FIP manifests in two forms: effusive, or "wet", and noneffusive, or "dry". The wet form comprises 60 to 70 percent of all cases and has a more rapid progression than dry FIP.
In wet FIP, fluid accumulates in body cavities—typically in the abdominal cavity, where it causes progressive, but generally painless, swelling; or in the thoracic (chest) cavity, where it causes respiratory distress due to compression of the lungs and fluid backing up into the airways.
In dry FIP, fluid buildup is minimal and symptoms depend on which organs are affected by lesions. About half of all dry cases produce eye inflammation or neurological problems: paralysis, unsteady gait, and seizures. Other lesions may cause kidney or liver failure. Weight loss, pancreatic disease, depression, anemia, and fever are usually present.
Despite these differences, wet and dry FIP are not two different diseases. The strength of a cat's immune system response appears to determine which form of the disease it develops. If the immune system reacts poorly, the cat probably will develop the wet form. A stronger response might result in the dry form. When the cat's immune system responds optimally, the animal develops neither form but may become a virus carrier for several years. A carrier cat whose immune system weakens over time eventually may develop the full-blown disease.