Has you vet ruled out ringworm, which is not a worm, but a fungus, the symptom in a circular bald or thinning spot. My sister's yorkie had this when she was a puppy. My sister caught it from her pup and she to be treated as well. If I remember correctly the vet did not diagnosis it either, I think it was not till my sister discovered she had that they realized it had come from the pup. The breeder was very involved in rescue so they think it came from one of the rescue dogs. It cost my sister a lot of money to get rid of it, so rule this out as quickly as you can
Canine Ringworm Infection | The Drake Center For Veterinary Care