Is there anyone in your social circle who is diabetic who might be able to pop round with their meter so that you can do some spot-check tests for BG levels? Diabetics may be quite stable for a good period of time but something out of the blue might start affecting their BG levels adversely and numbers start to climb and regulation deteriorates. This is one
potential reason why your kitty might be a bit 'off' at the moment. (Dental/gum problems and UTIs are very often the culprits; resolve those issues and regulation usually improves again.) FWIW, depression is a common clinical sign in poorly-regulated FD cats. Even a few spot checks would help you to gauge whether his BG levels are higher than normal.
The diminished interest in some feeds might be due to nausea. He has a history of pancreatitis so there is a possibility that his lethargy may be due to a flare-up in the pancreas. Try raising his food and water bowls up a few inches to see if he likes that better. Here is a useful document about nausea:
Nausea/inappetence - symptoms and treatments
If he's showing any signs of nausea when trying to eat then that can be addressed with prescription meds (e.g. GENERIC ondansetron - branded Zofran is very, very expensive but the generic should be much more affordable - available at human pharmacies with a written Rx from your vet). Get that under control and he might start feeling much brighter. A SNAP or Spec fPL test (if at all possible) could answer whether pancreatic inflammation is present, otherwise perhaps ask your vet to treat 'as if' pancreatitis is the problem. (It's impossible for me to say what's happening with Cosmo but, if it were the case that pancreatitis was at the root of his 'offness' we've seen cats here (my own included) who have been poleaxed by a flare but who make amazing recoveries when the right supportive treatments get put in place. Saoirse completely astounded me at times!) A course of B12 injections can give an under-the-weather cat with digestive system issues a major boost and anti-nausea treatment can do wonders.
Another thing I strongly recommend you try is testing your kitty's urine for ketones. See the following sticky:
Are you testing your cat for ketones?
If ketones were to be found at the root of Cosmo's 'offness' then they, too, can make a cat feel really lousy/lethargic/iffy about food but they are treatable (and treatment needs to be prompt to prevent DKA).
Also see if kidney issues/UTI might be the issue. Even a basic home urinalysis test may give you helpful info (e.g. whether there's blood / protein / glucose / ketones present; also how well/poorly urine is being concentrated (specific gravity) which may give a very rough indicator of whether kidney issues might be a factor driving the clinical signs you're observing).
Be sure to keep a close eye on Cosmo's hydration levels because dehydration can make a cat feel out of sorts (think what hangovers do to humans). The scruff will snap back into place and gums won't be sticky if a cat is well-hydrated. If dehydration is present the vet may be able to administer sub-q fluids (which can perk a cat up quite quickly) and, if regular sub-qs are needed going forward (e.g. for CKD issues) the vet may be able to teach you how to give sub-qs at home and members here should be able to give you tips on managing costs.
I hope the above isn't too overwhelming. I'm just brainstorming to try to give you suggestions of things to investigate and to discuss with your vet. Talking of which ...
The very best suggestion I can give to you is not to make any decision until Cosmo has been examined by the vet and try to get done whatever diagnostics you can manage on your funds. They will help Cosmo's body to 'speak' to you and you'll be able to make better informed choices for him.
Sending positive vibes for you both.
Mogs
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