Feline coronavirus (FCoV) consists of two biotypes: feline enteric coronavirus (FECV), causing mild systemic signs, and feline infectious peritonitis virus (FIPV), causing FIP. Two theories on FIP development exist. The mutation theory maintains that FIPV mutates from FECV in an infected cat and no horizontal transmission occurs. The circulating virulent-avirulent FCoV hypothesis claims that FECV and FIPV strains circulate in the environment, and FIP only results from infection with the FIPV strain. New developments indicate that the vasculitis occurring with clinical disease is driven by monocytes, and that endothelial cells can assume a primary role. In the wet (effusive) form of FIP, vasculitis is enhanced by antigen-antibody immune complex formation, resulting in antibody-dependent enhancement of monocyte infection and effusive disease. In the dry (proliferative) FIP form, a mixed Type I/Type II immune response results in granuloma and plaque formation on serosal surfaces of organs.
 
Concentrations of the acute phase protein (ie, α-1-acid glycoprotein) >3000 μg/mL are supportive of, but not definitive for, diagnosis. High globulin levels and a decreased albumin:globulin ratio are found most consistently. Fluid analysis can be useful. Histopathology remains the gold standard for diagnosis. Treatment is supportive and symptomatic, including immunosuppressive and antiinflammatory drugs. Although criteria for remission exist, FIP is usually progressive and fatal, and humane euthanasia may be considered when refractory disease is present.

Commentary
Despite much research about the natural biology of the causative virus and the disease, FIP remains a poorly understood and potentially frustrating disease. This welcome update reviewed how and why FIP develops in certain kittens but not in others, as well as the reliability of testing and new treatment approaches, such as polyprenyl immunostimulant. Further investigation for FIP treatment includes small interfering RNA molecules and drugs inhibiting the ability of the virus to evade immune response. Future combinations of therapeutic approaches may make FIP, particularly noneffusive FIP, a chronically manageable disease.—Susan Little, DVM, DABVP (Feline)

Source
Update on feline infectious peritonitis. German A. IN PRACT 34:282-291, 2012.