Last updated: April 6, 2026

Feline Infectious Peritonitis (FIP): A Complete Guide

Feline infectious peritonitis (FIP) has long been one of the most feared diagnoses in feline medicine — a disease that was, until very recently, considered uniformly fatal. For decades, a positive FIP diagnosis meant preparing to say goodbye. That reality has changed dramatically. The development of effective antiviral treatments has transformed FIP from a death sentence into a treatable disease, and thousands of cats worldwide have now been successfully cured. This guide covers everything cat owners need to know about FIP, from understanding the disease to navigating the increasingly hopeful treatment landscape.

FIP is caused by a mutation of feline coronavirus (FCoV), a common and usually harmless intestinal virus that infects a large percentage of cats, particularly those in multi-cat environments. In a small fraction of infected cats, the virus mutates within the body into a form that can infect and replicate within macrophages — a type of white blood cell. This mutated virus triggers a devastating immune-mediated inflammatory response that can affect virtually any organ system in the body.

What Is Feline Infectious Peritonitis?

FIP is a complex disease that begins with a simple, common virus and ends with a potentially fatal systemic illness. Understanding this progression is key to understanding the disease.

Feline enteric coronavirus (FECV) is an extremely common virus in the cat population. It spreads through the fecal-oral route and primarily infects the cells lining the intestine. Most cats that contract FECV either show no symptoms at all or develop mild, self-limiting diarrhea. The virus is shed in feces and is particularly prevalent in multi-cat environments — some studies estimate that 25 to 40 percent of household cats and up to 90 percent of cats in multi-cat settings have been exposed to feline coronavirus.

In a small percentage of infected cats — estimated at 5 to 10 percent — the virus undergoes spontaneous mutations that transform it from an intestinal pathogen into a macrophage-tropic pathogen. These mutations occur within the individual cat’s body and are not transmitted between cats. The mutated virus (which is the FIP-causing biotype) enters macrophages and uses them as Trojan horses to spread throughout the body via the bloodstream. As the virus-laden macrophages accumulate in various organs, they trigger an intense immune-mediated inflammatory response that produces the clinical disease known as FIP.

FIP manifests in two classical forms, though many cats show features of both. The effusive (wet) form is characterized by the accumulation of protein-rich fluid in the abdominal cavity, chest cavity, or both. The non-effusive (dry or granulomatous) form is characterized by granulomatous inflammatory lesions in various organs, including the kidneys, liver, spleen, lymph nodes, eyes, and central nervous system. The distinction between wet and dry forms is less rigid than once thought — they represent a spectrum of the same disease process, with the form determined by the balance between the immune system’s cell-mediated and humoral responses.

Causes and Risk Factors

The fundamental cause of FIP is the mutation of feline enteric coronavirus within an individual cat. Several factors influence which cats are most likely to develop FIP after coronavirus exposure.

Age. Young cats between three months and two years of age are most commonly affected. This likely reflects the combination of high coronavirus exposure in kitten-rearing environments and the still-maturing immune system’s less effective cell-mediated response to viral infection. A second, smaller peak occurs in cats over ten years of age, possibly related to age-related immune decline.

Multi-cat environments. Shelters, catteries, breeding facilities, and multi-cat households have higher rates of FIP because of higher coronavirus prevalence and viral loads. Cats in single-cat or low-density households have significantly lower risk. If you have recently adopted a kitten from a shelter, the kitten was almost certainly exposed to feline coronavirus, though the vast majority will never develop FIP.

Immune status. The cat’s immune response — specifically the balance between cell-mediated immunity (which is protective) and humoral immunity (which can be counterproductive in FIP) — determines whether coronavirus mutation leads to clinical disease. Cats with strong cell-mediated immune responses typically control the mutated virus before it can cause widespread disease. Cats with weaker cell-mediated responses are more vulnerable.

Stress. Stressful events — adoption, surgery, rehoming, introduction of new animals, transport — have been associated with the development of FIP, presumably by affecting immune function at a critical time.

Genetics. Certain breeds and bloodlines appear to have higher susceptibility to FIP. Abyssinians, Bengals, Birmans, Himalayans, Ragdolls, and Rex breeds have been reported with higher prevalence in some studies, though FIP can affect any breed.

