Feline infectious peritonitis (FIP) is a fatal, incurable disease that affects cats. It is believed by some to be caused by Feline Infectious Peritonitis Virus (FIPV), which is a mutation of Feline Enteric Coronavirus (FECV/FeCoV). Although there appears to be a connection between FIP and feline coronavirus, no clear cause and effect has yet been proven. Experts do not always agree on the specifics of FIP. However, the most common theory is that the normally benign FECV mutates into FIPV. The mutated virus has the ability to invade and grow in certain white blood cells, namely macrophages. The immune system's response causes an intense inflammatory reaction in the containing tissues. This disease is generally fatal However its incidence rate is roughly 1 in 5000 for households with one or two cats.
Despite the prevalence of FECV, most infected cats do not develop FIP. Often, exposure to FECV produces no clinical signs, but may cause a mild diarrhea. Therefore, a cat without clinical signs may still be an FECV carrier and may pass the virus to another cat. In any cat infected with FECV there is a chance that the virus may mutate into the FIP causing form. This chance is increased for cats that are immune compromised including very young and very old cats. There is also thought to be a genetic component to susceptibility to viral mutation.
There are two main forms of FIP: effusive (wet) and non-effusive (dry). While both types are fatal, the effusive form is more common (60-70% of all cases are wet) and progresses more rapidly than the non-effusive form.
The signs associated with FIP are often non-specific, which can cause diagnosis to be very difficult. There is as yet no definitive diagnostic test for FIP. Diagnosis may incude a combination strong clinical suspicion, physical examination findings, presence of adbominal fluid with characteristic chemistry changes and examination of affected tissues for the FIP virus (this is usually performed post-mortem), but can be performed via tissue biopsy. Histopathological examination of tissue samples is usually the cheapest available diagnostic test, but its sensitivity and specificity for FIP is questionable. A polymerase chain reaction test is also available for use with fluid or certain tissue samples; however, its efficacy is currently being reviewed.
More commonly, a presumptive diagnosis is made based on clinical signs and evaluation of abdominal or chest fluid, if available. Fluid caused by FIP tends to be yellow in color and have elevated protein levels. Blood tests can also be performed to bolster a presumptive diagnosis by looking for coronavirus antibodies and elevated protein. Coronavirus titers are NOT considered diagnostic in and of themselves due to the ubiquity of FeCoV, but may be used in conjunction with clinical symptoms to make an FIP diagnosis. It is important to note that cats with higher titres of FCoV are no more likely to develop FIP than those with lower titres.
Quarantine is not necessary as FIP is not truly an infectious disease. Feline Enteric Coronavirus is shed in the feces and can be passed on to other cats, however, it is the mutated form of the virus that leads to FIP. This form is only found in macrophages and is therefore not shed and not contagious.
As FIP signs can be easily overlooked, it is highly advised to have your cat examined by your family veterinarian at any signs of abdominal distention, changes in the eyes, chronic diarrhea, unusual lethargy or respiratory infection. While treatment will only be symptomatic, it may prolong the life of the cat as well as soften the blow to the owner.