Feline Infectious Peritonitis (FIP) in Cats: Symptoms, Diagnosis, and Treatment Options

TL;DR. Feline Infectious Peritonitis (FIP) is a complex, progressive viral disease caused by a mutation of a common feline coronavirus, leading to severe inflammation in either a "wet" (fluid-accumulating) or "dry" (organ-damaging) form.

FIP often begins with vague signs such as lethargy, decreased appetite, and weight loss.

What is it?

Feline Infectious Peritonitis (FIP) is an uncommon, progressive, and systemic viral disease of cats. To understand FIP, it helps to understand that it actually begins with a very common and usually harmless virus called feline coronavirus (FCoV). FCoV is widespread in the feline population, especially in environments where multiple cats live together, such as shelters, catteries, and multi-cat households. In most cats, FCoV causes either no symptoms at all or a mild, self-limiting bout of diarrhea.

However, in a small percentage of infected cats, this benign gastrointestinal virus undergoes a genetic mutation within the cat's body. This mutated version is known as the Feline Infectious Peritonitis Virus (FIPV). Unlike the original coronavirus, which stays primarily in the digestive tract, the mutated FIPV gains the ability to invade and replicate inside macrophages, which are key white blood cells of the cat's immune system. Instead of fighting the virus, these infected white blood cells carry the virus throughout the cat's entire body, triggering an intense, destructive inflammatory response.

FIP manifests in two primary clinical forms, though some cats can show signs of both:

• Effusive ("wet") FIP: This form is characterized by severe inflammation of the blood vessels, a condition known as immune-complex vasculitis. This inflammation causes the blood vessels to become leaky, allowing protein-rich, yellow fluid to accumulate in the cat's body cavities, such as the abdomen (peritoneal cavity) or chest (pleural cavity).
• Noneffusive ("dry") FIP: In this form, the inflammatory response is more localized, forming small clumps of inflammatory tissue called pyogranulomas or granulomas in various organs. These lesions can develop in the kidneys, liver, lymph nodes, eyes, and central nervous system (brain and spinal cord), gradually disrupting organ function.

Causes & risk factors

The primary trigger for FIP is the mutation of the feline coronavirus within an individual cat. It is a common misconception that FIP itself is highly contagious from cat to cat. While the parent virus, FCoV, is easily spread through contact with infected feces, the mutated FIPV that causes FIP is not easily shed and does not spread readily to other cats. As a leading veterinary critical care reference explains:

"Feline coronaviruses mutate readily, and researchers hypothesize that a relatively nonpathogenic feline coronavirus that replicates within the gastrointestinal tract (FCoV) mutates within the host to form virulent FIPV. Mutation occurs soon after infection with FCoV, or years later. Spread of FIP from cat to cat does not occur, so affected cats do not have to be isolated."

Several risk factors influence whether a cat will develop FIP after being exposed to the common feline coronavirus:

• Age: FIP is most commonly diagnosed in young cats under two years of age, as their immune systems are still developing. It can also occur in very old cats whose immune systems are declining.
• Stress: Stressful life events—such as rehoming, adoption, surgery (like spaying or neutering), or concurrent illnesses—can temporarily suppress the immune system, providing an opportunity for the virus to mutate and replicate.
• Genetics and Breed: While any cat can develop FIP, certain breeds appear to have a higher susceptibility, suggesting a genetic component to how their immune systems respond to the virus. Suspected breed predispositions include the British Shorthair, Devon Rex, and Abyssinian.

Signs to watch for

The clinical signs of FIP can vary widely depending on whether the cat has the effusive (wet) or noneffusive (dry) form, and which organs are affected. Many of the early signs are vague and mimic other common feline illnesses.

Common Signs

• Weight loss: Progressive loss of body condition and muscle mass.
• Fever: A persistent, fluctuating fever that does not improve with standard antibiotic therapy.
• Anorexia: A marked loss of appetite, leading to decreased food intake.
• Lethargy: General weakness, depression, and a lack of interest in normal activities.

