Hi,
I won't say that it is impossible that this is FIP, but I think its more likely that your vet is right and this is a serious bacterial infection. Here are my reasons for this: FIP more commonly causes fluid in the abdomen rather than the chest (though it can happen). The main reason I don't think its FIP is because usually once cats reach the stage where fluid is accumulating they don't improve, their condition just steadily declines no matter what you do. Here is a link to a page about FIP:
http://www.vet.cornell.edu/fhc/brochures/fip.html
Continue to monitor him closely. Its a good sign that he's still eating and acting good. If he should develop more fluid accumulation, you may want to have your vet do a culture and sensitivity test on the fluid to determine for sure if there's a bacterial infection and which antibiotics its sensitive to.
If you have further questions let me know. If this answers your question please click accept. Thank you and best wishes.
Lynn
Why does the vet think its FIP when they didn't think the fluid looked right for it?
Okay, given this additional information it is more likely that this is FIP. However, if he's still eating well and acting okay you certainly do not have to euthanize him tomorrow. I don't see any harm in continuing to try to treat him as long as he keeps eating. I would keep him away from the kitten if possible though. You could also get a second opinion from another vet if you don't feel confident in this one.
If you don't feel its the right time to put him to sleep, then it isn't. What was wrong with the kitten?
Anything is possible though I think your cat would have gotten sick sooner if he was going to catch something from the kitten. Illnesses are usually the most contagious shortly before symptoms show up, and then in the first few days of sickness. Even considering an incubation period it is still a long time. And, unfortunately every cat doesn't have every symptom of FIP. Websites usually just list all of them that are possible. I am NOT trying to convince you to put your cat to sleep though. I really think you would do best getting a second opinion. If your kitty is eating and acting okay then he is not suffering.
Veterinary Assistant
worked at a vet clinic for 15 yrs, BA in biology, have had cats most of my life
You're welcome. I'll keep my fingers crossed for him!