Okay, we have various issues at the shelter, ranging from food intolerance requiring limited ingredient/limited protein diets to nerve damage resulting in inability to self eliminate or the opposite- nerve damage requiring assistance eliminating, and nearly everything in between.
It just seems that that has been a lot of jumping from one thing to the next. Again, I'm not in your shoes, but my plotting of your kitty's medical journey read like this (and again, this is my easy synopsis, not what may have actually happened): sick kitty, immediately go on meds, try numerous diets, you are stressed, he is stressed. He doesn't eat well, at some point you mention he is on an appetite stimulant as well as a pain med plus whatever else the vets have prescribed. Pain meds often cause constipation. He gets an enema, requiring sedation, sedation also sometimes causes the GI system to slow down=constipation.
Our vets & specialists tend toward a slower ramp up of or changing of medications/foods. Our cats that do go through a diet restriction or change will change immediately to the new diet and stay on one form of that particular diet (be it protein restriction or ingredient restriction) for a minimum of 2 weeks; if no change, another food is considered, if it goes well, it would be another week or more before either the wet or dry variety of that food would be introduced. And for diet restriction, absolutely no treats or other food than the prescribed diet, because sometimes those treats may have a contaminant from a restricted ingredient/protein. Think of it like a kid with a life threatening peanut allergy; a food that doesn't list peanuts in the ingredients isn't enough, it needs to be made in a peanut free facility.
Instead of quitting the all of the meds, a more studied approach might have been warranted. I read, basically, the vet said he's doing great, stop everything (? again, I may have misread) and a weekish you are back at the vet with Walnut presenting other issues. Plus, have they tried expressing him instead of sedating for enemas? Sedating a cat is pretty taxing on his system, which by all accounts isn't running at full steam. We literally express one or more cats multiple times a day at the shelter and this is something that a trained volunteer can manage and isn't painful to the cat if done properly.
You mention that Walnut was given panacur. That's a dewormer. I don't know why a cat who is experiencing significant medical issues, who does not present with indications of internal parasites, would be dewormed in the middle of this process. It doesn't make sense.
Prednisone can be harsh on cats, but unfortunately is often the only real treatment for some conditions both long and short term. It is broken down in the liver, so the liver levels must be checked routinely. It can also cause lethargy, which you mentioned Walnut was experiencing. It can also cause nausea, which will make a cat disinclined to eat, which when added to the general "I don't feel good, my plumbing is backed up, they keep pushing pills down my throat, sedating me, etc" that Walnut might be feeling would certainly warrant at least a good pitty party/general bought of lethargy if not a general hunger strike (because, lets be honest, Fancy Feast is delicious according to 99% of the cats I know and if he's only getting the bland, diet restriction food that might be reason enough to cause a hunger strike).
So, that's my $0.02 worth. If I were Walnut's person, I would find one really great vet, probably a specialist, to be the go to person for his treatment. Some vets are great at thinking outside the box, some have a very specific treatment formula for each case of IBD, fatty liver, etc, no matter the patient and his reaction to the treatment formula. Those last types are fine vets 99% of the time, but for a perplexing case, you need the vet who will do research on their own time, who will think outside the box and will reach out to their colleagues for help and aren't scared to refer you to someone else.
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