I will be the first to admit that I am not a vet but it does sound as though your GSD has a unusual problem which does not mean that it can't be solved but that it possibly needs to be viewed from another aspect. There are many other alternatives to traditional veterinary medicine and I am firm believer in many of these alternatives.
With that being said perhaps you'd best be suited by checking into the advice of a Naturopathic Vet to find the real cause of the problem to solve - not just stopping the symptoms. And for that, I can't recommend you anyone any higher than a wonderful scientist from Australia named Sara Rooney.
She has a wonderful book, "Heal Your Dog Naturally", offering all sorts of advice on making your pet as healthy as possible and being able to treat them when they're not. If you want to find out more, check out the page at: http://www.total-german-shepherd.com/Natural-Dog-Health-Supplements.html
Click on the book photo at the top of the page and it will take you directly to Sara's page to learn more. I trust Sara implicitly - she is a knowledgeable scientist with many years of experience treating all sorts of animals.
The vets haven't been able to help you yet - why not give Sara a try?