FIV and FeLV. Concurrent infection with feline immunodeficiency virus or feline leukemia virus may increase susceptibility by compromising immune function.

Symptoms of FIP in Cats

FIP symptoms are variable and often vague in the early stages, making early diagnosis challenging. The clinical presentation depends on which form of FIP develops and which organs are primarily affected.

Early/non-specific signs. The initial symptoms of FIP are often subtle and non-specific: fluctuating fever that does not respond to antibiotics, decreased appetite, weight loss, lethargy, and a generally unthrifty appearance. These signs may persist for days to weeks before more specific symptoms develop.

Effusive (wet) FIP. The hallmark of wet FIP is the accumulation of fluid in body cavities. Abdominal effusion is most common and produces a progressively distended abdomen that may be mistaken for weight gain, pregnancy, or overeating. The fluid is characteristically straw-colored, viscous (sticky), and high in protein. Pleural (chest) effusion causes labored breathing, open-mouth breathing, and reduced exercise tolerance. Some cats develop both abdominal and pleural effusion. Wet FIP typically progresses more rapidly than dry FIP.

Non-effusive (dry) FIP. Dry FIP produces granulomatous inflammation in specific organs, and symptoms depend on which organs are affected. Kidney involvement causes enlarged, irregular kidneys and signs of kidney dysfunction. Liver involvement causes jaundice (yellow discoloration of the gums, ear skin, and whites of the eyes), enlarged liver, and elevated liver enzymes. Ocular involvement produces uveitis (inflammation inside the eye) — the eye may appear cloudy, the iris may change color, and deposits may be visible on the corneal surface. Neurological involvement causes seizures, incoordination, behavior changes, tremors, head tilt, nystagmus (rhythmic eye movements), or paralysis. Central nervous system FIP is particularly challenging to treat.

Mixed presentations. Many cats show features of both wet and dry forms. A cat may have abdominal effusion along with ocular or neurological signs, for example.

Diagnosis of FIP

FIP remains one of the most difficult diagnoses in feline medicine. There is no single simple, definitive test for FIP, and diagnosis typically relies on a combination of clinical findings, laboratory results, and imaging.

Clinical presentation. The combination of a young cat with persistent fever, weight loss, and either abdominal effusion or multiorgan involvement raises strong suspicion for FIP.

Effusion analysis. In cats with effusions, analysis of the fluid is one of the most useful diagnostic steps. FIP effusions are characteristically straw-colored to yellow, viscous and sticky (high fibrin content), high in total protein (typically greater than 3.5 g/dL), and have a low cell count relative to the protein content. The Rivalta test — a simple, in-clinic test where a drop of effusion is added to dilute acetic acid — produces a characteristic clumping pattern in FIP fluid. While not definitive, a positive Rivalta test combined with the appropriate clinical picture is highly suggestive.

Blood work. Laboratory findings that support an FIP diagnosis include hyperglobulinemia (elevated globulin levels), a low albumin-to-globulin ratio (less than 0.4 is highly suggestive), elevated liver enzymes, elevated bilirubin, lymphopenia (low lymphocyte count), and non-regenerative anemia. No single blood test confirms FIP, but the constellation of findings can strongly support the diagnosis.

Coronavirus antibody testing. Feline coronavirus antibody titers measure exposure to feline coronavirus but cannot distinguish between the harmless enteric form and the FIP-causing mutant. A positive titer means the cat has been exposed to coronavirus — not that it has FIP. Many healthy cats have positive titers. A negative titer makes FIP less likely but does not rule it out. Antibody testing alone is insufficient for FIP diagnosis.

Immunohistochemistry and PCR. Detection of feline coronavirus antigen within macrophages in tissue biopsies or effusion samples using immunohistochemistry is considered the gold standard for definitive diagnosis. PCR testing on effusion fluid for feline coronavirus RNA, particularly when combined with sequencing to identify the mutated biotype, is also highly specific. These tests provide the closest thing to a definitive antemortem diagnosis.

