Soil characteristics at artificial salt licks and their potential impacts on occurrence of chronic wasting disease

Abstract

Salt lick sites, where artificial salt blocks are placed at permanent locations, are common in summer grazing areas for free-ranging sheep in Norwegian mountains. These areas often overlap with areas used by wild reindeer, and reindeer are frequently observed at these salt lick sites. The first cases of chronic wasting disease (CWD) were discovered among Norwegian wild reindeer in 2016, and salt lick sites were presumed to be hotspots for the transmission of CWD. In this study, we compare soil properties at salt lick and nearby control sites not affected by salt blocks and review how salt-induced changes may influence the persistence and transmission of CWD. Three wild reindeer areas were studied: one CWD-affected area, Nordfjella, and two areas without CWD, Knutshø and Forollhogna. The soils at the salt lick sites were strongly influenced by dissolving salt blocks and increased animal activity. The salt lick sites had higher pH and ionic strength and increased levels of sodium (Na), chlorine (Cl), calcium (Ca), magnesium (Mg), zinc (Zn), manganese (Mn), and iodine (I), reflecting the composition of the salt blocks. The increased animal activity was reflected in eroded topsoil causing less soil organic matter (SOM), and there were higher amounts of elements related to defecation and urination, giving higher concentrations of inorganic nitrogen (Inorg-N), phosphate (PO₄-P), sulfate (SO₄-S), and potassium (K) as well as high gastrointestinal parasite frequency and diversity. The high salt content in the salt lick soils may stimulate geophagy, and as the soil is heavily contaminated by animal excretions, this may facilitate prion transmission. In addition, the high pH and ionic strength in the salt lick soils increase both the cation attraction and anion diffusion toward the soil particles, thereby facilitating both persistence and transmission of CWD. There was an increase in salinity at the salt lick sites in a gradient from west to east, most likely related to the coinciding decrease in precipitation. This suggests that if the use of permanent salt lick sites is discontinued, the salt lick sites in the east will maintain their attraction for congregating animals and geophagy longer than the western sites.