Feline Infectious Peritonitis (FIP)

As many readers of Felines will be aware, this disease occurs sporadically, and is usually fatal. This article is quite technical, and should be kept as a reference for time when your pet is thought to have FIP, so that the interested cat owner can understand what is happening to its pet, and why laboratory tests need to be done.

FIP is most common in cats between 6 months and 2 years old, and cats older than 12 years of age. The disease is caused by a corona virus. In my opinion feline corona viruses have caused more confusion in the diagnostic evaluation of sick cats than has any other organism. Feline Infection Peritonitis is an “immune mediated” disease. The primary lesions are caused by over-reaction of the body’s immune system to the virus inside the cat’s body’s own body cells, resulting ultimately in the death of the pet. The virus goes into the body and the body produces antibodies. But the antibodies are not protective and bind with the virus forming an antibody + FIP virus unit. This complex is destroyed by the immune system contributing to the disease syndrome of Feline Infectious Peritonitis.

The source of new infection is by mouth and most cats will then destroy the virus or get a mild gastro-intestinal infection. Respiratory infection with feline corona virus has not yet been confirmed. These recovered cats will show positive antibody test for FIP virus even though they are healthy. The problem is that “antibody level” related tests which can be significant in some diseases, are not diagnostic in this case because many healthy cats will show high levels of antibody. So we have to employ a number of tests that need to be used concurrently to reach a diagnosis.

There are two basic clinical forms of Feline Infectious peritonitis.

1. The wet form or fluid-filled belly form – is relatively easy to diagnose as the fluid has characteristics that practically no other feline disease would exhibit. The fluid is yellow, has a high protein content and often has fibrin streaks. I find that if shaken in a test tube, the “froth” that forms is still present the following day. If the fluid is sent for electrophoresis (a common laboratory procedure) a diagnostic protein pattern is found. Quoting a professor at the veterinary faculty, “Nothing else on God’s living earth would cause such an electrophoretic patter.”
2. The Dry form – here the only clinical signs are non-specific: the fluctuating high temperature and general malaise etc. Some cats show hindquarter unco-ordination and are hypersensitive to touch in that area.

There are five laboratory pointers to Feline Infectious Peritonitis

1. High serum proteins due to a high serum globulin (a type of antibody).
2. “Blood test” serum electrophoresis, showing typical electrophoretic pattern (a polyclonal gammopathy – or in English a large broad-based spike over the gamma globulin area. In all the tests the immune system is over-reacting producing excessive antibodies, that are measured as high blood proteins). A similar pattern may be seen in any long-standing disease.
3. Blood antibody levels to the FIP virus measured directly and often repeated 3 weeks later.
4. A liver biopsy is the next best test. Again the histopathology or microscopic changes seen are typical for FIP and we would confirm this disease on typical microscopic findings. Any other organ showing granulomatous reaction should be biopsied.
5. Post mortem diagnosis is usually accurate with very specific microscopic changes in the organs. In conclusion the diagnosis of FIP is based on a complete history, clinical exam, possibly liver and kidney function tests and possibly feline leukaemia virus and feline immuno-deficiency virus tests. All these tests should be combined, as no single test (except for histopathology) can yield a definitive diagnosis and your veterinarian may then be able to reach a diagnosis.

How a typical Feline Infectious Peritonitis case was worked up in our practice. An interesting case? Felix, a two-year old moggie was brought in to our hospital. His owner related a sick, lethargic cat. After one look I saw difficult breathing and after listening with a stethoscope it appeared that there was fluid in the chest and abdomen. After giving the owner the run-down on FIP, we admitted Felix for further tests. Sure enough, 5ml of clear yellow fluid was collected from the chest for analysis (I was almost certain FIP). Then 20ml of darker yellow fluid was collected for analysis from the abdomen (The odds of FIP are very high). The chest fluid was clear and had low proteins (probably a very early case). The abdominal fluid had typical high proteins (My diagnosis was still FIP, until proved otherwise). On drawing blood (while putting up a drip, as the cat was deteriorating), the blood protein was normal (Oh gosh!!! It is now half an hour after evening closing. Should I ??… What next!!)

Radiography or X-raying the stressed patient, as we do with any pet with breathing difficulty! Diagnosis: A diaphragmatic hernia, or tear in the diaphragm!! The organs were literally “squeezed” through the tear into the chest, causing oedema fluid to leak out (That explained the chest fluid). The abdominal fluid turned out to be urine! Treatment: Surgical repair after stabilization.

(Thank heavens Felix’s vet explored beyond the typical FIP symptoms, and discovered the real cause. Another example of not rushing into a hasty diagnosis.ED)

Written by Dr. Phil Sacks for a cat fancy magazine.