FIP – Feline Infectious Peritonitis

Feline infectious peritonitis, or FIP, is a serious systemic disease affecting cats of all ages, but most common in young cats. FIP is caused by a virus, although the type of virus is quite unique when compared to other feline viruses. The reason for this is that the causative virus for FIP is a mutated version of the common coronavirus. 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 multi-systemic disease that up until recently was considered invariably fatal.

For many years following its discovery, it was believed that the mutation that occurred 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 occurring 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 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 abdominal 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 nebulous and very much like other feline viral diseases. These include vomiting, diarrhea, decreased or lack of appetite, lethargy/depression, ocular inflammation, and neurological signs (seizures, 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 well 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 blood work. On blood chemistry, FIP patients often show significant elevations in immunoglobulins and total serum protein.

As previously mentioned, a straw colored abdominal 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 ocular form, the presence of retinal granulomas on routine retinal examination are supportive evidence for FIP.

There used to be no effective treatment for FIP and the disease was invariably fatal.  Within the past several years, antiviral agents GS-441524 (patented by Gilead Sciences) and GC376 (patented by Kansas State University) have been shown to reverse the progression of FIP in clinical trials involving several dozen infected cats and are now FDA approved for use in the US. The agents interfere with the virus’s ability to hijack the body’s immune system.

The names GS and GC that precede each FIP drug are derived from small molecules that easily pass from the bloodstream into infected cells. Once inside infected cells, they target specific viral proteins that control viral replication. GS inhibits the formation of viral RNA early on and GC inhibits the formation of mature viral proteins at a later stage. In trials so far, GS has been shown to be the more effective.

Once the disease is in a premises, there is a risk to other cats on premises, but the risk is considered low since, as previously mentioned, the coronavirus is likely what infects the host, with the mutation to FIP variant occurring within the host. However, since chronic exposure to coronavirus is the most likely mechanism for FIP, disinfection of the premises, including food bowls, bedding, and litter boxes is a good idea. Coronavirus is killed by most conventional household disinfectants.

While a FIP vaccine is available, it has not been proven effective and is not recommended by the American Association of Feline Practitioners Feline Vaccine Advisory Panel. While the vaccine may be safe, risks should be weighed carefully. Before starting any new vaccine or treatment, you should talk with your veterinarian.

Roger L. Welton, DVM

Founder and Chief Editor, Web-DVM.net

CEO Premier Veterinary Care

Host Space Coast Pet Podcast

Author, The Man in the White Coat

Article updated 6/3/2026