Understanding Feline Infectious Peritonitis: Symptoms, Diagnosis, and Treatment

Feline infectious peritonitis (FIP) is the diagnosis people dread hearing after weeks of vague signs: a young cat who won’t eat, a fever that won’t settle, a belly that suddenly fills with fluid, or a slow drift into weight loss and dullness.

The stakes are simple. Without effective antiviral treatment, FIP is often fatal. With the right tests and the right drugs, many cats now recover. The hard part is getting from “something’s wrong” to a confident diagnosis and a safe, legal treatment plan.

What FIP is (and what it isn’t)

FIP is an inflammatory disease triggered 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. They may have no signs at all, or a short bout of mild diarrhoea, then carry on as normal.^{1, 2}

Causes and transmission of feline coronavirus (FCoV)

Where FCoV 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—contaminated paws, grooming, shared litter trays, and the quiet background drift of everyday living.²

Why FIP isn’t “catching FIP”

The FCoV infection itself is contagious. FIP, in most cases, is the result of viral changes that occur within a cat after it has already been infected. That means one cat having FIP does not automatically mean other cats will develop FIP, although they may share (or already carry) FCoV because they share an environment.^{1, 2}

Symptoms and types of FIP

Common early signs

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 (yellow tinge to gums/ears) or a pot-bellied appearance

These signs overlap with many other conditions, which is why FIP is diagnosed by pattern and probability, not by one simple test.^{1, 2}

Effusive (“wet”) FIP

Effusive FIP is defined by fluid build-up (effusion), usually in the abdomen and/or chest. The fluid tends to be straw-coloured and sticky, and it can accumulate quickly. In the chest, it may cause rapid breathing or 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. This form can look like many other diseases until the pieces line up.

Some cats develop eye disease (uveitis, changes inside the eye) and/or neurological signs (wobbliness, behaviour change, seizures). These presentations tend to be more complex and may need higher antiviral dosing and closer monitoring.^{2, 6}

Diagnosis of FIP

Why diagnosis is rarely a single moment

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 effusion. Definitive confirmation often relies on finding coronavirus antigen within characteristic inflammatory lesions (typically via tissue sampling and specialist laboratory testing).^{1, 2}

What vets commonly use 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/antigen can be very informative.²
• Rivalta test on effusion in some settings: useful as a supportive piece, especially because a negative result can help make FIP less likely, but a positive result is not exclusive to FIP.^{4, 5}

When to push for urgent assessment

See a veterinarian promptly if you notice:

• fast or laboured breathing
• a suddenly enlarged abdomen
• yellow gums/skin (jaundice)
• new wobbliness, seizures, or sudden vision changes

These signs don’t confirm FIP, but they can indicate serious disease where delays matter.

Treatment options for FIP

Supportive care (still matters, but rarely enough on its own)

Supportive care aims to keep the cat steady while diagnostics are underway and, if appropriate, during antiviral therapy. It may include fluid support, nutrition support, management of nausea and pain, and careful use of anti-inflammatory medication when indicated. Alone, supportive care does not address the underlying viral replication that drives FIP.^{1, 2}

Antiviral therapy: the major change in the story

FIP was once described as almost universally fatal. That statement needs updating. Antivirals—particularly nucleoside analogues such as GS-441524 and the closely related drug remdesivir—have been associated with high recovery rates in many treated cats when properly diagnosed and monitored.^{2, 6, 7}

Published evidence includes experimental and clinical studies showing rapid improvement in fever, appetite, and effusions, with many cats achieving remission after a full treatment course, though relapse can occur and some cases (especially neurological/ocular disease) are more demanding.^{8, 6}

A note on legality and product quality (Australia)

In Australia, owners sometimes encounter imported or unregistered “GS” products online. Australian regulators have warned that GS-441524 products obtained this way are not APVMA-approved, may be unsafe or ineffective, and importing unregistered veterinary chemicals can be an offence.⁹

The practical takeaway is to involve a veterinarian 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 and management in multi-cat homes

You cannot fully “FIP-proof” a home, but you can reduce FCoV pressure and stress, which helps lower risk at a population level.

Reduce FCoV spread

• Keep litter trays very clean and placed in sensible numbers for the group.
• Avoid overcrowding where possible, especially with kittens and adolescents.
• Isolate new arrivals long enough to assess health and reduce sudden mixing stress.

These steps don’t eliminate FCoV, but they can reduce heavy exposure and continual reinfection.²

Vaccination

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

Impact on cat 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 other cats in the home. It is common to feel whiplash—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.

Final thoughts

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 veterinary-guided antiviral treatment can turn the outcome from inevitable loss into a realistic chance of recovery.^{2, 6}

References

1. Merck Veterinary Manual (Professional) — Overview of Feline Infectious Peritonitis (Reviewed/Revised Jan 2024; modified Nov 2025)
2. European Advisory Board on Cat Diseases (ABCD) Guidelines — Feline Infectious Peritonitis (Viruses, 2023)
3. Tasker S. — Diagnosis of feline infectious peritonitis: update on evidence supporting available tests (Journal of Feline Medicine and Surgery, 2018)
4. Hartmann K. et al. — Diagnostic accuracy of the Rivalta test for feline infectious peritonitis (Veterinary Clinical Pathology, 2013)
5. Hartmann K. et al. — Diagnostic accuracy of the Rivalta test for feline infectious peritonitis (full text via PMC)
6. Coggins S. et al. — Outcomes of treatment of cats with FIP using parenteral remdesivir, with or without transition to orally administered GS-441524 (Journal of Veterinary Internal Medicine, 2023)
7. The University of Sydney — New hope for cat health (remdesivir and/or GS-441524 for FIP) (7 Aug 2024)
8. Pedersen N.C. et al. — The nucleoside analog GS-441524 strongly inhibits FIP virus in tissue culture and experimental cat infection studies (Veterinary Microbiology, 2018)
9. Veterinary Practitioners Registration Board of Victoria (APVMA information) — Unregistered products: feline infectious peritonitis (GS-441524/GC376 not approved by APVMA)