Fig. 2   Map of Iceland depicting sample sites and main scrapie endemic region (hatched). Numbered sample sites correspond to numbering of farms on Table 1.

        Interestingly, there are some good examples of scrapie-free valleys found in the middle of the scrapie endemic zones which provide good opportunities for comparative studies. One fascinating example is demonstrated NW of Akureyri where the scrapie endemic valley 'Svarfadardalur' runs 15 miles parallel to the scrapie free valley 'Horgardalur' (see Fig. 3). Sheep from both valleys freely intermingle on the open mountain during summertime, suggesting that the mystery causal factor X associated with scrapie aetiology would be present in the specific valley homes where the scrapie affected flocks overwinter. Results of the author's study demonstrated an av level of 94 mg/kg Mn (dry basis) drawn from 4 test sites in the scrapie free valley and 223.4 mg/kg Mn from 10 sites in the scrapie valley. Interestingly, Barka was the only farm recorded in the scrapie free valley that has purportedly suffered a suspected outbreak of scrapie in 1949, perhaps explaining why the Barka sample demonstrated the highest Mn level in the valley:
 

Fig. 3   Map of Larimer County in Colorado Depicting location of sample sites across CWD endemic cluster region and where CWD affected cervidae have been found (58). Numbered sample sites correspond to numbering of locations on Table 3. Shaded circles represent CWD affected deer. Hatched circles represent CWD affected elk.

The recent fall in scrapie incidence in the scrapie-endemic regions must be partly due to the sharp decline in the total number of 'TSE susceptible' sheep due to the Icelandic government's scrapie slaughter policies (60. The fall could also be due to the virtual universal switch over from feeding hay to silage as winter fodder over the last ten years in Iceland. Various analytical studies have demonstrated increasing concentrations of Mn in the seed heads of grasses during the maturation process (31), confirming the fact that manganese concentrations are higher in hay than in silage (69); simply because it is customary to harvest grass for hay at a more advanced stage of maturity than the younger flowering stage required for the silage harvest. Hidiroglou et al. (66) measured serum Mn levels in different batches of cattle fed hay or silage, and concluded that the bioavailability of Mn is much greater in hay than in silage.
 
2. Colorado CWD cluster (Tables 2 & 3)
 
Herbage drawn from an 80 mile cross section of the CWD endemic cluster zone in North central Colorado (Fig. 4) consistently demonstrated excessive levels of the divalent cation, calcium, at 1.19% total dry metter. However, the levels of Mn recorded in this specific batch of herbage/soil samples were low; averaging out at 39.5 mg/kg in herbage and 9.3 ppm in the soil. These samples were drawn during the drought conditions of July 1998 following three months of dry weather. (NB. Soil Mn is rendered considerably less available during drought conditions (31) whilst the protracted daylight of the mid summer period decreases levels of Mn in plant tissues