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FIP – Feline Infectious Peritonitis

Feline infectious peritonitis, or FIP, is a serious systemic disease affecting cats of all ages, but most common in young or old cats. FIP is caused by a virus, although th type of virus is quite unique when compared to other feline viruses. The reason for this is that the causitive virus for FIP is a mutated version of the common coronvirus. Coronavirus, however, unlike the FIP variant, typically causes only mild, transient gastrointestinal upset, most commonly presenting with diarrhea, that rarely becomes serious. FIP, on the other hand, is a multisystemic disease that is considered invariably fatal.

For many years following its discovery, it was believed that the mutation that occured from coronavirus to FIP variant happened outside the host. However, there was no scientific evidence to explain precisely how and why coronavirus mutated into the FIP variant. Therefore, until recently, feline infectious peritonitis remained very much a mystery disease. New research, however, has led to the theory that FIP does not exist in the environment at all, but enters the host as a corona virus, with the mutation occuring within the host itself. The mechanism by which this happens is not certain, but believed to be a viral mutation due to a hyper-immune occurence brought on by heightened immune status from chronic exposure to coronavirus. This offers an explanation as to why FIP tends to be prevalent most commonly in multiple cat environments (catteries, shelters, large multi-cat households, etc.). Whatever the case, once a cat becomes infected with the FIP variant, two different clinical syndromes will occur.

FIP Wet Form

With the wet form of FIP, there is typically a collection of free fluid within the body cavities (abdomen and chest). These cats often present very sick, with a pot bellied appearance due to the abdomenal fluid accumulation, and/or having difficulty breathing due to chest fluid accumulation. The fluid is typically straw colored, and under microscopic analysis shows an abundance of two types of white blood cells, monocytes and neutrophils (termed a pyogranulomatous effusion).

One or multiple organs can be infiltrated and affected by the FIP variant virus, including the liver, kidneys, eyes, brain, spleen, lungs and GI tract.

FIP Dry Form

The dry form of FIP is clinically similar to the wet form, the difference being that there is no abdominal or chest fluid, and hence no pot belly appearance. Other than that, all of the same organ systems can potentially be affected.

Clinical signs of FIP can be quite nebulus and very much like other feline viral diseases. These include vomiting, diarrhea, decreased or lack of appetite, lethargy/depression, and neurological signs (siezures, trouble walking, or paralysis) if there is brain involvement. There is typically a severe fever.

Diagnosis of FIP is not as straight forward as other feline viruses, and can be quite challenging to say the least. The antibody and PCR blood tests, unfortunately do not differentiate between exposure to coronavirus vs. the FIP variant. Since as high as 80 % of cats have been exposed to coronavirus, while a negative blood test result is reliable, a positive blood test does not confirm a diagnosis of FIP. Therefore, if FIP is suspected and the blood test shows positive, more evidence is required to arrive at a diagnosis of the disease.

Further support for FIP can be found in routine bloodwork. On blood chemistry, FIP patients often show significant elevations in their immunoglobulins and total serum protein.

As previously mentioned, a straw colored abdomenal fluid showing microscopically to have a predominance of white blood cells called monocytes and neutrophils, is also indicative of FIP.

On ultrasound of the abdomen, the presence of multifocal nodules called granulomas, present on organs such as the liver, kidneys, and spleen, are also supportive of FIP.

In the occular form, the presence of retinal granulomas on routine retinal examination are supportive evidence for FIP.

At this time, no treatment exists for FIP. Therefore, once a diagnosis of FIP is attained, euthanasia is usually recommended for humane reasons. Once the disease is in a premises, there is a risk to other cats on premises, but the risk is considered rare since, as previously mentioned, the coronavirus is likely what infects the host, with the mutation to FIP variant occuring wthin the host. However, since chronic exposure to coronavirus is the most likely mechanism for FIP, then disinfection of the premises, including food bowls, bedding, and litter boxes is a good idea. Coronavirus is killed by most conventional houshold disinfectants.

There is an FDA approved vaccine whose manufacturer claims 60% – 70% effectiveness, but this claim has never been substantiated by any unbiased research. What’s more, now that it is postulated that heightened immunity may be the mechanism of FIP in the first place, many feel that it is even dangerous to give the vaccine. For all these reasons, the FIP vaccine is considered by most veterinarians to be ineffective, or even dangerous.


Roger L. Welton, DVM
Founder and Chief Editor,
President, Maybeck Animal Hospital

Article updated 10/1/2012

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