Imaging. Ultrasound can reveal abdominal effusion, organ enlargement, mesenteric lymph node enlargement, and granulomatous lesions. MRI is valuable for diagnosing neurological FIP.

Treatment of FIP: The New Era

The treatment of FIP has undergone a revolution in recent years. Where once the diagnosis was a death sentence, effective antiviral treatments now offer genuine hope for cure.

GS-441524. This nucleoside analog, closely related to the human antiviral remdesivir, has emerged as the primary treatment for FIP. It works by inhibiting viral RNA replication, stopping the virus from multiplying within macrophages. GS-441524 was first demonstrated to cure FIP in laboratory studies and has since been used to successfully treat thousands of cats worldwide. Treatment protocols typically involve daily injections or oral administration for 84 days (12 weeks), with dosing adjusted based on the form of FIP (neurological and ocular forms require higher doses). Success rates have been reported at 80 to 90 percent or higher for non-neurological forms when treatment is initiated promptly.

Molnupiravir. This antiviral, originally developed for human use, has shown efficacy against FIP in cats. It works by introducing errors into the viral RNA during replication, a mechanism called lethal mutagenesis. Molnupiravir is administered orally, which some owners and cats prefer over injections. Studies and clinical experience suggest efficacy comparable to GS-441524, though the evidence base is still growing.

Treatment protocols. Standard treatment involves 84 days of daily antiviral therapy. During treatment, cats are monitored with regular blood work and clinical assessments to track response. Most cats show dramatic improvement within the first one to two weeks of treatment — fever resolves, appetite returns, effusions begin to reabsorb, and energy levels improve. After completing the 84-day course, cats enter an observation period of at least 84 additional days during which they are monitored for relapse.

Relapse. Approximately 10 to 15 percent of treated cats experience relapse, typically within the first few months after completing treatment. Relapsed cats are usually retreated with a higher dose or a different antiviral, and many achieve remission with the second course.

Regulatory status. The regulatory landscape for FIP antivirals varies by country and is evolving rapidly. In some countries, GS-441524 and molnupiravir are now available through licensed veterinary channels. In others, they may still be obtained through less formal pathways. Veterinarians specializing in FIP treatment can guide owners through the available options in their region.

Supportive care. Alongside antiviral therapy, supportive care includes appetite stimulants if needed, anti-nausea medications, fluid therapy for dehydrated cats, therapeutic drainage of large effusions to ease breathing, and nutritional support. Good nutrition during treatment supports immune recovery and helps the cat rebuild strength.

Prevention of FIP

Preventing FIP is challenging because the causative virus — feline enteric coronavirus — is so widespread and the mutation that produces FIP occurs unpredictably within individual cats.

Reducing coronavirus exposure. In multi-cat environments, maintaining excellent litter box hygiene (scooping twice daily, complete litter changes weekly, having adequate numbers of boxes), reducing overcrowding, and isolating sick cats can reduce coronavirus transmission. However, complete prevention of coronavirus exposure in multi-cat settings is unrealistic.

Stress reduction. Minimizing stress in cats known to have been exposed to coronavirus may reduce the risk of FIP development. This is particularly relevant during high-stress periods such as adoption, surgery, or rehoming.

Early weaning. In breeding facilities with known coronavirus problems, early weaning and isolation of kittens from shedding queens has been practiced to reduce early-life exposure, though this approach has practical and welfare limitations.

Vaccination. An intranasal FIP vaccine (Primucell FIP) has been available for years but has not been widely recommended because of limited evidence of efficacy, the inability to protect cats already exposed to coronavirus (which includes most at-risk cats), and concerns about safety in some populations. The vaccine targets the nasal mucosa to prevent initial coronavirus infection, but since most cats at risk for FIP have already been infected, its practical utility is limited. It is not part of the core vaccination protocol.

Prognosis for Cats with FIP

The prognosis for FIP has been transformed by antiviral therapy. The key prognostic factors are the form of FIP, the severity at the time treatment begins, and access to appropriate antiviral medications.

