Feline Infectious Peritonitis (FIP): A Cat Owner's Guide

Few conditions worry cat owners as much as feline infectious peritonitis, usually shortened to FIP. For decades it carried a reputation as a mysterious illness that appeared suddenly in young, apparently healthy cats and progressed quickly. That reputation was earned, but it is no longer the whole story. Veterinary understanding of how FIP develops has improved substantially, diagnostic approaches have become more refined, and antiviral therapy has changed what a diagnosis can mean for some cats.

This guide explains what FIP is, why it happens, how it tends to look in real households, and what the diagnostic and treatment process generally involves. It is written for owners rather than clinicians, so it stays away from dosages, brand claims and numbers that vary from cat to cat. The goal is simply to help you recognise when something is wrong, ask better questions at the clinic, and understand the decisions your veterinary team may present.

If you live with several cats, have recently adopted a kitten from a shelter or rescue, or are watching a cat who has been quietly losing condition, the sections below will help you make sense of what you are seeing and what happens next.

Important: This article shares general educational information for pet owners and is not a substitute for an in-person veterinary examination, diagnosis, or treatment. For any medical concern or emergency, contact your veterinarian promptly.

What Feline Infectious Peritonitis Actually Is

FIP is not a virus you catch from another cat in the way that cat flu is. It is a disease that develops inside an individual cat as a complication of an extremely common, usually harmless infection. The starting point is feline coronavirus, a gut virus that circulates widely wherever cats share space. Most cats who encounter it shed it in their stool for a while, show either no signs at all or a brief bout of soft stool, and move on with life.

In a small minority of infected cats, the virus changes inside the body in a way that lets it move out of the intestinal lining and into certain immune cells. Once that happens, the cat's own immune response to the altered virus drives inflammation in blood vessels throughout the body. That inflammatory process — not the gut infection itself — is what produces the illness we call FIP.

This distinction matters for owners because it explains something that often causes confusion. A household can be full of cats carrying feline coronavirus, yet only one develops FIP. The disease is a complication, not a straightforward contagion, and a cat with FIP is not usually treated as a direct danger to housemates in the way an infectious respiratory case would be.

Which Cats Tend to Be Affected

FIP is most often diagnosed in younger cats, particularly those under roughly two years of age, although it can appear at any age and is occasionally seen in older cats. Kittens and young adults from environments where many cats share litter boxes and airspace — shelters, rescues, breeding catteries, feral colonies and busy multi-cat homes — are over-represented, because coronavirus exposure in those settings is close to universal.

• Age. Young cats and kittens are diagnosed most frequently, with a second smaller cluster in geriatric cats.
• Density. Homes and facilities with many cats sharing litter trays have higher coronavirus circulation.
• Stress events. Rehoming, surgery, weaning, shipping and illness are frequently reported in the weeks beforehand.
• Genetics and breed. Some pedigree lines appear more susceptible, which suggests an inherited component to the immune response.
• Immune status. Cats with other illnesses or immune suppression may be less able to keep the infection contained.

None of these factors predict FIP in an individual cat, and plenty of cats with every risk factor listed never develop it. They are useful mainly as context: if a young cat from a crowded background becomes unwell in a way that does not fit an obvious cause, FIP tends to move up the list of possibilities your veterinarian considers.

The Effusive or “Wet” Form

The effusive form is usually the faster-moving and more visually obvious presentation. Inflamed blood vessels leak protein-rich fluid into body cavities, and that fluid accumulates where gravity and anatomy allow. In the abdomen, this produces a belly that looks progressively rounder and feels fuller, often on a cat whose spine, hips and shoulders are simultaneously becoming bonier. That contrast — a swelling body with a thinning frame — is one of the more recognisable patterns.

When fluid collects in the chest instead, the picture changes. Rather than a swollen abdomen, owners notice breathing that becomes faster, shallower or more effortful. The cat may sit with elbows held away from the body, breathe with visible abdominal effort, or become reluctant to lie flat on one side. Any change in a cat's breathing pattern deserves same-day veterinary attention regardless of the suspected cause.

