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His signs are non-specific, but decreased appetite, vomiting, and changes in behavior are concerning.
I'm worried that he could have an underlying problem with a major organ like his livery or kidneys. He could also have pancreatitis.
I recommend having him evaluated by your local vet. I would start with a physical exam and blood testing including a fPL (feline pancreatic lipase).
Hopefully these will be enough to diagnose his problem, so that we can begin a targeted treatment plan.
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