strain-specific alteration of the starting ratios of conversion-competent HY and DY PrP^Sc can alter strain emergence. We next investigated whether environmentally relevant factors such as surface binding and weathering could alter strain emergence. Adsorption of HY and DY PrP^Sc to silty clay loam (SCL), both separately and combined, resulted in DY interfering with the emergence of HY in PMCAsi in a manner similar to that seen with unbound controls. Similarly, repeated cycles of wetting and drying of SCL-bound HY and DY PrP^Sc did not alter the emergence of HY PrP^Sc compared to untreated controls. Importantly, these data indicate that prion strain interference can occur when prions are bound to surfaces. Interestingly, we found that drying of adsorbed brain homogenate on SCL could restore its ability to interfere with the emergence of HY, suggesting a novel strain interference mechanism. Overall, these data provide evidence that the emergence of a strain from a mixture can be influenced by nonhost factors.

IMPORTANCE The prion strain, surface type, and matrix containing PrP^Sc can influence PrP^Sc surface adsorption. The cumulative effect of these factors can result in strain- and soil-specific differences in prion bioavailability. Environmental weathering processes can result in decreases in PrP^Sc conversion efficiency and infectivity. Little is known about how incomplete inactivation of surface-bound PrP^Sc affects transmission and prion strain emergence. Here, we show that strain interference occurs with soil-bound prions and that altering the ratios of prion strains by strain-specific inactivation can affect strain emergence. Additionally, we identify a novel mechanism of inhibition of prion conversion by environmental treatment-induced changes at the soil-protein interface altering strain emergence. These novel findings suggest that environmental factors can influence strain emergence of surface-bound prions.

Soil humic acids degrade CWD prions and reduce infectivity

Chronic wasting disease (CWD), an environmentally transmissible, fatal prion disease is endemic in North America, present in South Korea and has recently been confirmed in northern Europe. The expanding geographic range of this contagious disease of free-ranging deer, moose, elk and reindeer has resulted in increasing levels of prion infectivity in the environment. Soils are involved in CWD horizontal transmission, acting as an environmental reservoir, and soil mineral and organic compounds have the ability to bind prions. Upper horizons of soils are usually enriched with soil organic matter (SOM), however, the role of SOM in prion conservation and mobility remains unclear. In this study, we show that incubation of PrP^CWD with humic acids (HA), a major SOM compound, affects both the molecular weight and recovery of PrP^CWD. Detection of PrP^CWD is reduced as HA concentration increases. Native HA extracted from pristine soils also reduces or entirely eliminates PrP^CWD signal. Incubation of CWD prions with HA significantly increased incubation periods in tgElk mice demonstrating that HA can reduce CWD infectivity.

Chronic wasting disease (CWD) is a contagious prion disease affecting several species of captive and wild cervids. Environmental prion contamination plays a major role in increasing incidence of CWD, with CWD infectivity being released into the environment by decaying carcasses, or shedding of biological fluids including urine, feces, and saliva. Horizontal transmission of CWD involves soils as an environmental reservoir of infectivity. Here, we tested the role of a soil organic matter compound, humic acid, for its ability to bind CWD prions and impact infectivity. A wide range of humic acid concentrations were examined representing the extensive spectrum of humic acid levels present in native soils. We found that incubation of CWD prions with high concentrations of humic acids (>2.5 g L^-1) decreases the both CWD-prion signal and infectivity, whereas lower levels of humic acids did not significantly impact protein stability or infectivity. Our study provides new insights into soil-prion interactions, prions persistence in soil, and their bioavailability to grazing animals.

Dehydration of Prions on Environmentally Relevant Surfaces Protects Them from Inactivation by Freezing and Thawing

Chronic wasting disease (CWD) is an emerging prion disease in North America. Recent identification of CWD in wild cervids from Norway raises the concern of the spread of CWD in Europe. CWD infectivity can enter the environment through live animal excreta and carcasses where it can bind to soil. Well-characterized hamster prion strains and CWD field isolates in unadsorbed or soil-adsorbed forms that were either hydrated or dehydrated were subjected to repeated rounds of freezing and thawing. We found that 500 cycles of repeated freezing and thawing of hydrated samples significantly decreased the abundance of PrP^Sc and reduced protein misfolding cyclic amplification (PMCA) seeding activity that could be rescued by binding to soil. Importantly, dehydration prior to freezing and thawing treatment largely protected PrP^Sc from degradation, and the samples maintained PMCA seeding activity. We hypothesize that redistribution of water molecules during the freezing and thawing process alters the stability of PrP^Sc aggregates. Overall, these results have significant implications for the assessment of prion persistence in the environment.IMPORTANCE Prions excreted into the environment by infected animals, such as elk and deer infected with chronic wasting disease, persist for years and thus facilitate horizontal transmission of the disease. Understanding the fate of prions in the environment is essential to control prion disease transmission. The significance of our study is that it provides information on the possibility of prion degradation and inactivation under natural weathering processes. This information is significant for remediation of prion-contaminated environments and development of prion disease control strategies.

