I don't know, I am just not that up on my canine diseases. One thing I do know from my biology degree though, is that all populations go in cycles, literally feast and famine. Predator populations are tied to prey populations, and tend to follow those swings, but with somewhat of a delay.
As the prey population booms, the predators are living high on the hog with good health and large litters, and increase n population to follow the prey cycle. As the prey population peaks and reaches carrying capacity and beyond, disease due to high population density and starvation goes to work killing off large numbers of prey. This however is just fine with Mr. Coyote, as this is the situation he thrives in. Life is good, and he is still rolling fat and sassy. As the prey cycle continues to crash though, things begin to change. Now it is the coyote who is on the wrong end of the carrying capacity equation, and he follows the same route as the prey before him. Disease and starvation hit him hard, and his numbers crash as well. Of course being the adaptable scrounger and omnivore that he is, he can swap around and find different things to survive on, but sti;ll times are lean, very lean.
Eventually as the pendulum continues to swing, predator populations are still going down and prey populations have started to rebound back up. A little further down the timeline from that as predator population bottoms, I feel is the toughest time to call. At this time the numbers of predators are at their lowest point, yet prey has been rebounding for a couple years and food is plenty. the two combined can make for tough conditions.
Is this what is happening with you? I have absolutely no idea, but it does give some food for thought. And of course, it really isn't all that simple anyways because you aren't talking about a single prey species and a single predator species. There are multiple populations of both, all interacting the same time, and we haven't even figured in things like human manipulation! The whole system is incredibly complex and dynamic, ever changing.
Looking back over my fathers 20 year calling log, I can see some definite patterns. I am sure some of that coincides with busy times in his life where he wasn't able to call as much or could call more, but I can also see entries about how good or poor their condition was and so forth as well. We seem to be approaching a peak right now, times are flush and calling is good, coyotes everywhere, not much mange. I know it won't last though, you just can't have mountains unless you have valleys.