There are several diseases we commonly see in older cats. With the history you give about Pixy Bob, two come to mind right away. Hyperthyroidism and Diabetes. I will send you links to more information about both- http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=516
There are other diseases that we see in cats, but if I do an exam on an older kitty that is losing weight, but still eating well (and no concurrent vomiting or diarrhea), those are the first two that come to mind. I typically start with bloodwork- a CBC, Chemistry panel, and a T4 reading. The chemistry panel will give us a blood glucose reading. If it is elevated, we have our diagnosis of diabetes. The T4 helps us diagnose hyperthyroidism. Other diagnostics may also be warranted such as a blood pressure reading. Many of the hyperthyroid cats will also have hypertension. A fully chemistry panel and CBC are useful to make sure we aren't dealing with any other concurrent issues such as underlying kidney disease or anemia. Both diseases are controllable. I would recommend starting with a thorough exam by your vet and bloodwork. Your vet may recommend other diagnostics based on the exam findings.
I hope this information was helpful to you.
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