What owners typically notice first

• A waistline that disappears while the rest of the body loses muscle
• Persistent low-grade fever that does not respond to routine treatment
• Reduced appetite, slower eating, or interest in food that fades over days
• Lethargy, less jumping, more time spent hidden or tucked away
• Faster or more laboured breathing, or reluctance to move much at all

The Non-Effusive or “Dry” Form

The non-effusive form produces little or no free fluid. Instead, inflammation forms small nodular lesions in organs — commonly the kidneys, liver, lymph nodes, eyes, brain and spinal cord. Because these lesions can appear almost anywhere, the dry form is far more variable and often much harder to pin down. It tends to develop more gradually, sometimes over weeks, with vague signs that look like several other diseases.

Eye involvement is one of the more distinctive clues. Owners may notice that the colour of the iris looks different, that one pupil is a different size from the other, that the eye seems cloudy or that a fine haze appears within it. Neurological involvement can show up as wobbliness, unusual head posture, circling, tremors, altered behaviour or seizures. Any of these changes warrants prompt veterinary assessment.

Because the dry form is so variable, cats are sometimes investigated for weeks before the picture becomes clear. This is frustrating, but it reflects the genuine difficulty of the diagnosis rather than a failure of care.

How Veterinarians Approach the Diagnosis

There is no single blood test that confirms FIP on its own, and this is probably the most important thing for owners to understand before the process begins. Diagnosis is built from a combination of the cat's history, physical examination, laboratory patterns, fluid analysis where fluid is present, and imaging. Your veterinarian is essentially assembling a case rather than reading a yes-or-no result.

• History and signalment. Age, background, household density and recent stressors all feed into the assessment.
• Blood work. Certain patterns in protein fractions, red cell values, white cell counts and liver markers are commonly seen and help build the picture.
• Fluid sampling. When effusion is present, analysing its colour, protein content and cell population is one of the most informative single steps.
• Imaging. Ultrasound and radiographs help locate fluid, assess organ texture and identify enlarged lymph nodes.
• Targeted testing. Additional laboratory techniques on fluid or tissue may be used where available and appropriate.

A widely misunderstood point is the coronavirus antibody test. A positive result tells you the cat has met feline coronavirus at some stage, which is true of a very large proportion of cats living in groups. On its own it neither confirms nor rules out FIP, and interpreting it without the rest of the picture leads to unnecessary alarm.

How Treatment Has Changed

For most of the time FIP has been recognised, care was limited to supportive measures and comfort. That has shifted. Antiviral compounds developed in recent years have been shown to help many cats with FIP, and their use has become part of mainstream veterinary conversation in a growing number of countries. Availability, regulatory status and cost vary considerably by region, and the situation continues to evolve.

Because of that variability, the honest answer to “what is the treatment” is that it depends where you live and which options your veterinary team can legally and practically access. Treatment courses are typically prolonged, require monitoring, and are followed by an observation period afterwards. Some cats respond well; others do not, and outcomes still depend heavily on how advanced the disease is when it is identified.

Please do not attempt to source antiviral products independently through informal channels. Purity, concentration and safety cannot be verified outside a regulated supply chain, and a cat receiving unmonitored medication may deteriorate without anyone recognising why. Work through your veterinarian, and ask directly what is available in your area.

Supportive Care at Home

Whatever the treatment plan, day-to-day supportive care matters. A cat who feels unwell often stops doing the small things that keep them stable — eating enough, drinking enough, grooming, moving around. Gentle attention to those basics can make a real difference to comfort and to how well a cat tolerates a long treatment course.

Nutritional supplements sometimes come up in these conversations. If you are considering one, discuss it with your veterinarian first. Products of this kind are complements to veterinary care rather than substitutes for it, and during active treatment it is particularly important that nothing is introduced without your clinician knowing about it.

