A disease that strikes at young cats. A disease that has no definitive diagnostic tests. A disease that is nearly 100% fatal. A disease that has no effective treatment. A disease that is infectious but is not contagious. Cat owners know and fear the diagnosis. FIP is the feline practitioners chimera. A poorly understood untreatable and deadly disease that offers no warning that it might be on the horizon.
From what we currently understand, FIP is an inflammatory reaction to infection with the feline enteric coronavirus. The feline enteric coronavirus is not involved in SARS which is caused by a different species of coronavirus. The majority of cats infected with coronavirus typically encountered the virus in infected feces (litter box) and this typically occurs in environments where multiple cats are housed together. They will exhibit flu like symptoms for a few weeks maybe a few months and depending on the number of cats in the house they may or may not clear the virus. Homes with 5 or fewer cats seem to spontaneously clear the virus eventually but houses with more than 5 cats will almost never clear the virus. Cats that are exposed to and infected with coronavirus and cleared it are still susceptible to reinfection. They can easily pick up the virus again and again.
As long as the virus sticks to your cats gastrointestinal system your cat will not develop FIP and will most likely clear the virus, unless you have a lot of cats. There are multiple theories about how FIP occurs and how it spreads in the body. Today, in 2012, I can give you the most recent understanding we have and the most recent ideas being proposed about FIP.
It is currently thought that FIP is the body reacting to a mutated version of the Corona virus. We can't cause transmission of FIP by putting a cat with FIP into a densely packed cat environment. We can cause transmission by taking fluid from one cat and injecting it into another cat. This supports the disease being infectious (fulfills Koch's Postulates) but not actively contagious.
Assuming that we are correct about the virus that causes FIP being a mutated strain we can then make some inferences about how it causes disease. When the body is dealing with a virus it utilizes a particular cell called a macrophage. Macrophage means "big eater" in Greek and that is exactly what these cells do. The engulf cellular debris, foreign material, bacteria and viruses and package it into little sacks called phagostomes. The macrophage then joins forces with a cell called a lysome to form a phagocyte. This phagolysome fills the sack containing the invader with digestive enzymes and peroxides. Once the macrophage has ingested a foreign invader it will display a piece of the invader's surface on the outside of its cell. This encourages the body to create antibodies and allows for a sort of seek and destroy mission to commence within the body. This is where FIP becomes a problem.
In FIP the mutated virus is immune to the degradation activity of the macrophage. So the body still goes through the action of creating more and more antibodies and macrophages but they don't do anything and the virus continues to replicate. Increased numbers of viral particles cause an increase in the production of macrophages. These useless macrophages start to coalesce in the internal organs into tumors called pyogranulomas. FIP is the infiltration of these pyogranulomas into internal organs.
Diagnosing FIP is difficult to say the least. It typically is a rule out diagnosis meaning that even if that is what we are suspecting from the beginning, we still need to rule out just about everything else before we can be comfortable telling you your cat had FIP. This means blood work, sampling fluid from the abdomen and if possible taking a surgical biopsy. There are few diseases that cause the spread of pyogranulomas through the body and if we already suspect FIP for other reasons that would make me comfortable with the diagnosis.
Once we have a diagnosis treatment is limited or non existent. Because the pyogranulomas and inflammation are caused by the body's immune system the mainstay of therapy for a long time was suppressing the immune system. Immunosuppression will slow the progression of the disease but because FIP is also a viral infection immunosuppression is not going to be a cure. In cases where there is a lot of abdominal effusion or fluid removing the fluid may help to lessen the symptoms and may also slow the progression of the disease.
Right now research is looking into different anti-viral therapies to treat FIP, people are also working on breeding cats that are more resistant to FIP. Currently neither of these approaches have been successful yet. FIP continues to be a fatal disease for the affected cats.