Actually, I'm not in a heavy agricultural area. The farmers in my area have predominantly cattle, there's very little grain and subsequently a lot of heavy cover. In fact, immediately to the north, it's thousands of square miles of boreal forest with only a couple of roads intersecting that area before you reach the north pole. we have always had wolves and coyotes, though in the last 10 years or so both populations seem to have increased. I do believe it's partially due to the increase in the whitetail population and a decrease in the moose popualtion in northern areas. More wolves are moving further south.
There are always coyotes with mange around, some years it's worse than others. I would say currently a fair estimate would be that less than 5% of the population is affected. With wolves, it seems to affect about the same amount, perhaps a bit less. We do have fox as well, though not many, and in all the years I have never seen one with mange. My understanding is that they can't catch the strain coyotes and wolves carry.