FipEdit

Fip, short for feline infectious peritonitis, is a deadly disease affecting domestic cats and some wild felines. It results from a mutation of the common feline coronavirus (FCoV) inside a cat, transforming a usually mild gut infection into a severe, immune-mediated illness. For many years, Fip carried a grim prognosis, with most cases culminating in death. In the last decade, however, advances in veterinary science—especially in antiviral therapies and diagnostic methods—have begun to change the outlook, albeit with substantial variation by country, clinic, and individual cat.

From a practical, owner- and clinic-centered perspective, the response to Fip highlights broader debates about medical innovation, access to care, and the role of regulation in veterinary medicine. The private veterinary sector, charity-driven research, and international collaboration have been crucial in expanding treatment options, while concerns about cost, quality control, and evidence standards continue to shape how therapies are adopted in everyday practice.

Background

Feline coronavirus (FCoV) is widespread in multi-cat environments and often causes only mild or subclinical disease. In a minority of infections, the virus mutates within the host into a form that triggers an intense inflammatory response and multipart organ involvement characteristic of Fip. The disease can present in two major forms: a wet (effusive) form with fluid buildup in body cavities, and a dry (non-effusive) form with granulomatous lesions in organs such as the liver, kidneys, or brain. The pathogenesis involves a complex interplay between viral factors and the cat’s immune response, and cats can display varied signs depending on which organs are affected.

Because FCoV is ubiquitous, many cats in high-density settings—such as catteries or shelters—carry the virus without progressing to Fip. The transition from a benign viral carriage to deadly Fip is not fully predictable, which makes prevention and early detection challenging. Researchers and clinicians rely on a combination of clinical signs, laboratory data, imaging, and, in some cases, histopathology to reach a diagnosis. The disease is linked to housing, stress, nutrition, and overall health status, and it is a reminder of how ambient conditions can influence disease expression in companion animals.

In encyclopedia terms, Fip sits at the intersection of infectious disease, immunology, and veterinary medicine. It is also a case study in how new therapies migrate from experimental use to standard practice, and how pet owners navigate difficult medical decisions in partnership with veterinarians. For readers seeking deeper context on the viral precursor, see feline coronavirus.

Symptoms and forms

Fip can manifest with a range of signs, and the form of the disease influences the clinical picture.

Wet (effusive) FIP: This form is often marked by fluid accumulation in the abdomen (ascites) or chest, leading to abdominal distension or breathing difficulties. Cats may present with lethargy, weight loss, a fever that does not respond to antibiotics, poor appetite, and reduced activity. The effusions are typically viscous and can be detected through physical examination and ultrasound.
Dry (non-effusive) FIP: In this form, there is no sizeable fluid effusion. Instead, granulomatous lesions can affect the eyes (uveitis), nervous system (ataxia, seizures, behavior changes), kidneys, liver, or other organs. Neurologic signs or ocular abnormalities can be prominent early indicators.
Mixed or atypical presentations: Some cats exhibit features of both forms or symptoms that do not fit neatly into one category, complicating diagnosis and management.

Owners should consult a veterinarian promptly if a cat shows persistent fever, weight loss, reduced appetite, abdominal enlargement, breathing changes, vision problems, or behavioral changes. See feline infectious peritonitis for a general overview of disease progression and management challenges.

Diagnosis

Diagnosing Fip is complex because many of its signs mimic other feline diseases. A veterinarian typically combines history, physical examination, and targeted tests to assess the likelihood of Fip.

Blood work: Anemia, elevated proteins, and inflammatory markers can support suspicion, but are not definitive.
Imaging: Ultrasound or chest/abdominal imaging can reveal effusions consistent with wet FIP or organ changes associated with dry FIP.
Fluid analysis: In suspected wet FIP, analyzing the effusion can yield supportive clues (protein-rich fluid with certain cellular patterns). In dry FIP, tissue biopsy or fine-needle aspirates may be used to detect characteristic granulomatous lesions.
Serology and molecular tests: Antibody tests for FCoV can indicate exposure but do not confirm Fip. PCR-based assays aimed at FCoV RNA or specific mutation patterns in the virus within affected tissues can be informative, though interpretation requires veterinary expertise. The most reliable confirmation often comes from a synthesis of clinical picture and test results, and in some cases histopathology is considered the definitive standard.

For readers seeking more technical detail on diagnosis, see feline coronavirus and feline infectious peritonitis.

Treatment and prognosis

Treatment of Fip remains one of veterinary medicine’s more challenging frontiers. Historically, there was no cure, and most cases were managed with palliative and supportive care. In recent years, several antiviral strategies have shown promise, though access and regulatory status vary by jurisdiction.

