Soil clay content underlies prion infection odds

Abstract

Environmental factors-especially soil properties-have been suggested as potentially important in the transmission of infectious prion diseases. Because binding to montmorillonite (an aluminosilicate clay mineral) or clay-enriched soils had been shown to enhance experimental prion transmissibility, we hypothesized that prion transmission among mule deer might also be enhanced in ranges with relatively high soil clay content. In this study, we report apparent influences of soil clay content on the odds of prion infection in free-ranging deer. Analysis of data from prion-infected deer herds in northern Colorado, USA, revealed that a 1% increase in the clay-sized particle content in soils within the approximate home range of an individual deer increased its odds of infection by up to 8.9%. Our findings suggest that soil clay content and related environmental properties deserve greater attention in assessing risks of prion disease outbreaks and prospects for their control in both natural and production settings.

Figure 1. Spatial relationships between soil clay content and prion disease in mule deer.

(a) The average percentage of clay-sized particles contained in soils within the 9 km² area surrounding a sampled deer strongly influenced its odds of prion infection in northcentral Colorado, USA (Table 1). (b) A similar relationship between soil clay content and the odds of prion infection also was evident in deer sampled from northwestern Colorado (Table 1). In both panels, dots (•) represent locations of prion-infected deer presence and crosses (×) represent locations of apparently uninfected deer presence.

Figure 2. North American distribution of prion disease in deer.

In some areas, prion disease foci in deer (Odocoileus spp.) in western North America overlap landscapes with soils that have cation exchange capacity (CEC) values (tan areas in inset) suggestive of a relative abundance of clay minerals (phyllosilicates) and/or organic matter. Game management units (GMUs) are administrative boundaries used in hunting management; prion-infected GMUs are shown in light grey on the continental map and in cross hatching on the inset.