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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., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

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.

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.

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.

Scrapie and chronic wasting disease (CWD) are prion diseases of particular environmental concern as they are horizontally transmissible and can remain infectious after years in the environment. Recent evidence suggests that the N-terminus of PrPSC, the infectious conformation of the prion protein, plays an important role in the mechanism of sorption to soil particles. We hypothesize that, in a prion-infected animal carcass, a portion of the N-terminus of PrPSc could be cleaved by proteinases in the brain at ordinary temperatures. Hamster (HY transmissible mink encephalopathy-infected), transgenic mice (CWD-infected), and elk (CWD-infected) brain homogenates were incubated at 22 and 37 degrees C for up to 1 month and then analyzed by Western blot using N-terminal and middle region monoclonal anti-PrP antibodies. For all three systems, there was a very faint or undetectable N-terminal PrP signal after 35 days at both temperatures, which indicates that full-length PrPSc might be rare in the brain matter of animal carcasses. Future studies on prion-soil interactions should therefore consider N-terminal-degraded PrPSc in addition to the full-length form. Both mouse and elk CWD PrPSc demonstrated greater resistance to degradation than HY TME PrPSc. This indicates that the transgenic mouse-CWD model is a good surrogate for natural CWD prions, but that other rodent prion models might not accurately represent CWD prion fate in the environment.