This is my simplistic understanding of it*:-
The Coronavirus can easily be transmitted between cats, often through poo. It's likely that a lot of cats have it, or have had it at some point and in the majority of cases it doesn't cause any major issues.
FIP occurs due to the body’s reaction to the virus. From what I’ve read the FIP disease can not be passed on and will only occur in a small number of cases. It’s actually not that common. It seems to me that it is generally young kittens or older cats who are weak or those who have immunity issues who are more susceptible* and Dr Addie suggests that stress can also play a part.
Those with FIP will still be shedding coronavirus in their poo, so this is where the main cross contamination risk comes from. Cats do not become immune to it either, they can continue to re-infect each other, although there are ways to reduce the risks with good litter hygiene. In a multi cat house like mine it will be harder to eradicate.
There is no test for FIP only for Coronavirus, therefore Vets generally use blood tests and symptoms to make a diagnosis.
There are two types of FIP, wet and dry. The websites below give specifics on symptoms for each.
My Vet told me to isolate my kittens from the rest of the household. After researching I believe this is not necessary. Again I stress, you need to make your own decision on this.
*Please note that these are only my own interpretations – if you’re affected by this see the links below to the resources I used. I’d strongly suggest you read and make your own decisions – and if your pet is ill, see your vet
Where I found information to become more informed:-
Dr Diane D Addie PHD, BVMS is a veterinary surgeon and virologist who has spent the last 23 years researching feline infectious peritonitis (FIP) and feline coronavirus infection (FCoV) from the University of Glasgow. She has dedicated her website to information resources for cat owners, breeders and veterinarians which can be found at www.dr-addie.com
You will find a variety of free resources here, but also a downloadable booklet. If you can purchase the booklet please do, as the money received goes towards funding research into this horrible disease.
The Cornell University College of Veterinary Medicine published some interesting research information in June 2013 entitled:-
downloads page and look for the leaflet entitled Feline Coronavirus (FCoV) and (FIP) - PDF
Personal thanks to all these people/sites (and many more I came across) for putting this invaluable information out there. Vets don't always get it right so make sure you research.