ok, I gave you that link to listen to. I'm not saying you or your vet is doing something wrong. I'm not saying that you think i'm saying that either. lets try and figure out something to try. when dr stahl talks on that link. he states that

some cultures are done wrong.
meds are not giving long enough, but you should still see improvement
sometimes using more than one med at the same time is needed.

I figure if you listen to this, something might pop up on something miss or something you might want to try.
he's strickly exotics, and is well known. he gets reptiles shipped to him for treatment. my point is he seen a lot, and after trying everything you have, calling him might be your best option.