Amoxicillin is really not good for anything other that mild respiratory disease or urinary tract disease. A better, more wide spectrum antibiotic is called clindamycin. There are many disease of cats that can cause them to lose body mass. I would have the vet do additional tests for FIV, FIP, Toxoplasmosis, and if it is in your area, any of the deep fungi.
Have any X-rays of the chest or abdomen been done? Did the vet do a Complete Blood Count and Blood Chemistries?
You did not mention if the cat was eating or not. I will often use a steroid called dexamethasone to get the cat to eat and to decrease non-specific imflammation (103 fever).
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Dr. Louis Gotthelf
Animal Hospital of Montgomery
Pet Skin and Ear Clinic
