What actually causes FIP in a cat's body?

Feline infectious peritonitis develops when the ordinarily mild feline enteric coronavirus, which infects the gut and usually causes little more than mild diarrhea, mutates within an individual cat into a biotype that invades white blood cells called monocytes and macrophages. Cornell Feline Health Center explains that this mutated virus spreads through the body inside these immune cells, triggering an intense and misdirected inflammatory reaction rather than an effective defense (Cornell Feline Health Center, 2023). The resulting vasculitis, or inflammation of blood vessel walls, allows protein-rich fluid to leak into body cavities and damages organs including the liver, kidneys, eyes, and brain depending on where the inflammatory cells accumulate.

Because FIP arises from a mutation that happens inside one cat rather than from cat-to-cat transmission of a distinct FIP virus, it is not considered directly contagious in the way that feline leukemia virus is. The American Association of Feline Practitioners notes that the parent coronavirus is extremely common, with up to 90% of cats in multi-cat households or shelters testing positive for exposure at some point, yet only a small fraction ever develop FIP because the mutation and subsequent immune dysregulation are what actually cause disease (AAFP, 2022).

Which cats are most likely to develop FIP?

Age is the single strongest risk factor: most cases occur in cats younger than 2 years old, with kittens between 4 and 12 months especially overrepresented, because their immune systems are still maturing when coronavirus exposure typically occurs. Cornell Feline Health Center reports that stress from overcrowding, recent spay or neuter surgery, rehoming, or a shelter environment can trigger the shift from harmless coronavirus carriage to the mutated, disease-causing form (Cornell Feline Health Center, 2023).

Multi-cat households and shelters see disproportionately higher rates simply because coronavirus circulates more freely where several cats share litter boxes and close quarters. Certain pedigree breeds, including Abyssinians, Bengals, Birmans, Himalayans, Ragdolls, and Rex breeds, appear to be statistically overrepresented in some case series, suggesting a possible genetic susceptibility component that researchers are still working to characterize (Cornell Feline Health Center, 2023).

What are the first signs an owner usually notices?

Early FIP signs are frustratingly nonspecific: a persistent low-grade fever that doesn't respond to antibiotics, reduced appetite, lethargy, and weight loss or stunted growth in a kitten that should be gaining. The AVMA describes these prodromal signs as easy to mistake for a garden-variety upper respiratory infection or mild gastrointestinal upset, which is part of why FIP is so often diagnosed late (AVMA, n.d.).

As the disease progresses, it typically follows one of two patterns. The 'wet' or effusive form produces visible abdominal distension from fluid accumulation, sometimes with labored breathing if fluid also builds around the lungs. The 'dry' or non-effusive form instead produces granulomatous lesions in organs like the eyes, kidneys, or nervous system, causing signs such as cloudy or bloodshot eyes, seizures, wobbly gait, or jaundice depending on which organs are affected.

How do veterinarians actually diagnose FIP?

No single blood test confirms FIP, which is one of the most frustrating aspects of the disease for owners and clinicians alike. Veterinarians build a diagnosis from a combination of signalment, clinical signs, bloodwork showing a high globulin-to-albumin ratio, and, when fluid is present, analysis of that fluid for its characteristic high protein, straw-colored, viscous appearance. Cornell Feline Health Center notes that an albumin-to-globulin ratio above 0.8 makes FIP unlikely, while a ratio below 0.4 is strongly supportive of the diagnosis (Cornell Feline Health Center, 2023).

Confirmatory testing increasingly relies on reverse-transcriptase PCR to detect coronavirus RNA in effusion fluid or tissue, and immunohistochemistry on biopsy samples to identify viral antigen inside macrophages, which remains the gold-standard confirmatory test. The Merck Veterinary Manual notes that a positive coronavirus antibody titer alone is not diagnostic, since so many healthy cats carry antibodies to the harmless intestinal form of the virus (Merck Veterinary Manual, 2023).

Is FIP actually treatable now, and what does treatment involve?

Yes, and this is the biggest shift in feline medicine in the past decade. The nucleoside analog GS-441524, an antiviral related to remdesivir, has been used in field trials and published case series with reported cure rates of roughly 80 to 90% when treatment is started before a cat is critically debilitated. A landmark study led by Dr. Niels Pedersen at UC Davis first demonstrated this efficacy, and the drug is now available in the United States through veterinary prescription following FDA guidance changes that allowed broader legal access starting in 2024 (UC Davis School of Veterinary Medicine, 2019).

Standard protocols call for daily oral or injectable dosing for a minimum of 84 consecutive days, with dose adjustments based on body weight, disease form, and whether the brain or eyes are involved, since those cases typically require higher doses to cross the blood-brain and blood-ocular barriers. Regular bloodwork rechecks during and after treatment monitor organ values and confirm the albumin-to-globulin ratio and body weight are trending in the right direction before treatment is declared complete.

What does caring for a cat through FIP treatment look like day to day?

Owners administering GS-441524 at home need a reliable daily routine, since missed or inconsistent dosing risks treatment failure or relapse. Cats often need weight rechecks every one to two weeks so dosing keeps pace with growth, particularly in kittens who may gain a pound or more over the 12-week course. The University of Wisconsin School of Veterinary Medicine notes that injectable formulations can cause injection-site pain, so many owners transition to the equally effective oral tablet form once a cat is stable enough to eat reliably (UW Veterinary Care, 2023).

Supportive care during the early weeks may include appetite stimulants, anti-nausea medication, and nutritional support for cats that have lost significant body condition, since malnourishment can slow immune recovery. Isolating a treated cat from unexposed cats isn't required, since FIP itself doesn't spread cat to cat, though a stressed multi-cat household benefits from reducing crowding and litter box competition to lower the odds of coronavirus recirculating and mutating in another cat.

What is the long-term outlook after a cat completes treatment?

Cats that complete a full course and show sustained normalization of bloodwork, appetite, and body weight over an 84-day observation period afterward are generally considered cured, with published relapse rates in treated cohorts running under 10% when treatment was completed appropriately. Cornell Feline Health Center reports that relapses, when they occur, usually happen within the first 12 weeks after stopping treatment and often respond to a second round of therapy at an adjusted dose (Cornell Feline Health Center, 2023).

Long-term prognosis is now dramatically better than the historical near-100% fatality rate, but outcomes still depend heavily on how early treatment began and whether neurological or ocular disease was present, since those forms require higher, longer-lasting doses and carry a somewhat higher relapse risk. Cats that survive treatment and remain stable for six months or more after finishing therapy are generally not expected to need ongoing medication.