FIP or NOT ? please help him !!

Hello, I really need your advise if anybody had to deal with FIP before... or anyone really

My little Syrius is 4,5 months old, he was diagnosed by a first vet last week, saying he was 99% sure he has the wet type of FIP. Syrius had a lot of liquid in his abdominal area only (that showed a bit yellow-ish when the vet punctured it), was very skinny (but at first I thought maybe he had worms) and had a high fever (39,7°C). He gave him a shot of cortisone.

The vet ran some blood tests and Syrius was negative to all of the viruses, including the one that causes FIP. At first I thought that was good news, but the vet told me that since he was young, maybe he didn't have the time to develop the coronavirus antibodies in his blood yet or, maybe all the antibodies went to the ascites's liquid.

I went to another vet, the first one told me that he had to send the effusion to run some other test, though I thought he could have done it from the beginning instead of making me pay another time. 

I went to another vet, who punctured the effusion and sent it for further tests. This one gave Syrius antibiotics for 5 days and told me it should stop the infection from spreading (the cortisone suppresses the immune system, therefore shouldn't be given without antibiotics) but the fever should not go down.

After 3 days of antibiotics, I took his temperature and it was 38,5°C, no fever !

I thought the fever was supposed to be persistant with FIP ? Is this possible that he has FIP without having fever AND the antibodies in his blood ?

It's been 5 days now, he still has no fever, has a lot of appetite but since he has a lot of liquid in his belly, he can't eat as much as he wants, he is constipated too because of the liquid. I asked the vet what we could do about it, she told me to try some olive oil but since it's a mecanical thing it probably won't work... there aren't much things to do. She also told me to stop the antibiotics and if the fever comes back, I can start them again. ah and his pee is orange now too, so maybe really dehydrated or some renal failure I guess.

Can someone tell me if he ever were in this situation ?

She didn't run any further blood tests and the first vet didn't do all the blood tests (albumin ratio / globulins etc..)

I have had way too much experience with FIP, both the wet and the dry kind.  Sometimes draining the fluid from the belly will help them be more comfortable for awhile, but the fluid is really their own body trying to fight the virus that is invading their organs.  FIP is hard to diagnose perfectly and other than the fever and fluid, it can manifest in a lot of ways.  If the fluid is yellow and about the texture of egg white, I always assume it's FIP and that it's just a matter of time.  I spoil them rotten until they don't feel well anymore and don't want to eat, and then I have always put them to sleep before they can suffer.  I learned that with the first two, that you really can't bring them back once they get to the depressed and not eating stage, so I don't go beyond that. 

Since there is no treatment available yet other than supportive care, as you are doing, once you make sure he has no other infection that might be causing ascites (and those fluids usually look quite different than FIP fluid), just keep him comfortable and let him enjoy whatever time he has.  It's always possible that his body could beat it (rare but it happens) so keeping him as stress free as possible is the best way to go.

Here are some things you may want to read in order to better understand it.  (Note, the two cornell links are NOT the same article.)  The last link includes in it the link to the 2016 treatment update.  (A treatment has been discovered but it's still just in the laboratory phase.)

http://www.vet.cornell.edu/fhc/Health_Information/CW_FIP.cfm

http://www.vet.cornell.edu/fhc/Health_Information/brochure_ftp.cfm

http://www.2ndchance.info/fip.htm

I lost a kitten to FIP. We kept hoping that it wasn't FIP. It wasn't until his abdomen expanded overnight and i took him back to the vet. She did an ultrasound, saw many pockets of fluid, extracted it and it was yellow. She put some on her fingers to show me how syrup-like it is. She said it's mostly protein which is why it's so thick. And then she said this is a huge indication of FIP (wet form). And that it's the only disease that does this specific kind of yellow thick fluid. She was 99.99% sure. He had been running a fever, jaundice and anemia for 3 weeks before the abdomen swelled, though.