I've had the same problem with my 6 year old lurcher since he was a pup, end of every summer the fur on the his back legs goes really thin and fluffy and looks bald, it stays the same all season then grows back around May time, only to happen again come Sept/October, I went to vets thinking it could be mites or alopecia or something like that and they said to try different things including aloe vera gel (which didn't work). I've also tried nu-stock aswell as aload of other things but nothing works. It doesn't seem to bother the dog or his performance so I think it's just one of them things. I'