Penn is home again, and his belly is shaved from one side to the other, so he looks like a standard poodle shaving experiment gone horribly wrong. The vet isn’t sure, but this could be one of two things: Feline infectious peritonitis, which is an untreatable virus thing that cats can get at birth, or, more seriously, mysenteric cancer (a fancy way of saying he might have a nonspecific cancer throughout the sac that surrounds his organs).
The vet is sending some of the fluid that’s gathering in his abdomen out for some tests which might tell us something more conclusive, but the nature of FIP is such that they can’t be positive about it. The next steps will probably be some form of exploratory surgery to determine exactly what’s happening inside.
Neither one of these things is good news; FIP is lousy, and cancer is shitty. This sucks.