Protecting effect of PrP codons M142 and K222 in goats orally challenged with bovine spongiform encephalopathy prions

Breeding towards genetic resistance to prion disease is effective in eliminating scrapie. In sheep, classical forms of scrapie have been eradicated almost completely in several countries by breeding programs using a prion protein (PrP) gene (PRNP) amino acid polymorphism. For goats, field and experimental studies have provided evidence for several amino acid polymorphisms that are associated with resistance to scrapie, but only limited data are available concerning the susceptibility of caprine PRNP genotypes to BSE. In this study, goat kids representing five PRNP genotypes based on three polymorphisms (M142, Q211 and K222 and the wild type I142, R211 and Q222) were orally challenged with bovine or goat BSE. Wild type goats were killed with clinical signs between 24-28 months post inoculation (mpi) to both challenges, and goats with genotype R/Q211 succumbed between 29-36 mpi. I/M142 goats developed clinical signs at 44-45 mpi and M/M142 goats remained healthy until euthanasia at 48 mpi. None of the Q/K222 goats showed definite clinical signs. Taken together the highest attack ratios were seen in wild type and R/Q211 goats, and the lowest in I/M142, M/M142 and Q/K222. In all genotype groups, one or more goats remained healthy within the incubation period in both challenges and without detectable PrP deposition in the tissues. Our data show that both the K222 and M142 polymorphisms lengthen the incubation period significantly compared to wild type animals, but only K222 was associated with a significant increase in resistance to BSE infection after oral exposure to both BSE sources.

The Ecology of Prions

Chronic wasting disease (CWD) affects cervids and is the only known prion disease readily transmitted among free-ranging wild animal populations in nature. The increasing spread and prevalence of CWD among cervid populations threaten the survival of deer and elk herds in North America, and potentially beyond. This review focuses on prion ecology, specifically that of CWD, and the current understanding of the role that the environment may play in disease propagation. We recount the discovery of CWD, discuss the role of the environment in indirect CWD transmission, and consider potentially relevant environmental reservoirs and vectors. We conclude by discussing how understanding the environmental persistence of CWD lends insight into transmission dynamics and potential management and mitigation strategies.

Mitigation of prion infectivity and conversion capacity by a simulated natural process--repeated cycles of drying and wetting

Prions enter the environment from infected hosts, bind to a wide range of soil and soil minerals, and remain highly infectious. Environmental sources of prions almost certainly contribute to the transmission of chronic wasting disease in cervids and scrapie in sheep and goats. While much is known about the introduction of prions into the environment and their interaction with soil, relatively little is known about prion degradation and inactivation by natural environmental processes. In this study, we examined the effect of repeated cycles of drying and wetting on prion fitness and determined that 10 cycles of repeated drying and wetting could reduce PrP^Sc abundance, PMCA amplification efficiency and extend the incubation period of disease. Importantly, prions bound to soil were more susceptible to inactivation by repeated cycles of drying and wetting compared to unbound prions, a result which may be due to conformational changes in soil-bound PrP^Sc or consolidation of the bonding between PrP^Sc and soil. This novel finding demonstrates that naturally-occurring environmental process can degrade prions.

Prion diseases such as chronic wasting disease and scrapie are emerging in North America at an increasing rate. Infectious prions are introduced into the environment from both living and dead animals where they can bind to soil. Little information is available on the effect of prion inactivation under conditions that would be found in the natural environment. In this study, we exposed both unbound and soil-bound prions to repeated cycles of drying and wetting to simulate ambient environmental conditions. We found evidence of prion inactivation in both unbound and soil bound prions. The influence of repeated cycles of drying and wetting are dependent on the prion strain and soil type used and, interestingly, prions bound to soil were more susceptible to inactivation. This is the first report of natural environmental processes mitigating prion infectivity. This data suggests that the total environmental prion load is a balance between input and natural clearance.

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.

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.

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.

Superparamagnetic nanoparticle capture of prions for amplification

Prion diseases are associated with the presence of PrP(Sc), a disease-associated misfolded conformer of the prion protein. We report that superparamagnetic nanoparticles bind PrP(Sc) molecules efficiently and specifically, permitting magnetic separation of prions from a sample mixture. Captured PrP(Sc) molecules retain the activity to seed protein misfolding cyclic amplification (PMCA) reactions, enabling the rapid concentration of dilute prions to improve detection. Furthermore, superparamagnetic nanoparticles clear contaminated solutions of PrP(Sc). Our findings suggest that coupling magnetic nanoparticle capture with PMCA could accelerate and improve prion detection. Magnetic nanoparticles may also be useful for developing a nontoxic prion decontamination method for biologically derived products.

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.