Reply #20: I had a patient a few years ago that I was pretty sure had FIP:

[kestrel91316]

young cat from a rescue situation and overcrowding, came in with a persistent fever over a period of weeks and weeks,and poor appetite (except for clay cat litter, which she nibbled on aimlessly). I found absolutely nothing on blood panel and UA. Went to an internal medicine specialist who did a battery of complex tests: ultrasound, CT scan, CSF tap, biopsies of everything, blah blah blah. Nothing the least bit abnormal. Still the fever and pica persisted, and the cat wasted away. Finally euthanized. They did a full necropsy (these owners had essentially unlimited funds - famous musician with dozens of hits years ago) and the only thing they found at all was the brain tissue on special staining came back positive for FIP. THAT'S how hard this disease can be to confirm, sigh. I think they spent almost $10k, and no cat at the end of it.