Understanding Feline Infectious Peritonitis: Symptoms, Diagnosis, and Treatment

By Sophie Kininmonth · 19 Dec 2025 (updated 21 Jul 2026) ·7 min read

Feline infectious peritonitis, or FIP, is the diagnosis owners dread after weeks of vague signs: a young cat who won't eat, a fever that won't settle, a belly that fills with fluid, or a slow drift into weight loss and dullness. For years it was treated as a death sentence, and that is the part worth updating first. With the right tests and the right antiviral drugs, many cats now recover. The hard work is getting from "something's wrong" to a confident diagnosis and a safe, legal treatment plan, and that path runs through your vet.

What FIP is (and what it isn't)

FIP is an inflammatory disease. It starts when a common gut virus, feline coronavirus (FCoV), changes inside an individual cat and gains the ability to spread through the body and replicate in certain immune cells. The damage that follows is largely driven by inflammation around blood vessels, which is why so many different organs can be involved.^{1, 2}

Most cats exposed to FCoV never develop FIP at all. They may show no signs, or have a short bout of mild diarrhoea, then carry on as normal.^{1, 2}

How feline coronavirus spreads

FCoV is most common where cats share space: shelters, rescue foster networks, catteries, and multi-cat households. The virus is shed mainly in faeces, and cats pick it up through the faecal-oral route, by way of contaminated paws, grooming, shared litter trays, and the quiet background drift of everyday living.²

Here is the point that reassures owners most: FIP is not the same as "catching FIP". The FCoV infection itself is contagious, but FIP in most cases is the result of viral changes that occur within a single cat after it is already infected. One cat having FIP does not automatically mean other cats will develop it, although they may share, or already carry, FCoV because they live in the same environment.^{1, 2}

Symptoms and types of FIP

FIP often begins as a slow, unconvincing illness. The cat is present, but not quite themselves.

Persistent or recurring fever
Lethargy and reduced play
Poor appetite and weight loss
Sometimes jaundice (a yellow tinge to the gums or ears) or a pot-bellied appearance

These signs overlap with many other conditions, which is why FIP is diagnosed by pattern and probability rather than by one simple test.^{1, 2}

Effusive ("wet") FIP

Effusive FIP is defined by fluid build-up (effusion), usually in the abdomen and chest. The fluid tends to be straw-coloured and sticky, and it can accumulate quickly. In the chest it may cause rapid breathing or visible effort with each breath.^{3, 2}

Non-effusive ("dry") FIP

Non-effusive FIP may not produce obvious fluid. Instead, inflammation forms lesions in organs such as the kidneys, liver, lymph nodes, intestines, or brain, and this form can look like many other diseases until the pieces line up. Some cats develop eye disease (uveitis, or changes inside the eye) and neurological signs such as wobbliness, behaviour change, or seizures. These presentations tend to be more complex and may need higher antiviral dosing and closer monitoring.^{2, 6}

Diagnosing FIP

There is no single, perfect screening test that says "this is FIP" in every cat. Diagnosis is usually built from several layers: history, physical exam, bloodwork trends, imaging, and, when present, testing of the effusion. Definitive confirmation often relies on finding coronavirus antigen within characteristic inflammatory lesions, typically through tissue sampling and specialist laboratory testing.^{1, 2}

To build confidence, your vet may combine:

Blood tests to look for a typical inflammatory pattern (not diagnostic on its own).^{1, 2}
Ultrasound or chest imaging to look for effusion or organ changes.²
Effusion analysis if fluid is present: appearance, protein levels, cytology, and targeted testing for FCoV RNA or antigen can be very informative.²
Rivalta test on effusion in some settings, useful as a supportive piece. A negative result can help make FIP less likely, but a positive result is not exclusive to FIP.^{4, 5}

See a vet promptly if you notice fast or laboured breathing, a suddenly enlarged abdomen, yellow gums or skin (jaundice), or new wobbliness, seizures, or sudden vision changes. These signs do not confirm FIP, but they can point to serious disease where delays matter.

Treatment options

The story of FIP treatment has changed, and antivirals are the reason. Nucleoside analogues such as GS-441524 and the closely related drug remdesivir have been associated with high recovery rates in many treated cats when the disease is properly diagnosed and monitored.^{2, 6, 7} Published evidence, from both experimental and clinical studies, shows rapid improvement in fever, appetite, and effusions, with many cats reaching remission after a full treatment course. Relapse can still occur, and some cases, especially neurological or ocular disease, are more demanding.^{8, 6}

Supportive care still matters, though it is rarely enough on its own. It keeps the cat steady while diagnostics are underway and, where appropriate, during antiviral therapy, and may include fluid support, nutrition support, management of nausea and pain, and careful use of anti-inflammatory medication when indicated. On its own, it does not address the viral replication that drives FIP.^{1, 2}

Legality and product quality in Australia

In Australia, owners sometimes come across imported or unregistered "GS" products sold online. Australian regulators have warned that GS-441524 products obtained this way are not APVMA-approved, may be unsafe or ineffective, and that importing unregistered veterinary chemicals can be an offence.⁹ The practical takeaway is to involve a vet early. Diagnosis, dosing, monitoring, and access pathways differ by state and clinic, and a supervised plan is safer for both the cat and the household.

Prevention in multi-cat homes

You cannot fully FIP-proof a home, but you can reduce FCoV pressure and stress, which lowers risk across a group of cats. Keep litter trays very clean and provide sensible numbers for the group, avoid overcrowding where you can (especially with kittens and adolescents), and isolate new arrivals long enough to assess their health and reduce the stress of sudden mixing. These steps do not eliminate FCoV, but they can cut heavy exposure and continual reinfection.²

An FIP vaccine exists in some markets, but it is not widely used and is not considered a reliable core strategy. Most prevention advice focuses instead on managing exposure and recognising illness early, particularly in high-density cat environments.²

What this means for owners

FIP tends to arrive quietly and then demand decisions quickly: whether to pursue referral, how far to investigate, how to fund a course of treatment, and how to manage the other cats at home. It is common to feel whiplash, with hope and worry swapping places day to day, especially during the diagnostic phase. If FIP is suspected, ask your vet to explain the diagnostic probability (how likely it is, and why), the proposed treatment pathway, the monitoring schedule, and what changes would mean the plan needs to adjust. Clear checkpoints make the whole process less foggy and easier to sustain.

The bottom line

FIP is still one of the most serious diseases a cat can face, but it is no longer a dead end by default. When a cat's signs fit the pattern, fast diagnostics and vet-guided antiviral treatment can turn the outcome from near-certain loss into a realistic chance of recovery.^{2, 6}

References

Sophie Kininmonth

Dog lover and writer at My Pets Australia, sharing practical breed guides and care advice for Australian pet owners.