Wet FIP (feline infectious peritonitis) is a severe, inflammatory disease in cats caused by a mutated form of a common feline coronavirus. It’s called “wet” or “effusive” because the hallmark sign is fluid buildup in the abdomen, chest, or both. Until recently, wet FIP was considered universally fatal, but antiviral treatments developed in the last few years have dramatically changed the outlook for affected cats.
How a Common Virus Turns Dangerous
Most cats encounter feline coronavirus (FCoV) at some point, especially those living in shelters or multi-cat homes. In the vast majority, the virus causes mild or no symptoms at all, perhaps a brief bout of diarrhea. The problem begins when the virus mutates inside a specific cat’s body.
In roughly 10 percent of cats infected with feline coronavirus, one or more mutations change how the virus behaves. Instead of staying in the intestinal tract, the mutated virus infects white blood cells and hitches a ride through the bloodstream, spreading throughout the body. Once that happens, the virus is classified as FIPV, the strain responsible for feline infectious peritonitis.
The disease itself isn’t caused by the virus alone. It’s driven by the cat’s own immune response. An intense inflammatory reaction erupts around blood vessels wherever infected white blood cells settle, commonly in the abdomen, kidneys, or brain. In wet FIP specifically, this inflammation damages vessel walls so severely that plasma leaks into the spaces around organs. The leaked protein then draws even more fluid out of the bloodstream, causing the characteristic buildup in the abdomen or chest cavity.
Signs and Symptoms of Wet FIP
Wet FIP tends to develop and progress quickly compared to the dry (non-effusive) form. The earliest signs are vague: lethargy, loss of appetite, weight loss, and a persistent fever that doesn’t respond to antibiotics. In kittens, it may show up as stunted growth or failure to gain weight. These symptoms alone don’t point to FIP, which is part of what makes early diagnosis tricky.
The defining symptom is visible fluid accumulation. Most commonly, fluid collects in the abdomen, giving the cat a noticeably swollen, pot-bellied appearance even as the rest of the body loses weight. When fluid builds in the chest instead, the cat may struggle to breathe or breathe with obvious effort. Some cats develop fluid in both locations.
Eye problems can accompany the effusion. Inflammation inside the eye may cause cloudiness, color changes in the iris, or deposits on the inner surface of the cornea. On closer examination, a vet might see inflammation of the blood vessels in the retina or fluid accumulating behind it. Some cats also develop neurological signs like wobbliness, seizures, or behavioral changes, though these are more common in dry FIP.
How Vets Diagnose Wet FIP
No single test confirms wet FIP on its own, so veterinarians piece together multiple findings. The fluid itself provides the strongest clues. Classic wet FIP effusion has a distinctive egg-white consistency: sticky, yellow-tinged, high in protein, and prone to forming weak clots. It contains high levels of inflammatory cells and bilirubin. This type of fluid looks and behaves very differently from the watery, clear fluid seen in heart failure or certain cancers.
A simple in-clinic test called the Rivalta test can help distinguish inflammatory fluid from other types. A drop of the effusion is added to a tube of diluted acetic acid. If the drop holds together and floats down slowly (a positive result), it suggests an inflammatory cause. The Rivalta test catches about 91 percent of true FIP cases, but it’s not FIP-specific. Other inflammatory conditions can also produce a positive result, which is why vets don’t rely on it alone.
Blood work adds more context. A ratio of two blood proteins, albumin and globulin, below 0.45 is highly suggestive of FIP when combined with other findings like low lymphocyte counts and high fluid protein. For definitive confirmation, the fluid sample can be tested directly for the virus. A lab can check the cell pellet from centrifuged fluid for viral genetic material using PCR, or examine cells under a microscope using immunohistochemistry. A positive result on either test is considered definitive proof. However, up to 30 percent of confirmed FIP cases can produce a false negative because viral levels in the sample are too low to detect.
Not Every Swollen Belly Is FIP
Fluid in the abdomen or chest can have many causes in cats, including heart disease, liver disease, cancer, and bacterial infections. The character of the fluid helps vets tell these apart. Heart failure typically produces a thin, watery fluid with low protein (around 2.0 g/dL) and very few cells, essentially filtered interstitial fluid pushed out by high blood pressure in the veins. FIP effusion sits at the opposite end of the spectrum: protein-rich, cell-heavy, and sticky.
It’s worth noting that not all FIP effusions look the same either. Cats with more chronic infections can produce fluid that’s lower in protein and more watery, resembling the kind seen in heart failure. This makes diagnosis harder and can cause the Rivalta test to come back negative. In ambiguous cases, direct viral testing of the fluid becomes especially important.
Treatment With Antiviral Medication
The treatment landscape for FIP has transformed since 2019. An antiviral compound called GS-441524, which blocks the virus from replicating, has shown remarkable effectiveness. A related drug, molnupiravir, has also shown similar results. Standard treatment lasts 84 days (12 weeks), with dosing adjusted based on the form of disease. Cats with eye or neurological involvement typically receive higher doses.
In the United States, no FDA-approved drug exists specifically for FIP. However, the FDA announced in 2024 that it does not intend to enforce approval requirements for GS-441524 products compounded by pharmacies when prescribed by a veterinarian for a specific cat. This means your vet can legally prescribe it, even though the compounded drug is technically unapproved. The practical result is that GS-441524 is now widely accessible through veterinary clinics and compounding pharmacies across the U.S.
Treatment is given daily, usually as an oral tablet or liquid, though some cats start with injections. Most cats with wet FIP show visible improvement within the first week or two. Fluid accumulation often begins to resolve within days of starting treatment. Cats need regular veterinary checkups throughout the 84-day course and for several months afterward to monitor for relapse, which can occur if treatment is stopped too early or if the dose is insufficient.
What to Expect During Recovery
Cats on antiviral treatment typically regain their appetite and energy within the first few weeks. The abdominal or chest fluid gradually reabsorbs as the inflammation calms down. Weight gain follows. Blood work is monitored throughout to track protein levels and other markers that reflect disease activity.
The biggest risk is relapse. Some cats experience a return of symptoms during or shortly after the treatment course, particularly if the initial dose was too low. If relapse occurs, treatment is usually restarted at a higher dose. Cats with neurological or eye involvement have a somewhat higher relapse risk than those with straightforward effusive disease.
Before antiviral treatment existed, wet FIP was almost always fatal within weeks to months. The availability of GS-441524 and molnupiravir has turned it into a treatable condition for many cats, though treatment requires commitment: 12 weeks of daily medication, regular vet visits, and costs that can add up over the full course.
Which Cats Are Most at Risk
FIP most commonly strikes young cats. Kittens and cats under two years old account for a disproportionate share of cases, likely because their immune systems are still maturing. Cats in multi-cat environments, shelters, and catteries face higher exposure to feline coronavirus in the first place, which increases the odds of the critical mutation occurring.
Stress appears to play a role as a trigger. Rehoming, surgery, overcrowding, or the introduction of new cats into a household can precede the onset of FIP. Certain purebred lines also seem predisposed, suggesting a genetic component to susceptibility.
Because the precursor feline coronavirus spreads through feces, keeping litter boxes clean is the most practical prevention measure in multi-cat homes. Providing one litter box per cat (plus one extra), scooping daily, and periodically disinfecting boxes reduces the viral load in the environment. Minimizing overcrowding and reducing stress through adequate space, resources, and gradual introductions also lowers risk. There is no widely effective vaccine against FIP available in most countries.