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

Feline infectious peritonitis — FIP — has long been one of the most feared diagnoses in feline medicine. It develops from a virus that is astonishingly common and usually harmless, strikes young cats disproportionately, and for decades was considered essentially untreatable. That last part has been changing: the treatment landscape has evolved significantly in recent years, and a diagnosis that was once hopeless is now, in many cases, a disease that veterinarians actively treat. That makes understanding FIP — and catching it early — more valuable than ever.

This guide explains where FIP comes from, why only some cats develop it, how the 'wet' and 'dry' forms differ, what diagnosis involves, and what owners of multi-cat households can do to reduce risk. It is general education, not a substitute for a veterinary work-up — FIP is precisely the kind of disease that demands professional diagnosis.

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.

Where FIP Comes From: A Common Virus With a Rare Twist

FIP begins with feline coronavirus (FCoV) — a virus unrelated to human COVID-19 in its effects — that circulates widely among cats, especially where many cats share litter boxes. Most infected cats experience nothing more than mild, transient diarrhea or no signs at all, and many cats worldwide have been exposed at some point in their lives, with exposure especially common in shelters, catteries, and multi-cat homes.

In a small minority of infected cats, the virus mutates inside the body into a form that infects immune cells called macrophages and spreads beyond the intestine. Combined with an ineffective immune response, this mutated infection drives widespread inflammation of blood vessels and organs — the disease we call FIP. The mutation happens within the individual cat; FIP itself is generally not considered directly contagious from cat to cat in the way the original intestinal virus is.

Which Cats Are at Highest Risk

• Young cats. The majority of FIP cases occur in cats under about two years of age, with kittens and adolescents most affected; a smaller second peak occurs in elderly cats.
• Cats from dense multi-cat environments. Shelters, catteries, and large multi-cat households have higher coronavirus circulation, which raises the odds that some cat, somewhere, develops the unlucky mutation.
• Recently stressed cats. Rehoming, weaning, surgery, overcrowding, or concurrent illness frequently precede FIP onset — stress suppresses immune defenses.
• Some pedigree lines. Certain purebred lines appear more susceptible, suggesting genetic influences on the immune response.
• Immunocompromised cats. Cats with retroviral infections or other immune burdens are less equipped to contain the mutated virus.

Risk factors raise suspicion but never make the diagnosis — plenty of shelter kittens never develop FIP, and the occasional only-cat from a quiet home does.

Effusive ('Wet') FIP: Fluid Where It Should Not Be

The effusive form progresses fastest and is often the easier form to recognize. Inflamed blood vessels leak protein-rich fluid into body cavities. In the abdomen this produces the classic pear-shaped, swollen belly — often startling on a kitten whose spine and ribs remain bony. When fluid accumulates in the chest instead, the primary sign is labored or rapid breathing.

Alongside the fluid, affected cats typically show persistent fever that does not respond to antibiotics, fading appetite, weight loss despite the swelling belly, lethargy, and sometimes pale or yellow-tinged gums. Wet FIP tends to declare itself over days to a few weeks. Any young cat with a swelling abdomen and fever needs veterinary attention urgently — and fluid analysis is one of the most informative early tests.

Non-Effusive ('Dry') FIP: The Great Imitator

The non-effusive form produces little or no free fluid. Instead, inflammatory nodules called pyogranulomas form in organs — kidneys, liver, intestines, lymph nodes, eyes, and brain — and the signs depend entirely on which organs are involved. This makes dry FIP one of veterinary medicine's great imitators.

• General signs: waxing-waning fever, poor appetite, weight loss, dull coat, stunted growth in kittens.
• Eye involvement: color change in the iris, cloudiness, bleeding inside the eye, or sudden vision problems.
• Neurologic involvement: wobbliness, hind-end weakness, head tremors, behavior changes, or seizures — FIP is a leading infectious cause of neurologic disease in young cats.
• Organ involvement: enlarged abdominal lymph nodes or irregular kidneys a vet may feel on palpation, jaundice, or digestive signs.

Dry FIP evolves more slowly than wet FIP, sometimes over weeks to months, and some cats show overlapping features of both forms.

How Veterinarians Diagnose FIP

There is no single, perfect FIP test in routine practice, which is why diagnosis is built from converging evidence. A widely-cited frustration deserves emphasis: a positive coronavirus antibody test alone does not diagnose FIP — a large share of healthy cats have antibodies from ordinary, harmless exposure.

