In a small percent of infected cats (5 to 10 percent), either by a mutation of the virus or by an aberration of the immune response, the infection progresses into clinical FIP... An intense inflammatory reaction occurs around vessels in the tissues where these infected cells locate, often in the abdomen, kidney, or brain. It is this interaction between the body’s own immune system and the virus that is responsible for the disease. Once a cat develops clinical FIP involving one or many systems of the cat’s body, the disease is progressive and is almost always fatal.
Very young and very old cats are most suseptible. After exposure it can take 4-6 months to show signs; but once symptoms appear, it progresses very rapidly, and there isn't anything to be done.
We thought we were going to have to take him in this morning, to have him put to sleep. Turned out not to be necessary. He was 5 months old.
This virus can be easily spread between cats in a shelter that is not scrupulous about procedures: quarantining new cats, scrubbing down and disinfecting cages between uses, etc. We've learned that the shelter he came from has been shut down in the past for just such carelessness. Apparently that lesson didn't sink in. I cannot begin to tell you how angry that leaves me. Maybe the call that the vet is making to the sheriff will get the point across this time.
Rest in Peace, Junior.