For more than fifty years, a diagnosis of feline infectious peritonitis was one of the worst things a cat owner could hear. The disease was almost universally fatal, often killed young cats within weeks of the first vague symptoms, and offered no meaningful treatment beyond supportive care. That changed, suddenly and decisively, in 2019 — when an experimental antiviral called GS-441524 was shown to cure the majority of treated cats in a clinical trial at UC Davis. Six years on, FIP is no longer a death sentence. It is a treatable disease, and as of 2024 the medication that cures it is legally available by prescription in the United States and the United Kingdom.
That progress is extraordinary, but FIP remains a disease most owners and even many veterinarians find confusing. It is caused by an extremely common virus that almost never actually causes FIP. It shows up in wildly different ways in different cats. And the diagnosis still cannot be made by a single blood test. Here is what cat owners should know.
What FIP Actually Is
FIP is caused by a mutated form of feline coronavirus (FCoV) — a virus unrelated to the human SARS-CoV-2 and unable to infect people. The common, non-mutated form of feline coronavirus (sometimes called feline enteric coronavirus) lives quietly in the intestines of vast numbers of cats. Surveys have found FCoV antibodies in roughly 50–90% of cats in multi-cat households, shelters, and catteries, and 10–60% of household cats worldwide. In most infected cats it causes mild, transient diarrhea or no symptoms at all.
In a minority of those cats — fewer than 5–10% by most estimates — the virus mutates inside the individual cat. The mutated form gains the ability to infect immune cells called macrophages and to spread throughout the body, triggering a destructive inflammatory response known as a vasculitis. That cascade is FIP. The mutation is not contagious; cats with FIP do not spread FIP itself to other cats, only the harmless intestinal form of the virus.
Who Is at Risk
FIP can strike any cat, but a few patterns are well established:
- Age. The vast majority of cases occur in cats under two years old, with a peak around 3–16 months. A second, smaller bump occurs in geriatric cats whose immune systems are waning.
- Genetics and breed. Purebreds appear to be over-represented, including Birmans, Ragdolls, Bengals, Abyssinians, Himalayans, Devon Rex, and Persians. Heritability studies suggest a genetic component to susceptibility, though no single gene has been pinpointed.
- Environment. Crowded multi-cat environments — shelters, breeding catteries, and rescues — concentrate FCoV exposure and create the conditions in which the virus is most likely to mutate.
- Stress. Recent rehoming, surgery (including spay/neuter), travel, vaccination during illness, or a change in household composition are all reported as triggers around the time of symptom onset.
- Sex. Some studies report a modest male predisposition.
The Two Faces of FIP
Clinicians traditionally divide FIP into two presentations. Most affected cats actually show features of both.
Wet (effusive) FIP accounts for the majority of cases and tends to progress more quickly. The vasculitis leaks protein-rich fluid into the abdomen, the chest, or occasionally around the heart. Owners may notice a pot-bellied or swollen abdomen that does not match weight loss elsewhere, labored or rapid breathing, decreased activity, fever that does not respond to antibiotics, and progressive weight loss. The fluid itself is highly characteristic: thick, sticky, straw-yellow, and protein-rich.
Dry (non-effusive) FIP is more insidious. There is little or no fluid; instead, granulomas form in solid organs — kidneys, liver, lymph nodes, intestines, eyes, or the central nervous system. Cats may present with persistent fever, weight loss, jaundice, kidney enlargement, or organ-specific signs. Ocular involvement is common: a sudden change in iris color, blood in the front of the eye, or inflammation called uveitis can be the first clue. Neurologic FIP shows up as ataxia (wobbliness), seizures, tremors, or behavioral changes, and it is one of the harder forms to treat.
Warning Signs to Watch For
Early FIP is unfortunately easy to dismiss. The most common opening symptoms are nonspecific:
- Persistent fever (often 103–106°F / 39.5–41°C) that does not respond to antibiotics.
- Loss of appetite and progressive weight loss, often with a dull, unkempt coat.
- Lethargy or hiding, especially in a previously playful kitten or young adult.
- A swollen, fluid-filled abdomen without obvious cause (wet form).
- Labored breathing from fluid in the chest (wet form).
- Eye changes — color change in the iris, cloudiness, blood, or visible inflammation (dry form).
- Neurologic signs — wobbliness, head tilt, seizures, or sudden behavioral changes (neurologic dry form).
- Jaundice — yellowing of the gums, ears, or whites of the eyes.
Any combination of these signs in a cat under two years old — particularly one from a multi-cat environment — should prompt a workup for FIP rather than another round of empiric antibiotics.
