CONDITION

Feline Infectious Peritonitis (FIP)

Feline infectious peritonitis is a condition in which a cat's immune system responds in an overwhelming, damaging way to infection with a particular coronavirus. Rather than clearing the virus quietly, the immune response itself becomes the main driver of illness, causing inflammation that can affect many parts of the body — the abdomen, chest, eyes, brain, or internal organs. The virus involved is related to, but distinct from, coronaviruses that cause mild gut upset in many cats. Owners often arrive at this page because their cat has become quieter, less interested in food, or has developed a persistent fever that does not respond predictably to treatment. Some cats develop fluid in the abdomen or chest; others show signs that are harder to pin down, such as changes in behaviour, difficulty walking, or cloudiness in one or both eyes. The pattern can be gradual or relatively swift, and often follows a period of stress or change in the household. This page explores the signals that can raise suspicion, what is understood about how the condition develops, the diagnostic picture that veterinary surgeons piece together, and the approaches that exist. It does not replace assessment of an individual cat, but it can help make sense of what may be unfolding and what questions may be useful to hold.

Why this matters now

FIP most commonly affects young cats, typically between three months and two years of age, though cats of any age can develop the disease. It tends to occur in environments where feline coronavirus circulates readily — multi-cat households, catteries, and shelters — though cats in single-cat homes can also be affected if they were exposed during kittenhood. The condition develops when a common and usually harmless feline enteric coronavirus undergoes specific mutations within an individual cat, transforming it into the FIP-causing biotype. This mutation event is unpredictable and occurs in only a small proportion of cats infected with feline coronavirus. Stress, immune compromise, and genetic factors may influence susceptibility, but the precise triggers for mutation remain incompletely understood. Recent developments in antiviral therapies have substantially changed the outlook for cats diagnosed with FIP.

FIP typically progresses through an initial phase of non-specific illness — fever, lethargy, and reduced appetite — before evolving into one of two recognised clinical forms, though overlap between them is common. The effusive (wet) form is characterised by the accumulation of protein-rich fluid in the abdominal cavity, thoracic cavity, or both, often producing visible abdominal distension or breathing difficulty. The non-effusive (dry) form involves the formation of granulomatous inflammatory lesions in organs including the eyes, brain, kidneys, and liver, producing signs that depend on which organs are affected. Neurological signs, ocular changes, and organ-specific dysfunction may develop. Without treatment, FIP has historically been considered almost universally fatal, typically progressing over weeks to months. However, the introduction of antiviral therapies has offered the possibility of remission and potential cure in many cases, fundamentally altering the disease's trajectory for treated cats.

Signals & patterns

Early signals

Persistent or fluctuating fever

A fever that does not respond to standard antibiotic therapy and may fluctuate over days is one of the more characteristic early findings in FIP. The fever reflects the intense systemic inflammatory response driven by immune complex formation and vasculitis.

Progressive weight loss despite variable appetite

Cats developing FIP often show gradual weight loss that may begin before other signs become apparent. Appetite may be reduced, fluctuating, or initially maintained while weight still declines, reflecting the catabolic effects of systemic inflammation.

Lethargy and withdrawal

A general reduction in activity and social engagement, sometimes subtle at first, may be among the earliest behavioural changes noticed. Young cats that were previously playful and energetic may become quieter and less interactive.

Subtle abdominal changes

In the early stages of effusive FIP, mild abdominal distension may develop that is initially attributed to weight gain or intestinal gas. The gradual nature of fluid accumulation can make early detection challenging without deliberate assessment.

Later signals

Visible abdominal or thoracic effusion

In the effusive form, significant accumulation of straw-coloured, viscous fluid in the abdominal cavity produces a characteristic pear-shaped abdominal distension. When fluid accumulates in the thoracic cavity, progressive breathing difficulty develops as the lungs become compressed. The fluid has a distinctive high-protein, low-cellularity character.

Neurological signs

The non-effusive form frequently involves the central nervous system, potentially producing ataxia, tremors, seizures, behavioural changes, nystagmus, or progressive hind limb weakness. These signs reflect granulomatous inflammation affecting the brain, spinal cord, or both, and may develop gradually or appear relatively suddenly.

Ocular changes

Eye involvement in non-effusive FIP may manifest as changes in iris colour, pupil irregularity, cloudiness within the eye, or visible inflammatory deposits. These changes may affect one or both eyes and often accompany neurological involvement, reflecting the shared blood-brain and blood-ocular barrier vulnerability to FIP-related vasculitis.