Occasional Signs

• Abdominal distention: A progressively swelling or "pot-bellied" appearance caused by fluid accumulation in the abdomen (common in the wet form).
• Dyspnea (difficulty breathing): Labored or rapid breathing caused by fluid accumulation in the chest cavity compressing the lungs.
• Icterus (jaundice): A yellow tinge to the gums, skin, or the whites of the eyes, indicating liver involvement.
• Anterior uveitis, iridocyclitis, and chorioretinitis: Inflammation within the eyes that can cause cloudiness, color changes in the iris, bleeding inside the eye, or vision loss.
• Ataxia (uncoordinated movement): Wobbliness, weakness in the hind limbs, or difficulty walking.
• Nystagmus and Seizures: Involuntary, rapid eye movements or convulsions, indicating that the virus has affected the central nervous system.
• Renomegaly and Splenomegaly: Enlargement of the kidneys or spleen, which your vet may detect during a physical exam.
• Vomiting and Diarrhea: Gastrointestinal upset, which may occur intermittently.

Anterior uveitis, characterized by inflammation and cloudiness in the eye, is an occasional sign of noneffusive FIP.

How vets diagnose it

Diagnosing FIP is notoriously challenging because there is no single, non-invasive blood test that can definitively confirm the disease in every patient. Instead, your vet will gather a puzzle of clinical signs, history, laboratory tests, and imaging to reach a diagnosis.

To begin, your vet will perform a thorough physical exam and routine blood tests. Cats with FIP often show a characteristic pattern on their bloodwork, including a high level of globulins (inflammatory proteins) and a low level of albumin (a blood protein). This results in a low serum albumin-to-globulin (A:G) ratio. A very low A:G ratio (typically below 0.4 to 0.8) is highly suggestive of FIP.

If your cat has the effusive form, your vet will likely perform a fine-needle aspiration to collect a sample of the accumulated fluid from the abdomen or chest. FIP fluid is typically thick, straw-colored or yellow, and high in protein. Your vet can perform several tests on this fluid:

• Rivalta test: A simple, rapid chemical test performed in the clinic to help differentiate FIP fluid from other types of inflammatory fluids.
• RT-PCR (Reverse Transcription Polymerase Chain Reaction): A molecular test used to detect coronavirus genetic material in the fluid.
• Immunocytochemistry: A specialized test that uses antibodies to detect the presence of coronavirus proteins inside the macrophages within the fluid.

If your cat has the noneffusive form, diagnosing the disease is even more complex because there is no fluid to analyze. Your vet may recommend advanced imaging, such as an ultrasound, to evaluate internal organs and look for enlarged lymph nodes or abnormal tissue structures.

To evaluate exposure to the virus, your vet may run a coronavirus antibody titer. However, this test must be interpreted with caution. A positive titer only proves that the cat has been exposed to feline coronavirus, not that they have developed FIP. As noted in a leading veterinary internal medicine reference:

"In one study, titers of 1 :1600 or greater in cats that were suspected to have FIP had a 94% chance of truly having FIP, but cats that had any coronavirus antibody titer had a 44% chance of truly having FIP... However, considerable interlaboratory variation in assay results occurs, so this may not be true for serology performed at all laboratories."

The absolute gold standard for diagnosing FIP is the histopathologic examination of tissue biopsy specimens combined with immunostaining. This requires surgically obtaining a small tissue sample from an affected organ or lesion. Pathologists then examine the tissue under a microscope to identify the characteristic pyogranulomatous inflammation and use special stains to confirm the presence of the virus within the tissue cells.

Treatment options

Historically, FIP was considered an almost universally fatal disease, and traditional veterinary treatments have focused on supportive and palliative care to manage symptoms and improve quality of life.

First-Line Therapy: Glucocorticoids

Corticosteroids, such as prednisolone, are commonly used as a first-line therapy to suppress the overactive, destructive immune response and reduce blood vessel inflammation. While these medications can temporarily improve appetite, reduce fever, and make the cat feel more comfortable, they do not cure the underlying viral infection. A prominent veterinary internal medicine textbook discusses this approach:

"Low-dose prednisolone (1-2 mg/kg orally [PO] q24h) may lessen clinical manifestations of noneffusive FIP. However, the use of immunosuppressive drugs is controversial because cats with FIP have impaired immune responses. The use of prednisolone and feline interferon has been promoted for the treatment of both effusive and noneffusive FIP (Ishida et al, 2004)."

