this is well known. Co-grazing scrapie sheep will cross over into cows as mad cow disease. However, co-grazing CWD deer will not cross over into cows or sheep

this is well known. Co-grazing scrapie sheep will cross over into cows as mad cow disease. However, co-grazing CWD deer will not cross over into cows or sheep (this is also well known). The determinants of why X can or can't cross over into Y are not well-understood, yet.