With antiviral treatment. Cats with non-neurological, non-ocular FIP that begin GS-441524 or molnupiravir treatment promptly have reported cure rates of 80 to 90 percent or higher. Cats with ocular FIP respond well to higher-dose protocols, with success rates of approximately 70 to 85 percent. Neurological FIP is the most challenging form to treat due to the blood-brain barrier, but success rates of 50 to 70 percent have been reported with high-dose protocols. Early initiation of treatment, before severe organ damage occurs, is associated with better outcomes.

Without treatment. Untreated FIP remains fatal in virtually all cases. Wet FIP typically progresses to death within days to weeks. Dry FIP may progress more slowly, over weeks to months, but the outcome without treatment is the same.

Long-term outcomes. Cats that successfully complete treatment and remain disease-free through the observation period appear to be genuinely cured. Long-term follow-up of treated cats shows that recurrence after successful completion of treatment is uncommon (beyond the initial relapse window). Many treated cats go on to live normal, healthy lives with normal life expectancies.

The message for owners of cats diagnosed with FIP is one of genuine hope. What was once the most devastating diagnosis in feline medicine now has effective treatment. If your cat is diagnosed with FIP, seek out a veterinarian with experience in FIP treatment to discuss antiviral therapy as promptly as possible.

Frequently Asked Questions

Is FIP contagious to other cats?

This is a nuanced question. The mutated FIP-causing virus itself is not believed to be directly transmissible between cats. However, feline enteric coronavirus — the precursor virus from which FIP develops through mutation — is highly contagious between cats through the fecal-oral route. A cat with FIP may shed the enteric coronavirus that other cats could potentially be infected with, but each cat’s risk of developing FIP from that coronavirus depends on whether the virus mutates within their own body. The practical implication is that a cat with FIP does not need to be isolated specifically to prevent FIP transmission, though basic hygiene measures are reasonable.

How much does FIP treatment cost?

The cost of 84 days of antiviral treatment varies significantly depending on the country, the specific drug used, the cat’s weight, the form of FIP (neurological and ocular forms requiring higher doses cost more), and the source of the medication. Costs typically range from $2,000 to $8,000 for a complete treatment course, including diagnostics and monitoring blood work. While this represents a significant financial commitment, it has made a previously fatal disease treatable. Some FIP support communities and organizations offer financial assistance or guidance on affordable treatment options.

Can a cat survive FIP without treatment?

Spontaneous remission from FIP is extremely rare — documented cases exist but represent a tiny fraction of affected cats. Without antiviral treatment, FIP is considered uniformly fatal. Supportive care (fluids, nutrition, anti-inflammatories) may temporarily improve comfort but does not stop disease progression. If FIP is diagnosed or strongly suspected, antiviral treatment should be pursued as promptly as possible for the best chance of survival.

Should I get another cat after my cat had FIP?

If your cat passed away from FIP, the feline coronavirus that caused it may persist in the environment for several weeks. Thorough cleaning of all surfaces with a 1:32 bleach solution, discarding porous items like cat trees and bedding, and waiting four to eight weeks before introducing a new cat reduces the risk of coronavirus transmission to the new cat. That said, coronavirus is so common in the cat population that most new cats will likely encounter it eventually regardless. Adopting a cat that has already been exposed to coronavirus (virtually all shelter cats) means the environmental risk is less relevant. FIP development is ultimately about the individual cat’s immune response, not just exposure.

Can indoor cats get FIP?

Yes. Most cats are exposed to feline coronavirus early in life, often in the shelter, breeding facility, or multi-cat household where they were born or initially lived. A cat can carry the coronavirus for months or years before it mutates into the FIP-causing form. An indoor cat living in a single-cat household can develop FIP from a coronavirus infection acquired long before it came to your home. Indoor-only status reduces ongoing exposure but does not eliminate the risk from prior exposure.

Is there a test that can predict if my cat will develop FIP?

Currently, no test can predict whether an individual cat carrying feline coronavirus will develop FIP. Coronavirus antibody titers indicate exposure but do not predict disease development — most cats with positive titers will never develop FIP. Research into genetic markers and immune profiling that might predict susceptibility is ongoing but has not yet produced a clinically available predictive test. The best approach is awareness of the early signs and prompt veterinary attention if your cat develops unexplained fever, weight loss, or other suspicious symptoms.