Antiviral therapies: Nucleoside analogs such as GS-441524 have demonstrated substantial efficacy in many cats with Fip when used under veterinary supervision. These drugs are not universally approved for veterinary use in all countries, and availability is often through private veterinary practice or international suppliers. When administered under appropriate medical guidance, some cats experience remission or significant improvement, especially when started early in the disease course.
Regulatory and quality considerations: Because some antivirals are not licensed for Fip in certain markets, veterinarians may use them under off-label conditions or rely on compassionate-use frameworks. This has spurred debates about safety, sourcing, dosing standards, and the ethics of access, particularly given the high cost of treatment and the risk of counterfeit products.
Supportive care: Regardless of antiviral use, cats typically require supportive therapy, including fluid therapy for hydration, appetite stimulants or hand-feeding, anti-nausea medications, analgesia, nutritional support, and monitoring for organ involvement. In the dry form, management focuses on supporting organ function and mitigating inflammation.
Prognosis: The prognosis varies with form, overall health, and response to treatment. Wet FIP often progresses rapidly, but some cats respond well to antiviral therapy. Dry FIP prognosis is more heterogeneous and depends on which organs are affected and the cat’s response to supportive care and antivirals.

Owners considering treatment should discuss with a veterinarian the expected benefits, costs, potential side effects, and the likelihood of relapse or progression. See antiviral therapy and GS-441524 for a look at the evolving pharmacological landscape.

Vaccination, prevention, and public health considerations

Prevention primarily focuses on reducing exposure to FCoV and managing household factors that may influence disease risk. FCoV is common in multi-cat environments, and its presence does not inevitably lead to Fip; instead, the mutation and immune interactions determine disease development.

Vaccination: A vaccine intended to reduce the risk of Fip exists in some markets under the name PureVAX FIP. Its efficacy, safety profile, and recommended use vary, and many veterinarians consider vaccination only for particular risk groups or households with elevated exposure risk. The decision to vaccinate is typically made on a case-by-case basis after weighing potential benefits and uncertainties, rather than as a routine measure for all cats. See PureVAX FIP for more detail.
Household management: Reducing stress, maintaining good nutrition, and ensuring good hygiene in multi-cat settings are common-sense steps that support overall health and may influence how cats respond to FCoV exposure. When new cats join a household, testing and gradual introduction can help minimize risk to resident cats.
Screening and testing: Routine testing for FCoV antibodies is not a reliable predictor of Fip risk, and veterinarians generally resist broad, non-targeted testing as a public-health measure. The focus tends to be on clinical monitoring and targeted diagnostics when symptoms arise.

See also feline coronavirus for the broader virology and epidemiology context.

Controversies and debates

Fip sits at the center of several debates about veterinary medicine, clinical evidence, and how best to deploy limited resources.

Efficacy and use of vaccines: Proponents of vaccination argue that any reduction in risk is valuable, particularly in high-exposure settings. Critics contend that the current data on efficacy are inconclusive, that the vaccine can produce adverse reactions in some cats, and that vaccination should not be routine without carefully assessing risk factors. The decision to vaccinate is often framed as a balance between practical protection and scientific uncertainty.
Access to antivirals and regulatory status: The emergence of antiviral drugs capable of stopping Fip in some cats has prompted a debate about how quickly new therapies should be integrated into standard practice. Advocates emphasize the urgency of providing access to life-saving options, especially in cases with poor prognosis. Critics worry about quality control, consistent dosing, regulatory oversight, and the risks posed by non-validated products or inconsistent supply chains. From a market-oriented perspective, people argue for clearer pathways to approval, robust clinical trials, and transparent pricing rather than broad restrictions that hamper access.
The role of veterinary professionals vs. patient choice: Some observers argue that veterinarians should guide decisions based on the best available evidence, while others stress pet owners’ autonomy to seek out experimental or off-label options when conventional care has failed. The practical tension is between ensuring safety and enabling access to potentially life-saving therapies, a tension that plays out differently in different jurisdictions.
Criticism and discourse: Critics who frame debates in broad, ideologically driven terms sometimes conflate the complexities of evidence with broader political narratives. From a pragmatic, market-informed view, policy discussions should emphasize rigorous, peer-reviewed research, reproducible results, and patient-centered outcomes rather than slogans. Proponents of rapid innovation argue that the high mortality of Fip warrants pursuing promising therapies while maintaining safety checks; detractors warn against rushing unproven interventions that could erode trust in veterinary science.
The role of evidence in decision-making: Supporters of a robust evidence base emphasize controlled studies, real-world outcome data, and standardized dosing to ensure that therapies perform as advertised. Critics of the current pace may claim that waiting for perfect certainty delays access to effective care. The balance struck in different regions reflects local regulatory climates, professional standards, and the financial realities faced by cat owners.

In short, the Fip discourse is as much about medicine, markets, and patient choice as it is about the biology of the disease. It reflects broader questions about how quickly innovation should be adopted in animal health, how to ensure patient safety, and how to allocate scarce resources in a way that serves as many animals as possible without compromising standards.