If I remember my history, the division between the northern herd and the southern herd came about not by disease, but by the transcontinental railroad. For some reason the herds refused to cross the tracks and broke up into the two groups. I have seen the same reaction in cattle.
If we loose these, there are still large numbers of Wood Buffalo in Canada and Alaska.
Cute story but aren't you forgetting about gullies and rivers and overland bridges?