Its a little hard to tell in the photo but has the hair actually come out or is it just lighter? I can't offer any advise but my mini gets these small circular spots over summer where the hair doesn't seem to grow and they are more obvious when she is wet. The vet couldn't find any mites or anything that might cause them and at the end of summer the hair grows out again. I did wonder if it was from ticks but no patches ever started in the places where I had removed them from. She only gets them on her belly, rump and chest.