Transport of the pathogenic prion protein through soils

Transmissible spongiform encephalopathies (TSEs) are progressive neurodegenerative diseases and include bovine spongiform encephalopathy of cattle, chronic wasting disease (CWD) of deer and elk, scrapie in sheep and goats, and Creutzfeldt-Jakob disease in humans. An abnormally folded form of the prion protein (designated PrPTSE) is typically associated with TSE infectivity and may constitute the major, if not sole, component of the infectious agent. Transmission of CWD and scrapie is mediated in part by an environmental reservoir of infectivity. Soil appears to be a plausible candidate for this reservoir. The transport of TSE agent through soil is expected to influence the accessibility of the pathogen to animals after deposition and must be understood to assess the risks associated with burial of infected carcasses. We report the results of saturated column experiments designed to evaluate PrPTSE transport through five soils with relatively high sand or silt contents and low organic carbon content. Protease-treated TSE-infected brain homogenate was used as a model for PrPTSE present in decomposing infected tissue. Synthetic rainwater was used as the eluent. All five soils retained PrPTSE; no detectable PrPTSE was eluted over more than 40 pore volumes of flow. Lower bound apparent attachment coefficients were estimated for each soil. Our results suggest that TSE agent released from decomposing tissues to soils with low organic carbon content would remain near the site of initial deposition. In the case of infected carcasses deposited on the land surface, this may result in local sources of infectivity to other animals.