Fate and Transmission of Prion Diseases in the Soil Environment

Samuel Saunders, University of Nebraska-Lincoln

A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Interdepartmental Area of Engineering, Under the Supervision of Professor Shannon Bartelt‐Hunt. Lincoln, Nebraska: March, 2011

Copyright 2011 Samuel Saunders

Abstract

Prion diseases are fatal, transmissible neurodegenerative diseases including bovine spongiform encephalopathy, sheep scrapie, cervid chronic wasting disease (CWD), and Creutzfeldt-Jakob disease (CJD) in humans. The infectious agent of prion diseases is the prion protein (PrP^Sc). Scrapie and CWD are horizontally transmissible and can remain infectious after years in the environment. Soil reservoirs of prion infectivity may facilitate a sustained incidence of CWD in free-ranging cervid populations and complicate efforts to eliminate scrapie and CWD in captive herds.

Maximum PrP adsorption to soil in a competitive brain homogenate matrix requires days or weeks, depending on the soil or mineral, and is two to five orders of magnitude lower than reported in previous studies using purified PrP^Sc or recPrP. Strain and species differences in PrP adsorption over time and as a function of aqueous concentration were observed, as was variance in PrP adsorption in different adsorption solutions. The N-terminal region of PrP may enhance adsorption to clay but hinder adsorption to sand.

Degradation of soil-bound and unbound prions by a subtilisin enzyme was characterized. The enzyme effectively degraded PrP^Sc adsorbed to a wide range of soils and soil minerals below the limits of western blot detection, indicating prions can be effectively degraded when bound to soil. Additional study results suggest large (10⁴ to >10⁶-fold) decreases in soil-bound prion infectivity following enzyme treatment, demonstrating a mild enzymatic treatment could effectively reduce the risk of prion disease transmission via environmental surfaces.

Prions bound to a silty clay loam soil had a greater than one-log decrease in infectious intracerebral titer and an approximately equivalent decrease in replication efficiency compared with unbound prions. The replication efficiency of soil-bound prions varied with soil type, where prions bound to clay and organic surfaces exhibited significantly lower replication efficiencies compared with sand-bound and unbound controls. Replication potential was conserved for soil-bound prions after incubation for 1 year, although PrP^Sc levels greatly decreased. Rumen digestion does not degrade unbound or soil-bound PrP^Sc but does inhibit replication and may alter prion strain properties.