Managing Risk in Multi-Cat Households

Because FIP develops from a common gut virus, the practical goal in a multi-cat home is reducing coronavirus load and reducing stress rather than trying to isolate one sick individual. Coronavirus spreads mainly through faecal material, which means litter tray management is the single highest-value intervention available to most households.

1. Provide roughly one litter tray per cat plus one spare, spread across different locations rather than lined up together.
2. Scoop at least twice daily and wash trays regularly, using a scoop that does not travel between trays without cleaning.
3. Site trays well away from food and water bowls so that litter is not carried into feeding areas.
4. Use a mat or low-tracking litter to limit how far granules spread across the floor.
5. Keep the overall number of cats within what the space can comfortably support, with enough resting spots, high perches and quiet retreats.

Reducing stress matters too. Cats cope better when resources are plentiful and predictable: multiple feeding stations, vertical space, hiding places and a stable routine. Odour control and clean surfaces help the humans keep up with the routine, and a litter deodorising product such as MetaPet's activated carbon litter deodorizer can be part of a regular hygiene routine, alongside — never instead of — frequent scooping and proper cleaning.

A vaccine against feline coronavirus exists in some markets, but it is not generally recommended by expert advisory panels for routine use, and it is not a substitute for hygiene and sensible household management. If you are curious about it, raise it with your veterinarian, who can explain the current position where you live.

Myths and Facts

“FIP is contagious between cats.”

Feline coronavirus is contagious; FIP itself is generally not considered directly transmissible from cat to cat. Housemates are typically already exposed to the same coronavirus and the vast majority never develop the disease.

“A positive coronavirus test means my cat has FIP.”

It does not. Antibody positivity is common in group-housed cats and simply reflects exposure. Interpretation requires the full clinical picture.

“There is nothing that can be done.”

This was closer to true in the past. Antiviral options now exist in a growing number of regions, and early identification improves the odds. Ask your veterinarian what applies locally.

“If one cat gets it, the rest will follow.”

Multiple cases in one household do occur, particularly in high-density settings, but they are the exception. Most housemates remain well.

When to Contact Your Veterinarian

Cats are practised at hiding illness, which means the window between “something seems slightly off” and “something is clearly wrong” can be short. With FIP in particular, early assessment gives the most options. Contact your veterinary practice promptly if you notice any of the following.

• Breathing changes. Faster, shallower or effortful breathing, or open-mouth breathing, is an emergency — go immediately.
• Abdominal swelling. A belly that is enlarging while the rest of the body loses condition needs prompt assessment.
• Persistent fever. A temperature that stays elevated and does not settle with routine treatment.
• Eye changes. Colour changes, cloudiness, unequal pupils or apparent visual difficulty.
• Neurological signs. Wobbliness, circling, tremors, seizures or marked behavioural change.
• Ongoing weight loss. Steady loss of condition despite reasonable food intake.

It is always reasonable to ring the clinic and describe what you are seeing, even if you are not sure it is significant. Veterinary teams would far rather assess a cat who turns out to be fine than see one arrive several weeks later.

Living With the Uncertainty

FIP is one of the harder diagnoses to sit with, partly because the path to confirming it can be indirect and partly because outcomes still vary. If your cat is being investigated, it is entirely reasonable to ask your veterinarian direct questions: what are the leading possibilities, what would each of the next tests actually change, what would treatment involve here, and what would we monitor to know whether it is working.

Owners often find it helps to keep a simple daily log — weight once or twice a week, appetite, energy, litter tray output and any new signs. It turns vague impressions into a record your veterinary team can use, and it makes the small improvements easier to see during a long treatment course.

Finally, be gentle with yourself. FIP is not caused by anything an owner did or failed to do. It arises from a common virus behaving unusually in one particular cat, and the most useful thing you can offer is attentive observation, prompt veterinary involvement and steady, quiet care at home.