Thank you for the additional information.
The reason I asked about his signs and the work-up done by his vet is because there is no one test for FIP. When diagnosing it, we tend to have to look at a number of factors (ie nature of the fluid withdrawn, the virus titre, and the white cell and inflammatory protein levels). That said, if the vet has tapped the fluid from the chest then they will have a fair idea if FIP is present (since bacterial, trauma, lymph, or heart based fluids have significantly different appearances to the fluid we see with FIP).
Now your vet was not exaggerating about the lack of cure for this disease. This is a horrific disease that targets young cats his age and sadly doesn't yet have a cure. The virus damages the blood vessels, causing them to leak, which leads to the fluid you have seen fill the chest (and that will be doing so again now). We do find that we can buy some time and slow this disease using immunosuppressive doses of steroids and feline antivirals. Still even with these treatments, we only get days to weeks of survival time for them. And the additional problem for your wee one is that if the fluid is accumulating in the chest (rather then the belly), he will likely have respiratory issues even with treatment and likely will be on the lower end of the survival spectrum even with treatment. So, if the diagnosis is confirmed by the lab, then I am very sorry to say that his prognosis is grave.
In regards XXXXX XXXXX other cat, since this disease syndrome is thought to occur in the face of individual viral mutation and certain components of a individual cat's immune system, this isn't something that is thought to be spread in the traditional sense. It is quite likely the other cat is already carrying or has been exposed to the virus (feline corona virus, FCoV). That said, it doesn't mean that he will experience this disease syndrome (since the virus may not mutate in him or he may not have a genetic immune weakness like the other kitten does). Furthermore, a study on this very topic found that only ~12% of cats exposed to another cat with active FIP went on to develop it themselves. So, we cannot say that this second cat is not going to develop this disease but that he is more likely not to then to develop it.
In regards XXXXX XXXXX and lessening this risk, if the results confirm FIP, there will be some steps you can take for the second cat. First, to appreciate his exposure, you may want to have a FCoV antibody titre checked on the 2nd cat. This will likely be positive but will give you a base line to monitor from periodically until he potentially reverts back to a 0 titre. Alternatively, you assume that he is positive now and then potentially check the titre at 6 months to a year later to see if he has become negative or is still positive for exposure (which that far on does make carrier status more likely). This can be helpful since a significant number of cats will show a steady decline of the titre with time and a reversion to seronegativity (0). But to just give us an extra wee stress, some cats will be have titres that can take years to drop if ever (Despite never developing FIP in their lives). So, it gives us some information (and gives a great sigh of relief if/when they are seronegative) and allow us to monitor his level of exposure and likihood of being a carrier. And it is something that can be useful if you did consider introducing another cat down the line (so you could appreciate the risk you may be exposing them to).
As well, if FIP is confirmed, then we do want to make sure that we practice great hygiene in the house. The virus is passed via fecal-oral exposure and is a virus that can live in the environment for a month or more. Therefore, we want to make sure litter boxes are clean and routine cleaning (with dilute bleach)/hoovering is performed to keep household contamination of the virus as low as possible.
Overall, FIP is that one viral infection of our generation (where feline leukemia virus was the bane of cat vets 20+ years ago) that we all struggle with. Researchers are trying to get to the bottom of how this virus works and how to stop it but so far progress has been limited. This means that when this virus rears its head, we have very little to stop it, nothing to cure it, and therefore it is often a fatal disease of the young cat. So, I do hope that the results don't support FIP for your wee one because if they do, then his prognosis is poor to grave and euthanasia may be the most humane option for him.
Please take care,
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