Hun, I wouldn't bother testing Jeremy. They can only test for Corona virus (which mutates into FIP). Corona virus is highly contagious, the horse has already bolted.
They've already been living together for way long enough to cross infect each other months ago. I don't see the point in keeping them seperate now.
I can understand the frustration but if you want or expect a definitive diagnosis to say one way or another, I'm afraid you won't get one to say 'FIP' as it can only really be confirmed at postmortem.
If it matters at all, I doubt it really is FIP that's effecting Rolo as I've never heard a cat with certain FIP improve the way Rolo seems to be improving.
Corona virus more often than not is spread by cats sharing a litter tray and is much more common when many cats live together. Rescues almost expect to see it in over-crowded houses or houses where cats in poor condition or health are sharing facilities. In fact, Corona virus and the risk of FIP is one of the real biggies to look for when assessing cats that come into rescue with obvious health issues from a crowded and neglected or run down condition. I have no doubt Corona virus is the root cause behind many of the horror stories going around about mass euthanasia in hourding cases. It's a vile, vile disease.
Unfortunately it will never go away until the Corona virus goes away and sadly there is no vaccine.
The only way to prevent it is to segregate known carriers from other cats and do not allow them to share trays and to not breed from affected cats. In Rolo's case (and if it IS the dreaded Corona virus at the root of this), either he was born with it or he has in fact caught it from Jeremy.
No one knows what makes the Corona virus mutate and trigger FIP, although Rolo does fit the profile for developing full blown FIP being still quite young. But one thing is sure.... many, many cats carry the Corona virus without ever developing FIP. It is posible Rolo is carrying the Corona virus but it is something totally different and so far undiscivered that if making him ill... So what's the point of testing Jeremy? What are you going to do if he's positive other than worry? Even if he is positive, he might never fall sick.
The only circumstances I would test Jeremy is if at some point in the future you want to get another cat. Then naturally I would say if Jeremy is positive, don't adopt another cat while Jeremy is still with you.
Good luck, hun... believe it or not your case is frequently on my mind as I can see how scared you are and just wish I could offer a magic answer.
