Feline FIP: Causes and Prognosis

Key points (TL;DR)

• Feline infectious peritonitis (FIP) is a serious viral disease that mainly affects young cats and is caused by a mutation of feline coronavirus. It appears in two main forms: wet FIP, with fluid buildup in the abdomen or chest, and dry FIP, which affects internal organs.
• FIP is not contagious in its mutated form. Only the non-mutated feline coronavirus can spread from cat to cat through the fecal-oral route. The mutation that leads to FIP happens inside the cat’s body and is not transmitted to other cats or to other species.
• The prognosis remains serious, but the antiviral GS-441524 offers a genuine chance of recovery, especially when started early. Treatment is long, expensive, and requires close veterinary monitoring, while prevention depends on controlling coronavirus spread in group-living environments.

What is FIP in cats?

Among the diseases specific to cats, feline infectious peritonitis (FIP) stands out as one of the most severe. It is a serious viral disease and is considered a vices rédhibitoires, meaning a hidden defect of major importance in veterinary terms.

FIP develops from a coronavirus that is naturally present in a cat’s intestines but has undergone a mutation. Kittens and young cats are most often affected, as are very old cats (> 10 years) living in groups.

FIP results from a mutation of feline coronavirus (FCoV) in the intestines, transforming the virus into a virulent form capable of spreading throughout the body. Studies have identified specific mutations in the spike protein gene associated with FIP. These mutations alter the virus’s ability to infect cells and trigger a systemic inflammatory response.

There are two forms of FIP: the wet form, in which the virus causes fluid to accumulate in the abdominal and thoracic cavities, and the dry form, in which several organs stop functioning properly and a variety of clinical signs appear.

How much does FIP treatment in cats cost?

Treating feline infectious peritonitis (FIP) can be expensive. The final cost depends on several factors, including the cat’s weight, the form of the disease, the length of antiviral treatment, and the tests needed for follow-up. The protocol generally lasts 8 to 12 weeks.

Several complementary tests and examinations may be carried out in a veterinary clinic to help suspect or confirm the disease. Like consultations, they may be covered and reimbursed by Santévet cat insurance.

Transmission and spread of feline coronavirus

The feline coronavirus (FCoV) linked to feline infectious peritonitis (FIP) spreads mainly through the fecal-oral route. The key points are:

• Source of infection: The virus is found in the feces of infected cats, including cats with no visible symptoms.
• How infection happens: A cat becomes infected by licking litter trays, bowls, toys, or surfaces contaminated with feces from a carrier cat.
• Factors that increase spread: Group-living environments such as shelters, catteries, and breeding facilities raise the risk because cat density and shared litter boxes make transmission easier.
• Direct transmission: Direct spread through saliva or close contact is rare, unless the virus is present on a soiled coat or in the mouth of an infected cat.

Most FCoV infections do not lead to FIP. The disease develops when the virus mutates inside the cat and triggers an abnormal immune response.

Is FIP contagious?

Feline infectious peritonitis (FIP) is not contagious from one sick cat to another.

• FIP results from an internal mutation of feline coronavirus (FCoV) inside an infected cat’s body.
• The original feline coronavirus (FCoV) is contagious and spreads between cats, mainly by the fecal-oral route, but the mutated form that causes FIP does not spread directly.
• As a result, a cat with FIP cannot pass the disease on to another cat, even by sharing litter boxes or food bowls.

FIP is severe and often fatal, but the disease itself is not transmissible; only the initial FCoV infection is contagious.

FIP is a disease specific to cats and does not spread to other species. The non-mutated feline coronavirus can circulate among cats, but it does not infect dogs, humans, or other animals.

At what age can a cat develop FIP?

FIP (feline infectious peritonitis) can occur at any age, but it mainly affects young cats:

• Cats under 2 years old: Most cases occur in this age group, especially between 6 months and 2 years, because the immune system is still maturing and these cats are more vulnerable to coronavirus mutation.
• Adult cats: Less commonly, cats aged 3 to 5 years may develop FIP, especially when stress, concurrent infections, or a collective living environment are present.
• Older cats: Cases are rare and are usually linked to immune disorders or coinfections.

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The table below helps distinguish the two forms of FIP and identify symptoms in cats:

The presence of abdominal or thoracic fluid is the most suggestive sign of wet FIP, while varied, progressive symptoms such as weight loss, persistent fever, and neurological signs point more toward dry FIP.

