I had a red fox on my home property this summer that had a terrible case of the mange. I contacted the DNR and asked them what could be done about it. I was hoping that there would be a animal sanctuary that could trap the animal and cure it with antibiotics. No such luck. The red fox was showing up in the daylight hours and bedding down with 25 yards of my house. It lost it's ability to hunt. It never ran away when we went outside. It was scrounging under my bird feeders. It did not act like a typical red fox. This is the first red fox I have seen in 34 years at my place. The DNR person said that few animals with mange can survive a winter. It was mid summer and the red fox already looked quite skinny. I did the right thing and put it down with a single shot from my .22 magnum. Per DNR instructions, I put it in a burlap bag and buried it quite deep in my back yard. I put rocks on top of it before I covered it with dirt to keep any other animal from digging it up.
Mother Nature can be very cruel regarding how animals in the wild can die.