How Vets Diagnose FIP
There is still no single blood test that proves a cat has FIP, and antibody titers to feline coronavirus are unhelpful because so many healthy cats carry the virus. Diagnosis is built from a pattern of findings:
- Complete blood count and chemistry panel. Suggestive findings include non-regenerative anemia, lymphopenia (low lymphocytes), high total protein driven by very high globulins, a low albumin-to-globulin (A:G) ratio (a value under about 0.4 is strongly supportive), and elevated bilirubin.
- Analysis of effusion fluid (wet form). An abdominal or chest tap that yields thick, yellow, high-protein fluid (often more than 35 g/L) with a low cell count and a low A:G ratio is highly suggestive. The bedside Rivalta test is a useful screening tool.
- RT-PCR for feline coronavirus on effusion or tissue. Detection of FCoV RNA in fluid, particularly with mutation-specific assays, is one of the most useful confirmatory tests.
- Immunohistochemistry (IHC) on biopsy. The historical gold standard — staining for coronavirus antigen inside macrophages in a tissue biopsy — but it is invasive and rarely necessary now that PCR is widely available.
- Imaging. Abdominal ultrasound can identify free fluid, enlarged abdominal lymph nodes, kidney lesions, and intestinal masses typical of dry FIP. MRI is used for suspected neurologic cases.
The Treatment Revolution: GS-441524
For decades, FIP was treated with corticosteroids and supportive care, and almost every cat died. That changed when a UC Davis team published results in 2019 showing that an antiviral nucleoside analog, GS-441524 (the active metabolite of the human drug remdesivir), produced lasting remission in the majority of treated cats with naturally occurring FIP.
For several years the drug was available only through an unregulated online market, and desperate owners paid thousands of dollars for vials of unknown provenance. The legal landscape shifted in 2021 when the U.K. veterinary regulator authorized a Bova-compounded oral formulation. In June 2024, Stokes Pharmacy in the United States, in partnership with Bova, began offering a legally compounded oral GS-441524 by veterinary prescription, after the FDA's Center for Veterinary Medicine confirmed it would not enforce against veterinarians prescribing the drug specifically for FIP. For the first time, U.S. cats with FIP can be treated with a legal, quality-controlled medication.
The standard protocol is a single daily oral dose of GS-441524, weight-adjusted, given for 84 days. Higher doses are typically used for ocular and neurologic forms, because penetration into the eye and central nervous system is reduced. Clinical studies in the U.S., U.K., and Australia consistently report cure rates above 80%, with many series reporting greater than 85%. A 2024 study suggested that 42 days of treatment may be sufficient for cats with effusive FIP, though longer courses remain standard for non-effusive and neurologic disease.
Most cats improve dramatically within the first 1–2 weeks: fever resolves, appetite returns, and effusion clears. After the 84-day course, cats enter a 12-week observation period; relapses, when they occur, almost always happen within the first 3–6 months and are usually treated successfully with a second, often higher-dose course.
Prevention: What Actually Works
There is no reliable way to prevent the mutation that causes FIP in an individual cat. The available FIP vaccine (an intranasal product) has not demonstrated convincing efficacy in field studies and is not recommended by most feline-medicine specialists. What does help is reducing FCoV exposure and the stress that seems to tip susceptible cats into disease:
- Limit household density. Risk rises sharply in households with more than 5–6 cats and in shelter or cattery environments.
- Keep litter boxes clean and well-separated. FCoV spreads through fecal-oral contact. Scoop daily, provide at least one box per cat plus one extra, and keep boxes away from food.
- Quarantine new cats for 2–4 weeks before integration into a multi-cat household.
- Minimize stress around vulnerable life events — surgery, rehoming, weaning — when the immune system is most challenged.
- Choose breeders carefully. Lines with documented FIP cases warrant caution; responsible catteries actively monitor for FCoV shedding.
The Bottom Line
FIP is no longer the disease it was when most current veterinary textbooks were written. A young cat with persistent unexplained fever, weight loss, abdominal swelling, eye changes, or neurologic signs deserves a full FIP workup, not a wait-and-see plan — because for the first time in veterinary history, the answer can be a cure rather than a goodbye. If your cat is diagnosed, ask your veterinarian about legally prescribed compounded GS-441524 and, if needed, seek referral to a feline-medicine or internal-medicine specialist with experience in modern FIP protocols. The treatment is not cheap, the 84 days are not easy, but the outcome — for the majority of treated cats — is full recovery.