Second-Line Therapy: Immunomodulators & Antivirals

• Interferon Omega: This is an antiviral cytokine used to help regulate the immune response and inhibit viral replication. It is sometimes used in combination with corticosteroids.

Supportive & Adjunctive Therapies

• Propentofylline: A vasodilator and xanthine derivative that can help improve blood flow and tissue oxygenation, particularly in organs affected by chronic inflammation.
• Stanozolol: An anabolic steroid that may be prescribed in some chronic cases to help combat severe muscle wasting (cachexia) and stimulate appetite.
• Aspirin: A nonsteroidal anti-inflammatory drug (NSAID) that may be used in very low, carefully controlled doses to reduce platelet aggregation, helping to prevent the formation of tiny blood clots in inflamed blood vessels.
• Ascorbic Acid (Vitamin C): An antioxidant used to support cellular health and reduce oxidative stress associated with chronic inflammation.

Note: While novel antiviral therapies (such as GS-441524 and similar compounds) have shown highly promising results in recent clinical trials and are rapidly changing the treatment landscape for FIP, availability and legal status vary by region. Your veterinarian will discuss the most current, legally available treatment pathways for your specific location.

Prognosis

Historically, the overall prognosis for cats diagnosed with FIP has been grave, particularly for those with the effusive (wet) form. Without advanced antiviral intervention, most cats with effusive FIP deteriorate rapidly, with survival times ranging from days to a few weeks or months after diagnosis. Euthanasia is often recommended to prevent suffering once quality of life declines significantly.

For cats with the noneffusive (dry) form, the progression of the disease is typically slower. Survival times can be highly variable, and some cats can maintain a reasonable quality of life for several months, and occasionally up to two years, if the disease is detected early and supportive care is initiated promptly. However, under standard palliative therapies, the disease remains progressive and ultimately fatal.

Prevention

Preventing FIP is highly challenging because the feline coronavirus (FCoV) is so common and easily transmitted. In multi-cat environments, managing the spread of FCoV is the primary focus. This involves maintaining strict hygiene, providing ample litter boxes (ideally one per cat plus one extra), keeping litter boxes far away from food and water bowls, and cleaning them daily to reduce fecal-oral transmission.

It is important to understand that cats can shed the virus intermittently and can easily become reinfected even after they seem to have cleared it. A veterinary study highlights this complex shedding pattern:

"In one study of 155 pet cats with naturally occurring FECV infection, viral RNA was shed continuously ( n = 18) or intermittently ( n = 44) in the feces of some cats (Addie et al, 2001). Others were initially shedding viral RNA and then ceased shedding ( n = 56), and some were resistant to infection ( n = 4). The cats that stopped viral shedding were susceptible to reinfection."

Because the mutation from FCoV to FIPV occurs internally within an individual cat, minimizing stress is one of the most effective ways to reduce the risk of mutation in coronavirus-positive cats. Avoid unnecessary changes in the household, manage concurrent health issues promptly, and ensure a stable, enriched environment.

When to call your vet

Because FIP can progress rapidly, you should contact your veterinarian if you notice any changes in your cat's health, especially if they have recently experienced a stressful event or live in a multi-cat household.

Seek immediate emergency veterinary care if your cat exhibits any of the following red-flag signs:

• Difficulty breathing (labored, rapid, or open-mouthed breathing)
• Seizures or sudden, severe neurological changes
• Complete refusal to eat for more than 24 to 48 hours
• Extreme lethargy or inability to stand
• Sudden onset of severe yellowing of the skin, gums, or eyes (icterus)

For specific breeds

If you own a British Shorthair, Devon Rex, or Abyssinian, it is important to be aware of the suspected breed predisposition to FIP. While there is currently no genetic screening test that can predict whether an individual cat will develop FIP, breeders and owners of these breeds should be extra vigilant.

If you are adopting a kitten of these breeds, ask the breeder about their coronavirus management protocols and whether they have had cases of FIP in their breeding lines. Minimizing stress during the transition to a new home and maintaining excellent veterinary care are particularly vital for these genetically susceptible breeds.

Sources

• Small Animal Critical Care Medicine, pp. 564
• Internal Medicine, pp. 1378, 1380