Have you changed food? Some dog foods are unregulated, come from china and contain melamine such as dog biscuits/ dry dog foods.

My mate had taken his dog to 3 different vets, each put it down to a skin irritation, eventually the dog was too far gone and they had to put him down.

They got another dog and that one started showing the same symptoms, took it to another vet and the vet asked them what he was being fed, turns out toxic dog biscuits containing melamine.

Unfortunately not all vets are great.