Feline infectious peritonitis (FIP) is a serious, potentially fatal disease in cats caused by a mutated form of a common feline coronavirus. Most cats exposed to feline coronavirus experience nothing more than mild diarrhea or no symptoms at all. But in a small percentage of cats, the virus mutates inside the body and triggers an overwhelming inflammatory response that can damage organs, the brain, and the eyes. Until recently, FIP was considered a death sentence. Today, antiviral treatments cure the majority of cats who survive the first days of therapy.
How FIP Develops From a Common Virus
Feline coronavirus is extremely widespread, especially in multi-cat environments like shelters, catteries, and households with several cats. Cats pick it up through the fecal-oral route, typically by sharing litter boxes. In most cases, the virus stays in the intestinal tract and causes either mild digestive upset or no symptoms whatsoever.
FIP develops when that intestinal coronavirus mutates within the cat’s body. The leading theory is that genetic changes, likely in the spike protein and other parts of the virus, allow it to infect a type of immune cell called macrophages. Once the virus can hijack these immune cells, it spreads beyond the gut and into organs throughout the body. This triggers a massive, misdirected immune response that causes the damage we call FIP. The exact mutation responsible hasn’t been pinpointed, and researchers note the theory, while widely accepted, has never been explicitly confirmed.
One important detail for cat owners: the mutated FIP virus is not considered contagious from cat to cat. The original feline coronavirus spreads easily in shared environments, but the dangerous mutation happens individually inside each cat. You don’t need to isolate a cat with FIP from other cats to protect them from the mutated form of the virus, though the underlying coronavirus may already be circulating among housemates.
Wet Form vs. Dry Form
FIP shows up in two main patterns, often called “wet” and “dry,” though some cats have features of both.
The wet (effusive) form causes fluid to accumulate in the abdomen, the chest, or both. A cat with wet FIP often develops a visibly swollen belly that feels tight. If fluid builds up around the lungs instead, the cat may struggle to breathe. This form tends to progress quickly, sometimes over days to weeks.
The dry (non-effusive) form doesn’t produce obvious fluid buildup. Instead, clusters of inflammatory cells form in organs like the liver, kidneys, eyes, and brain. Dry FIP is harder to spot because its symptoms, such as weight loss, fever that doesn’t respond to antibiotics, lethargy, and poor appetite, overlap with many other illnesses. It also tends to develop more slowly than the wet form.
Neurological and Eye Symptoms
FIP can affect the eyes and central nervous system, either alongside other symptoms or on their own. Eye involvement may include bleeding or pus visible inside one or both eyes, cloudiness, and in some cases blindness. These changes can appear suddenly.
When FIP reaches the brain and spinal cord, the most common early sign is poor coordination that progresses to a wobbly, unsteady gait. As it advances, cats may develop seizures, tremors, personality changes, or unusual sensitivity to being touched. Neurological FIP is the most challenging form to treat because the virus is harder to reach behind the blood-brain barrier, but it is no longer the automatic death sentence it once was.
How Veterinarians Diagnose FIP
No single test can confirm FIP in a living cat, which makes diagnosis one of the trickiest parts of the disease. Veterinarians piece together the picture from a cat’s history, physical exam, blood work, and sometimes fluid analysis.
A key blood marker is the albumin-to-globulin (A:G) ratio. This compares two types of protein in the blood. An A:G ratio below 0.4 strongly suggests FIP, while a ratio above 0.6 to 0.8 makes the diagnosis highly unlikely. It’s a useful screening tool, but not definitive on its own since other inflammatory conditions can shift the ratio too. Veterinarians combine it with total protein levels, coronavirus antibody tests, and PCR testing that detects viral genetic material.
When fluid is present (in the wet form), analyzing it provides some of the strongest diagnostic clues. FIP fluid is typically straw-colored, sticky, and high in protein. A simple bedside test called the Rivalta test is particularly helpful: a positive result has high sensitivity for FIP, while a negative result essentially rules it out. This makes it a quick, practical first step before more expensive lab work.
Antiviral Treatment and What to Expect
The breakthrough that transformed FIP from a fatal diagnosis into a treatable one is an antiviral compound called GS-441524, developed through research at UC Davis. Treatment protocols follow a standard 12-week (84-day) course of daily medication, though some younger cats with wet FIP may be cured in as few as 8 weeks.
Dosing depends on which form of FIP a cat has. Cats with wet or dry FIP that hasn’t reached the eyes or brain typically start at 4 to 6 mg/kg daily. If the eyes are involved, the starting dose increases to 8 mg/kg. Cats with neurological symptoms require 10 mg/kg to push enough drug past the blood-brain barrier. Veterinarians adjust doses throughout treatment based on how the cat responds.
The first 10 days are the most critical and dangerous window. In a large clinical study of 118 cats, about 17% of cats died during treatment, and most of those deaths happened within the first 10 days. Cats that survive this early period have excellent odds. Among cats that completed the full treatment course, virtually 100% achieved remission, and the vast majority were still alive one to nearly three years later.
One challenge owners face is knowing when to stop. There’s no simple test that confirms a cat is truly cured, and the fear of relapse often drives owners and veterinarians to extend treatment beyond the standard 84 days. Relapse is possible but uncommon in cats who complete the full course and show sustained improvement in bloodwork and symptoms.
Which Cats Are Most at Risk
FIP most commonly strikes young cats, typically under two years old, though it can occur at any age. Cats in multi-cat households, shelters, and breeding catteries face higher risk because the underlying feline coronavirus circulates more readily in crowded environments. Stress from rehoming, surgery, or other illnesses may also play a role in triggering the mutation, though the exact relationship isn’t fully understood.
Purebred cats appear to be affected more often than mixed-breed cats, though it’s unclear whether this reflects true genetic susceptibility or simply the fact that purebreds are more likely to come from multi-cat breeding environments. There is a vaccine for FIP, but it is not currently recommended for general use. The decision to vaccinate is something to discuss with a veterinarian based on a cat’s specific living situation and risk factors.
Signs That Should Prompt a Vet Visit
The early symptoms of FIP are frustratingly vague: a persistent fever, declining appetite, weight loss, and low energy. What sets FIP apart from routine illness is that these symptoms don’t improve with standard treatments like antibiotics. A young cat with a stubborn fever and a pot-bellied appearance should raise immediate suspicion. So should sudden eye changes like cloudiness or visible inflammation inside the eye, or any new wobbliness or coordination problems. Early detection matters more now than it ever has, because the sooner antiviral treatment begins, the better the chances of survival.