After you have the vet check and rule out mange, it is likely that it is just from pressure compaction and dirt in the hair folicles, commonly from laying on hard surfaces, but you can even see it when a dog lays on bedding. It wouldn't hurt to wash and rinse the dog very well, scrub the whole body well but especially the bare spots, and do this every week. Often adding anything to the coat/skin will just continue to stifle the healthy new growth. You could at least ask the vet but it is a possibility and easy to reverse, just takes growth time. The more the dog is up on its feet the bette