This article is from the Feline Infectious Peritonitis FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
FIP usually appears in one of two forms: Effusive (wet) and Non-Effusive (dry). It should not be thought, however, that there are two different FIP diseases. The results of the infection are a continuum on a scale, with the 'wet version' being one end, the 'dry version' being in the middle, and a 'carrier' being the other end (a carrier is where the cat has successfully fought off the disease but may still be able to expose other cats to the virus). The way this happens is when a cat is exposed to FIPV, if its immune system gives a poor response, the wet form will develop. If it gives a better response, the dry form will develop. In the best responses, the cat will not develop either form of FIP, although it may be a carrier of the FIP virus.
The wet form is more common, and more rapid in progression than the dry form. It is characterized by the abdomen and/or chest progressively but painlessly distending with fluid. If this occurs in the chest, respiratory distress can occur due to compression of the lungs and release of fluid into the airways. The lining of the affected cavity will be covered with white, fibrin-containing areas (fibrin is a protein that is the center of a blood clot), often on the liver and spleen. Certain types of lymph nodes may be enlarged. Other signs include jaundice; mild anemia; and gastrointestinal, ocular (e.g. eye ulcers or severe conjunctivitis), and neurological signs may also occur.
The dry form is more rare (but appears to be becoming more common), and more slow in progression, often making diagnosis difficult. There is minimal fluid build-up, although weight loss, depression, anemia, and fever are almost always present. Signs of kidney failure, liver failure, pancreatic disease, neurologic disease or ocular disease may be seen in various combinations.
Often the organs in question develop a characteristic pyogranulomatous inflammation (this is a chronic inflammation resulting in a thickening of the tissue and local accumulation of white blood cells). Unfortunately biopsy of these lesions is the only definitive way to diagnose this form of FIP and is usually done in the form of a post-mortem diagnosis.