Environmental context. Chronic wasting disease (CWD) and other ‘prion’ diseases tend to cluster geographically, a characteristic that may indicate environmental risk factors, yet the biogeochemical features of regions with high prion disease incidence have rarely been investigated. This study, presenting soil, water and forage trace element analyses for a CWD cluster in Wisconsin, suggests that trace metal toxicity is unlikely to be a disease factor, but further indicates that willow browse could present a risk for copper deficiency in deer owing to high foliar sulfur concentrations. As copper deficiency leads to impaired immune function, it is hypothesised that the risk for CWD could be increased in environments where deer browse willow heavily.

Abstract. In an effort to determine whether incidence of chronic wasting disease (CWD) in wild cervids is linked to biogeochemical risk factors of affected regions, soils, plants and surface waters in the Mt Horeb, Wisconsin CWD cluster were sampled and analysed for trace metals as well as major elements. The results revealed normal (relatively low) levels of trace metals in the soils, water and most plants, with no evidence of widespread heavy metal contamination in this region. It was concluded that incidence of CWD in Mt Horeb is unlikely to be linked to abnormally high concentrations of potentially zootoxic metals, including Pb, Zn, Cd or Mn, in soils waters or forages of the region. However, shrub willow, a common browse plant in the area, had high S levels in its leaves compared with other potential forage plants. Bioaccumulation of S was species-specific, with one willow species in particular (Salix exigua) having high foliar S in comparison with other species both in controlled greenhouse experiments as well as in the field near Mt Horeb. The willow S concentrations in the Mt Horeb CWD locus, averaging in excess of 4500 mg kg^–1 in 2003 and remaining high in 2004, were high enough to have contributed to hypocuprosis in deer, and possibly to have caused polioencephalomalacia (PEM). It is hypothesised that conditions favourable to intensive deer browsing on shrub willow may exist periodically in Mt Horeb, leading to hypocuprosis and increased susceptibility to CWD.