Soil as an Environmental Reservoir of Prion Diseases

Prions are recognized as misfolded, pathologic isoforms of the normal mammalian prion protein, which can uniquely cause infectious inherited or spontaneous disease. They are agents of transmissible spongiform encephalopathies (TSEs). The normal, benign, host-encoded forms of PrP are denoted PrPC, and the infectious disease-associated, misfolded conformers are designated PrPTSE. Earlier it was hypothesized that TSEs were caused by a new type of “slow virus” that was too small to purify but had virus-like phenotypes such as transmissibility and heritability. This was the predominantly held theory until 1982, when Stanley Prusiner proposed that the causative agent was exclusively a protein. He solidified the “protein-only” hypothesis which had been developing and termed the agent proteinacious infectious only, or prion. Prion diseases are a family of inevitably fatal neurodegenerative disorders affecting a variety of mammalian species, including human diseases such as Creutzfeldt-Jakob disease (CJD), variant CJD (vCJD), Kuru, Fatal Familial Insomnia (FFI) and Gerstmann-Straussler-Scheinker Syndrom (GSSS). Animal prion diseases include chronic wasting disease (CWD) in North American deer, elk and moose; scrapie in sheep and goats, bovine spongiform encephalopathy (BSE: “mad cow” disease) in cattle, and transmissible mink encephalopathy (TME) in mink. These diseases are characterized by long incubation periods, spongiform degeneration of the brain and accumulation of an abnormally folded isoform of the prion protein, designated PrPSc, in brain tissue. The unusual nature of prions has created a formidable challenge for detection and study of the agent. Studies have shown prions to adsorb strongly to soil components, remain infectious and persist for years. Indirect transmission most likely occurs through incidental and geophagic ingestion of soil or other contaminated fomites, as well as deer sign-post behavior such as scraping and marking overhanging branches.