Sewage Sludge Expands Prion Pathways

Soil may serve as an environmental reservoir for prion infectivity and contribute to the horizontal transmission of prion diseases (transmissible spongiform encephalopathies [TSEs]) of sheep, deer, and elk. TSE infectivity can persist in soil for years, and we previously demonstrated that the deadly form of the prion protein binds to soil particles and prions adsorbed to the common soil mineral montmorillonite (Mte) retain infectivity following intracerebral inoculation. Here, we assess the oral infectivity of Mte- and soil-bound prions.

We establish that prions bound to Mte are orally bioavailable, and that, unexpectedly, binding to Mte significantly enhances disease penetrance and reduces the incubation period relative to unbound agent. Cox proportional hazards modeling revealed that across the doses of TSE agent tested, Mte increased the effective infectious titer by a factor of 680 relative to unbound agent. Oral exposure to Mte-associated prions led to TSE development in experimental animals even at doses too low to produce clinical symptoms in the absence of the mineral.

We tested the oral infectivity of prions bound to three whole soils differing in texture, mineralogy, and organic carbon content and found soil-bound prions to be orally infectious. Two of the three soils increased oral transmission of disease, and the infectivity of agent bound to the third organic carbon-rich soil was equivalent to that of unbound agent. Enhanced transmissibility of soil-bound prions may explain the environmental spread of some TSEs despite the presumably low levels shed into the environment. Association of prions with inorganic microparticles represents a novel means by which their oral transmission is enhanced relative to unbound agent.

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Transmissible spongiform encephalopathies (TSEs) are a group of incurable neurological diseases likely caused by a misfolded form of the prion protein. TSEs include scrapie in sheep, bovine spongiform encephalopathy (“mad cow” disease) in cattle, chronic wasting disease in deer and elk, and Creutzfeldt-Jakob disease in humans. Scrapie and chronic wasting disease are unique among TSEs because they can be transmitted between animals, and the disease agents appear to persist in environments previously inhabited by infected animals.

Soil has been hypothesized to act as a reservoir of infectivity and to bind the infectious agent. In the current study, we orally dosed experimental animals with a common clay mineral, montmorillonite, or whole soils laden with infectious prions, and compared the transmissibility to unbound agent. We found that prions bound to montmorillonite and whole soils remained orally infectious, and, in most cases, increased the oral transmission of disease compared to the unbound agent. The results presented in this study suggest that soil may contribute to environmental spread of TSEs by increasing the transmissibility of small amounts of infectious agent in the environment.

Citation: Johnson CJ, Pedersen JA, Chappell RJ, McKenzie D, Aiken JM (2007) Oral Transmissibility of Prion Disease Is Enhanced by Binding to Soil Particles. PLoS Pathog 3(7): e93. doi:10.1371/journal.ppat.0030093

Introduction

Bovine spongiform encephalopathy, human Creutzfeldt-Jakob disease and kuru, sheep scrapie, and chronic wasting disease of deer, elk, and moose belong to the class of fatal, infectious neurodegenerative diseases known as transmissible spongiform encephalopathies (TSEs) or prion diseases. The precise nature of the etiological agent of these diseases remains controversial, but most evidence points to a misfolded isoform of the prion protein (PrPTSE) as the major, if not sole, component of the pathogen.

Sheep scrapie and cervid (deer, elk, and moose) chronic wasting disease are distinct among TSEs because epizootics can be maintained by horizontal transmission from infected to naïve animals, and transmission is mediated, at least in part, by an environmental reservoir of infectivity. The presence of an environmental TSE reservoir impacts several epidemiological factors including contact rate (the frequency animals come in contact with the disease agent), duration of exposure (time period over which animals come in contact with the pathogen), and the efficiency of transmission (the probability that an exposed individual contracts the disease).

The oral route of exposure appears responsible for environmental transmission of chronic wasting disease and scrapie; the propagation of bovine spongiform encephalopathy epizootics (feeding TSE-infected meat and bonemeal to cattle); the appearance of variant Creutzfeldt-Jacob disease in humans and feline spongiform encephalopathy in cats (presumably by consumption of bovine spongiform encephalopathy–infected beef); the spread of kuru among the Fore of Papua New Guinea (ritualistic endocannibalism); and outbreaks of transmissible mink encephalopathy (TME) in farm-reared mink. Following consumption, TSE agent is sampled by gut-associated lymphoid tissue, such as Peyer’s patches or isolated lymphoid follicles, and accumulates in lymphatic tissues before entering the central nervous system via the enteric nervous system. While ingestion is a biologically relevant TSE exposure route, oral dosing is a factor of ~105 less efficient than intracerebral inoculation in inducing disease in rodent models. The amounts of TSE agent shed into the environment are presumably small. The assumed low levels of TSE agent in the environment and the inefficiency of oral transmission have led to uncertainty about the contribution of environmental reservoirs of infectivity to prion disease transmission.

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We and others have hypothesized that soil may serve as a reservoir of TSE infectivity. Deliberate and incidental ingestion of soil by ruminants can amount to hundreds of grams daily. Prions enter soil environments via decomposition of infected carcasses, alimentary shedding, deliberate burial of diseased carcasses/material and possibly, urinary excretion. TSE agent persists for years when buried in soil. The disease-associated prion protein sorbs to soil particles, and the interaction of PrPTSE with the common aluminosilicate clay mineral montmorillonite (Mte) is remarkably avid. Despite this strong binding, PrPTSE–Mte complexes are infectious when inoculated into brains of recipient animals.

For TSEs to be transmitted via ingestion of prion-contaminated soil, prions bound to soil components must remain infectious by the oral route of exposure. We therefore investigated the oral infectivity of Mte- and soil-bound prions.

Prions Bound to Whole Soils Remain Orally Infectious. Some Soils Increase Transmission.

Three soils (Dodge, Elliot, and Bluestem) were incubated in the presence of purified PrPTSE. The samples were orally dosed into hamsters and found to remain orally infectious. Agent association with Elliot and Bluestem soils increases disease incidence, whereas Dodge soil does not influence disease transmission. Animals dosed with soil alone remained healthy throughout the course of the experiment (unpublished data).

These experiments address the critical question of whether soil particle–bound prions are infectious by an environmentally relevant exposure route, namely, oral ingestion. Oral infectivity of soil particle–bound prions is a conditio sine qua non for soil to serve as an environmental reservoir for TSE agent. The maintenance of infectivity and enhanced transmissibility when TSE agent is bound to the common soil mineral Mte is remarkable given the avidity of the PrPTSE–Mte interaction [22]. One might expect the avid interaction of PrPTSE with Mte to result in the mineral serving as a sink, rather than a reservoir, for TSE infectivity. Our results demonstrate this may not be the case. Furthermore, sorption of prions to complex whole soils did not diminish bioavailability, and in two of three cases promoted disease transmission by the oral route of exposure.

In conclusion, our results provide compelling support for the hypothesis that soil serves as a biologically relevant reservoir of TSE infectivity. Our data are intriguing in light of reports that naïve animals can contract TSEs following exposure to presumably low doses of agent in the environment. We find that Mte enhances the likelihood of TSE manifestation in cases that would otherwise remain subclinical, and that prions bound to soil are orally infectious. Our results demonstrate that adsorption of TSE agent to inorganic microparticles and certain soils alter transmission efficiency via the oral route of exposure.

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Gary Chandler is a prion expert. He is the CEO of Crossbow Communications, founder of Sacred Seedlings and Earth News, and author of the Language and Travel Guide To Indonesia and several other books about health and environmental issues around the world. Chandler also is connecting the dots to the global surge in neurological disorders.