Some cats may carry the virus without developing the disease: they are asymptomatic.

Diagnosis and testing for FIP

Diagnosing FIP in cats can be difficult, because its symptoms can resemble those of other feline diseases. In addition, there is no single test that can confirm the disease with certainty.

Blood tests

Veterinarians may perform blood tests to look for antibodies against coronavirus in the cat. However, these results are not always reliable, because cats may carry the intestinal coronavirus without developing FIP.

Ultrasound

Ultrasound can be used to detect fluid buildup in the abdomen or thoracic cavity (effusions), which is characteristic of the wet form of the disease.

Biopsy

A biopsy of affected tissues can help confirm the diagnosis of FIP by revealing lesions typical of the disease.

Fluid analysis

The veterinarian may collect fluid from the thoracic cavity or abdomen to look for signs of the disease.

PCR

This test can be used to detect viral RNA in tissues or biological samples.

Santévet’s view: diagnosing FIP relies on a combination of tests, including PCR to detect viral RNA, analysis of abdominal or pleural fluid, blood tests, and biopsies. Distinguishing between wet and dry FIP is essential for prognosis and treatment.

Can FIP be treated?

Treatment for feline infectious peritonitis (FIP) has made major progress in recent years thanks to the introduction of specific antivirals. FIP, a serious viral disease caused by feline coronavirus (FCoV), was once considered incurable. More recent research has led to effective veterinary medicines, especially GS-441524, a remdesivir analog, which has shown promising results in the treatment of this disease.

Antiviral treatment with GS-441524

GS-441524 is an antiviral that inhibits viral replication by targeting the RNA polymerase enzyme, which is essential for FCoV multiplication. It is administered as an oral paste or by subcutaneous injection, usually for 12 weeks. Studies have shown that this treatment can lead to complete recovery in most cats with FIP, regardless of the clinical form of the disease.

Treatment protocol

Treatment begins with an initial daily dose. The dosage may be adjusted depending on the cat’s clinical response and the possible presence of neurological or eye involvement, which may require higher doses. It is crucial to follow the protocol strictly and attend regular veterinary check-ups to assess the treatment’s effectiveness and adjust the dose if needed.

Possible side effects

Although GS-441524 is generally well tolerated, some cats may develop side effects such as gastrointestinal upset (vomiting, diarrhea) or liver and kidney abnormalities. It is important to monitor the cat closely throughout treatment and contact the veterinarian if any concerning signs appear.

Treatment for FIP has therefore made major progress thanks to GS-441524. Although costly, it offers a real chance of recovery for cats affected by this serious disease. A veterinarian should be consulted to discuss treatment options and determine the most appropriate protocol for each individual case.

GS-441524 has shown significant effectiveness in the treatment of FIP. A review of 11 studies* reported an overall therapeutic success rate of 84.6%.

Life expectancy and prognosis for FIP

The prognosis for an animal with FIP is always guarded, and death is inevitable. Most cats with the wet form survive only a few weeks to a few months after diagnosis.

Cats with the dry form may survive somewhat longer, sometimes for several months or even several years, with appropriate palliative care. This disease is covered by cat insurance, and veterinary expenses are reimbursed according to the chosen plan, up to 100%.

Is there a vaccine against FIP?

No vaccine is currently available in France. Prevention therefore relies only on screening in catteries and breeding facilities.

If you have several cats and one of them develops FIP, there is a strong chance that the others have been exposed, but on average only 1 cat in 5 carrying coronavirus develops the disease.

Feline infectious peritonitis (FIP) is therefore a fatal viral disease that affects cats worldwide. It is caused by feline coronavirus and can take two different forms: wet and dry. Unfortunately, there is currently no curative treatment for FIP in cats, and the prognosis is generally poor for affected cats. It is therefore important to take preventive steps to protect cats from this disease, including keeping them in appropriate sanitary conditions.

Sources:

• Gokalsing E, et al. Efficacy of GS-441524 for Feline Infectious Peritonitis: A Systematic Review (2018–2024). Viruses. 2025;16(7):1144.
• Yong-Yu Gao, Qian Wang, Xiang-Yu Liang, Shuang Zhang, Di Bao, Han Zhao, Shao-Bai Li, Kai Wang, Gui-Xue Hu, Feng-Shan Gao, An updated review of feline coronavirus: mind the two biotypes, Virus Research.

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