Bulldogs World

I have a major problem with my 7 year old male bulldog.  About two years ago he was treated with Mitaban because he had demodex mange on his feet.  He went through approx. 12 dips total because it came back.  Shortly after his last dip he started having diarrhea.  We obviously took him to the vet when it didnt clear up in a couple days.  He was prescribed an antibiotic which didnt help so she prescribed another, then another.  After trying three different antibiotics and putting him on a bland diet and these things not helping she had us put him on prescription diet z/d.  That helped a little bit but he still had loose stool.  We gave him about three or four months to see if the food would help and it did get better, but not great.  We then decided to change vets.  He has now gone through blood work, many different antibiotics, supplements, ultrasounds etc. etc, etc.  The only thing that has ever been found was that he had inflammation of the lower intestine. He is now on prednisone which did really help for a while, but now its not helping at all.  The next step is to do a intestinal biopsy, but vet has said since he has been on prednisone that we probably wouldnt get a conclusive result because his body is altered by pred.  I am at a loss as to what else to do.  Does anyone have any experience with  this?  He is otherwise happy," healthy" (looks, acts and every test is coming back fine).  He is eating and drinking fine.