Jaundice

Yellowing of the skin, gums, and whites of the eyes may develop as hepatic involvement progresses, reflecting inflammation and dysfunction of the liver — one of the organs commonly affected by FIP-related granulomatous lesions.

Click to read about the biological mechanisms

How this is usually investigated

Diagnosing FIP has historically been one of the more challenging tasks in feline medicine, as no single test provides a definitive diagnosis in a living cat. Instead, diagnosis typically relies on combining clinical signs, laboratory findings, effusion analysis, and imaging to build a weight of evidence. Recent advances in molecular testing have improved diagnostic accuracy, though interpretation still requires clinical context.

Effusion analysis

Purpose: When fluid is present in the abdominal or thoracic cavity, analysis of this fluid provides some of the most useful diagnostic information. FIP effusion is characteristically straw-coloured to golden, viscous, and high in protein content with a low cell count — a pattern that differs from most other causes of effusion.
Considerations: The Rivalta test, a simple in-clinic assessment of effusion protein content, can help differentiate FIP effusion from other types. Cytology and protein analysis of the fluid provide additional information. However, not all cats with FIP develop effusion (non-effusive form), and fluid characteristics, while suggestive, are not exclusively diagnostic of FIP.

Blood biochemistry and haematology

Purpose: Certain patterns in blood results may support a suspicion of FIP, including elevated total protein with high globulin levels, low albumin-to-globulin ratio, elevated bilirubin, and non-regenerative anaemia. These findings are suggestive but not specific to FIP.
Considerations: An albumin-to-globulin ratio below 0.4 is often considered suggestive, though this threshold is not absolute. Hyperbilirubinaemia in the absence of haemolysis or obvious hepatic disease adds further supporting evidence. These laboratory changes reflect the systemic inflammatory and immune-mediated nature of the disease but can overlap with other conditions.

Coronavirus antibody titres

Purpose: Blood tests measuring antibodies against feline coronavirus indicate exposure to the common enteric form of the virus, which is widespread in cat populations.
Considerations: These tests cannot distinguish between harmless enteric coronavirus and the mutated FIP-causing biotype, significantly limiting their diagnostic utility. A high titre in a sick cat may support a suspicion of FIP but is not diagnostic, while a negative result may occur in terminal cases where antibody is consumed by immune complex formation. The widespread prevalence of feline coronavirus means many healthy cats have positive antibody titres.

Immunohistochemistry and PCR on tissue or effusion

Purpose: Detection of feline coronavirus antigen within macrophages in tissue samples or effusion, using immunohistochemistry or RT-PCR, provides the strongest ante-mortem evidence for FIP. PCR on effusion samples has become increasingly available and can offer high specificity.
Considerations: Tissue samples may require biopsy, which involves procedural considerations in a cat that may be systemically unwell. PCR on effusion is less invasive and increasingly used as a key diagnostic tool. False negatives can occur, and results must be interpreted alongside the broader clinical picture.

Advanced imaging

Purpose: Ultrasonography can detect abdominal effusion, organ changes, and mesenteric lymph node enlargement. MRI may be considered for neurological FIP to assess brain and spinal cord involvement.
Considerations: Imaging findings are supportive rather than diagnostic, as the changes seen in FIP overlap with other conditions. Ultrasonography is widely available and non-invasive, making it a practical component of the diagnostic workup. MRI provides detailed assessment of neurological involvement but is not universally available.

Options & trade-offs

The management landscape for FIP has undergone a remarkable transformation in recent years. Previously considered untreatable, the development and increasing availability of antiviral nucleoside analogues has offered the possibility of remission and cure for many affected cats, though treatment involves significant considerations around access, cost, duration, and monitoring.

Antiviral nucleoside analogues

Drugs such as GS-441524 and related compounds have demonstrated significant efficacy against FIP by directly inhibiting viral RNA replication. Treatment typically involves daily administration over a course of 12 weeks or longer, with the specific drug, dose, and route varying depending on the form of FIP and its severity. Response rates have been encouraging, with many cats achieving full remission.

