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Hello, I'm Dr Gary. I've been practicing veterinary medicine since 2007. I look forward to helping with your questions/ concerns.
The decreased appetite and weight loss could be one of many things. Hopefully it's just some type of GI bug or virus and some symptomatic treatment will do the trick.
I would definitely want to rule out kidney disease and diabetes. These are two common conditions that we see in older cats with weight loss and GI signs. I would also want to try to rule out an abdominal mass that could be Lymphoma.
If the bloodwork and exam are normal, then I would treat with some sub-Q fluids, appetite stimulants (Mirtazapine) and supportive care.
If anything is abnormal, then you treat more specifically. The first step is the bloodwork and exam to see what's going on.
Let me know if you have any other questions and I'll try to help more.