You can do the food trials yourself, if you think perhaps it is a food allergy. I would NOT suggest starting with the "low allergen" prescription food, not for a case this severe. Ask your vet if he thinks it will help! The most common food alleries are to grains, and to proteins. The prescription food choices are venison, duck, quail, rabbit, all proteins which are chemically quite different from those found in traditional dog foods (chicken, turkey, fish, beef and lamb). As for the grains, the prescription foods are all grain free, as far as I know. But there is one commercially available food, Nature Variety Raw Instinct, which is grain free. Their web site has a dealer locator, so you can look to see if there is someone nearby who stocks it.

Here is the link:
http://www.naturesvariety.com/

It didn't work for my cat, however I DID see an improvement. So I learned he is allergic to grains and . . . something else. That is when I went to the dermatologist for the testing. She didn't do any testing! Reviewed what had been done by the vet (the scrapings, antibiotics etc.) and said the next thing to try is a prescription food to remove possible protein allergies. Only after you exhaust all that do they do the allergen tests (this method saves the owner money in the long run, as most pet allergies ARE to foods. So why do the allergy tests for $300 or so, when you can do the food trials first. There are no reliable allergen tests for proteins.)

Just a thought. Throwing out ideas for you to consider.