• Signalment and history: a young cat from a multi-cat background with persistent fever raises the index of suspicion.
• Bloodwork patterns: classic findings include elevated total protein with low albumin-to-globulin ratio, anemia, and changes in white cell counts.
• Fluid analysis: in wet FIP, the characteristic straw-colored, sticky, protein-rich effusion is highly informative.
• Imaging: ultrasound helps find fluid, enlarged nodes, and organ changes, and guides sampling.
• Advanced testing: PCR testing for viral genetic material in fluid or tissue and immunostaining of biopsy samples add confidence; each has limitations your vet will weigh.
• Specialist input: neurologic and ocular cases often benefit from referral-level diagnostics.

Because each test has blind spots, expect your veterinarian to talk in terms of probability and accumulating evidence rather than a single yes/no result.

Treatment: A Landscape That Has Changed

For decades FIP was managed only with supportive and anti-inflammatory care, and the disease was almost always fatal. That picture has changed substantially: antiviral therapies developed in recent years have produced remission in many treated cats, and in a growing number of countries veterinarians can now legally prescribe such treatment. Availability, formulations, protocols, and legal status vary by region and continue to evolve.

This is exactly the conversation to have with your veterinarian — and, where available, a feline medicine specialist — rather than with the internet. Treatment courses are typically long (measured in weeks), require monitoring bloodwork, and work best when started before the cat is severely debilitated, which is one more reason early diagnosis matters. Supportive care — nutrition, hydration, anti-nausea support, and comfort — remains an important companion to any antiviral plan.

Reducing Risk in Multi-Cat Homes and Catteries

Because the root virus spreads mainly through shared litter boxes (fecal-oral route), hygiene and density are the levers owners actually control:

• Litter box hygiene: scoop at least daily, wash boxes regularly, and locate them away from food and water.
• Enough boxes: the classic rule — one per cat plus one extra — reduces both stress and contamination.
• Reasonable group sizes: coronavirus circulation climbs with density; keeping stable, small groups lowers viral load.
• Minimize stress: stable routines, resources spread through the home, and gradual introductions support the immune resilience that keeps FCoV harmless.
• Sensible caution with newcomers: quarantining new kittens briefly and keeping them parasite-free and well-fed supports their defenses during the highest-risk window.
• Breeding decisions: catteries that have produced multiple FIP kittens should discuss line management with their veterinarian.

No strategy eliminates risk entirely — the mutation is partly a matter of chance — so owners who have done all of the above and still face an FIP diagnosis should not blame themselves.

Caring for a Cat During Diagnosis and Treatment

• Prioritize eating. Warmed, aromatic, high-quality food in frequent small offerings; report any full day of not eating to your vet promptly — inappetence is dangerous in cats.
• Keep life low-stress. A quiet recovery room, familiar bedding, and gentle routines support both comfort and immunity.
• Track objective measures. Weekly weights, appetite scores, and activity notes give your vet real data at rechecks.
• Keep housemate cats healthy. Routine care for the other cats — parasite prevention, vaccination per your vet's schedule, and stress reduction — is sensible; discuss any testing questions with your clinic.
• Ask about follow-up bloodwork. Falling globulins and rising albumin are among the markers vets track to confirm a cat is responding.

Frequently Asked Questions

My cat tested positive for coronavirus antibodies. Does that mean FIP?

No. Antibody positivity is extremely common in healthy cats and by itself indicates exposure to the ordinary intestinal coronavirus, not FIP. Diagnosis rests on the whole clinical picture.

Can my other cats catch FIP from my sick cat?

The mutated, disease-causing form is generally considered not directly transmissible; what spreads between cats is the common intestinal coronavirus most multi-cat households already share. Good litter hygiene remains wise, and your vet can advise on your specific situation.

Is there a vaccine?

An intranasal vaccine exists in some markets, but its effectiveness is debated and major veterinary guidelines have generally not recommended it as a core vaccine. Ask your veterinarian what is appropriate where you live.

Is FIP still a death sentence?

Not the way it once was. Outcomes depend on the form of disease, how early treatment starts, and access to appropriate antiviral therapy — but many treated cats now achieve lasting remission. Speed matters: pursue a diagnosis promptly rather than waiting on a vague fever.

The Bottom Line

FIP is the rare, unlucky outcome of a very common virus: a mutation inside one cat's body that turns a trivial intestinal infection into a serious inflammatory disease. It disproportionately strikes the young and the recently stressed, wears two faces — fluid-building 'wet' disease and organ-focused 'dry' disease — and resists casual diagnosis, demanding a veterinarian who assembles evidence piece by piece.

The two most powerful things an owner can do are mundane and vital: keep litter boxes clean and cat groups low-stress to reduce risk, and act quickly on the warning combination of persistent fever, fading appetite, weight loss, swelling belly, breathing changes, or neurologic signs in a young cat. With today's evolving treatment options, early action has never mattered more.