Trade-offs: Access to these medications varies considerably by region and regulatory framework. Treatment courses are prolonged and require regular monitoring to assess response and detect relapse. Cost can be substantial over a full treatment course. Some cats may experience relapse after completing treatment and require retreatment. The neurological form may require higher doses or longer treatment courses due to the challenge of achieving adequate drug concentrations across the blood-brain barrier.

Supportive care during treatment

Alongside antiviral therapy, supportive measures including nutritional support, fluid therapy if needed, management of secondary symptoms, and monitoring of organ function help maintain the cat's condition during the treatment period.

Trade-offs: Cats with advanced disease at the time of diagnosis may require more intensive initial supportive care. The balance between the stress of repeated veterinary visits and medication administration and the potential benefit of treatment requires individual assessment, particularly in cats with significant neurological or organ involvement.

Drainage of effusions

In cats with significant abdominal or thoracic effusion, therapeutic drainage may provide temporary relief of symptoms — particularly breathing difficulty from thoracic fluid — while antiviral therapy takes effect.

Trade-offs: Effusion drainage provides symptomatic relief but does not address the underlying disease process. Repeated drainage without concurrent antiviral treatment is unlikely to alter the disease course. Thoracic drainage may be particularly valuable for immediate quality of life improvement in cats with compromised breathing.

Palliative care

In situations where antiviral treatment is not accessible, affordable, or appropriate for the individual case, palliative care focuses on maintaining comfort and quality of life. This may include anti-inflammatory medications, appetite stimulants, nutritional support, and management of specific symptoms.

Trade-offs: Without antiviral treatment, FIP typically progresses, and palliative care aims to optimise quality of life rather than achieve remission. The decision between pursuing treatment and focusing on comfort involves complex considerations including the cat's current condition, the form and stage of disease, and individual circumstances.

Common misconceptions

Misconception:

"FIP is directly contagious between cats"

Reality:

FIP itself is not directly transmitted from cat to cat. The common feline enteric coronavirus that cats spread between each other is usually harmless. The mutation from harmless coronavirus to FIP-causing biotype occurs within an individual cat and is not transmitted in its mutated form to other cats. While living in a multi-cat environment increases coronavirus exposure and therefore the statistical likelihood of a mutation event occurring, a cat developing FIP does not pose a direct FIP risk to housemates.

Misconception:

"FIP is always fatal and untreatable"

Reality:

While FIP was historically considered nearly universally fatal, the development of antiviral nucleoside analogues has fundamentally changed this outlook. Many cats treated with these medications achieve complete remission, with studies reporting high success rates particularly for the effusive form. Treatment is demanding in terms of duration and monitoring, and access varies by region, but the characterisation of FIP as untreatable no longer reflects the current state of veterinary knowledge.

Misconception:

"A positive coronavirus test means a cat has or will develop FIP"

Reality:

Feline coronavirus infection is extremely common, with the majority of cats in multi-cat environments testing positive for coronavirus antibodies at some point. Only a very small proportion of coronavirus-infected cats — estimated at around 5-10% — go on to develop FIP through the internal mutation process. A positive coronavirus antibody test indicates exposure to the common enteric virus, not infection with the FIP-causing biotype, and most cats with positive titres will never develop FIP.

Related conditions

Feline Leukaemia Virus (FeLV)

Both are significant viral diseases of cats, and FeLV-associated immunosuppression historically increased vulnerability to serious coronavirus outcomes. The two are also considered alongside one another when investigating fever, weight loss, and effusions in young cats.

Feline Immunodeficiency Virus (FIV)

FIV compromises immune function, and the immune response plays a central role in determining whether feline coronavirus infection progresses to FIP. Both viruses feature in the diagnostic picture for chronically unwell cats.

Lymphoma in Cats

Lymphoma is one of the main differentials for FIP, as both can produce abdominal masses, effusions, weight loss, and persistent malaise in cats, sometimes at similar ages.

Toxoplasmosis

Toxoplasmosis shares several presentations with the dry form of FIP, including uveitis and neurological signs, and the two are often investigated together in cats with inflammatory ocular or neurological disease.

Understanding FIP involves appreciating both the complexity of its pathogenesis and the rapidly evolving treatment landscape. The transformation from an untreatable diagnosis to a condition where remission is achievable represents one of the more significant recent advances in feline medicine. Recognising the distinction between common feline coronavirus and the FIP-causing biotype, understanding the diagnostic challenges, and being aware of available treatment options all contribute to a more informed perspective when navigating this condition.

Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS