Chronic wasting disease (CWD) is a transmissible spongiform encephalopathy (TSE), or prion disease, of deer and elk. Other TSEs include sporadic, familial and variant Creutzfeld-Jacob disease in humans, bovine spongiform encephalopathy (BSE) of cattle and scrapie in sheep and goats. The infectious agent that leads to these diseases is an abnormally folded protein (PrPSc), which is extremely resistant to inactivation and may associate with soil particles. The increase of chronic wasting disease in wildlife is increasing the potential for interaction of wildlife and domestic species with contaminated lands and infectious carcasses. The persistence of CWD represents a potentially significant factor in disease transmission. In CWD endemic areas, deer and elk share rangeland with domestic livestock, raising the concern that the disease may be transmitted to cattle.
Epidemics of CWD and scrapie are perpetuated by horizontal (i.e., animal-to-animal) transmission. Although the route of horizontal transmission is poorly understood, the accumulation of abnormal proteins in lymphoid tissues associated with the alimentary or digestive tract suggest that the TSE agent may be shed by infected animals in urine, feces and/or saliva. In both CWD and scrapie, oral exposure appears to play an important role in transmission.
Upon exposure of soil to an infectious agent, the accessibility of PrPSc to species that may ingest soil depends in part on the retention of PrPSc near the soil surface. The objective of this study was to investigate two factors believed to be key to the transport of CWD agent in soil: 1) electrostatic interactions between PrPSc and soil particles, and 2) the residence time of the infectious agent in the soil column. The researchers employed relatively simple saturated sand columns to allow them to examine the influence of single variables on PrPSc transport. The studies were designed to provide baseline information that would be critical for conducting subsequent, increasingly complex leaching studies exploring a range of factors that may influence PrPSc retention near the soil surface.
The TSE infectious agent (PrPSc) was isolated from research animals. The researchers examined the effects of pH, ionic strength and flow rate of PrPSc in an experimental column made of Teflon and containing a porous media (medium quartz sand, 0.50 to 1.1 mm). Three buffers were used in the pH experiment and experiments were conducted at pH 4.0, 7.0 and 9.0. Sodium chloride was added to the columns to adjust ionic strength for that portion of the analysis. All of the experiments were carried out at room temperature.
The amount of PrPSc transferred was quantified using a double-antibody “sandwich” type enzyme immunoassay (EIA). The EIA recognizes specific amino acids of the prion protein. All of the samples were run in duplicate. The columns were spiked with either a brain sample homogenate or a prion enrichment.
The recovery of PrPSc was higher for the columns spiked with the prion enrichment (88 percent) than those spiked with brain homogenate (50 percent). A significant fraction of the infectious agent did not migrate through the model soil. Prion retention and transport was shown to be influenced by the acidity and concentration of salts in the soil solution. Initial breakthrough of PrPSc occurred earlier at higher pH values and recovery of PrPSc decreased with decreasing pH. Recovery rates were 10.9 percent, 37.7 percent and 45.0 percent for pH 4.0, 7.0 and 9.0. The charges on soil mineral surfaces and the protein appeared to influence prion retention in soil. Peak breakthrough occurred earlier at the lowest ionic strength and was similar at medium and high ionic strengths. Recovery of PrPSc in column effluents decreased with increasing ionic strength and was 55.8 percent, 43.0 percent and 28.2 percent for ionic strengths of 10, 100, and 500 mM. The findings from this study indicate that some prion proteins may be held near the soil surface.
In these experiments, the researchers employed relatively simple saturated sand columns as a means to examine the influence of various variables on PrPSc transport through soil. These initial studies help establish information critical for conducting subsequent and more complex leaching studies. Since soils are not typically composed of pure quartz sand, the influence of other soil minerals that differ in particle size and surface charge would also be important to understand PrPSc transport through naturally occurring soils. In addition, the role of dissolved and particulate organic matter on prion retention near the soil surface should also be examined.