What is Feline Infectious Peritonitis (FIP)?

Feline Infectious Peritonitis – usually shortened to FIP – is one of the most feared diseases in cats. For many years it was considered an automatic death sentence. Today, we understand it much better, and new antiviral treatments are offering real hope in many countries.

What exactly is FIP?

FIP is a serious disease caused by a type of feline coronavirus (FCoV).

Most cats that come into contact with feline coronavirus do not get FIP. In fact:

• Many cats carry feline coronavirus in their intestines

• They may have no symptoms, or only mild tummy upsets like soft stools or occasional vomiting

• In most cats, the virus stays in the gut and eventually disappears or becomes very low level

FIP happens when, in a small number of cats, this common coronavirus mutates (changes) inside the body and turns into a more dangerous form. This new form can:

• Live and multiply inside certain immune cells (monocytes and macrophages)

• Travel through the bloodstream

• Trigger a strong, harmful immune reaction causing inflammation of blood vessels and organs

So FIP isn’t usually “caught” from another FIP cat. Instead, a cat:

1. Picks up the harmless intestinal coronavirus

2. In rare cases, that virus mutates inside that cat

3. The mutated virus plus the cat’s immune response leads to FIP

How common is it – and which cats are at risk?

Feline coronavirus itself is very common, especially where lots of cats live together:

• Single-cat homes: roughly 15% may have been exposed

• Multi-cat homes, shelters, breeders, rescues: 50–90% may have been exposed

But only an estimated 5–12% of infected cats go on to develop FIP.

Certain factors make FIP more likely:

• Group housing (shelters, catteries, multi-cat homes)

• Young cats (under 2 years) and older cats (over 10 years)

• Stress – rehoming, surgery, illness, boarding, overcrowding

• Males and some purebred cats (e.g. Abyssinian, Bengal, Birman, Ragdoll, Rex breeds)

• Cats with retroviruses like FIV or FeLV

If your cat is in one or more of these categories and has worrying symptoms, it’s especially important to speak to your vet.

Is FIP contagious to humans or dogs?

Good news:

• FIP and feline coronavirus are not considered a risk to humans.

• FIP is a cat disease, affecting domestic and wild cats.

There are coronaviruses that infect humans (like COVID-19), but they are different viruses.

 Types of FIP: “Wet” and “Dry”

FIP doesn’t look the same in every cat. Vets generally talk about two main forms – although they can overlap:

Wet (effusive) FIP

This form involves fluid build-up in the body cavities due to leaky blood vessels:

You might notice:

• Swollen, pot-bellied tummy from fluid in the abdomen (ascites)

• Breathing problems if fluid builds up in the chest (pleural effusion):

  • Fast breathing

  • Effortful breathing

  • Muffled heart / lung sounds detected by your vet

Dry (noneffusive) FIP

This form causes inflammatory “lumps” (granulomas) in organs rather than a lot of free fluid. Signs depend on which organs are affected:

• Eyes: cloudy eyes, colour changes in the iris, bleeding in the eye, visual changes, even blindness

• Brain and nerves: wobbliness, seizures, behaviour changes, head tilt, strange movements, trouble walking or jumping

• Kidneys, liver, intestines: weight loss, poor appetite, vomiting, diarrhoea, enlarged organs

Shared signs in both forms

Regardless of wet or dry type, many cats with FIP show:

• Lethargy – just not themselves, sleeping more

• Poor appetite or not eating at all

• Weight loss or failure to grow (kittens)

• Fever that tends to come and go and does not respond well to antibiotics

If your cat has a combination of these signs for more than a few days, especially with a swollen belly or eye/neurological changes, your vet will likely consider FIP as one of the possibilities.

Why is FIP so hard to diagnose?

One of the most important points for owners to understand is:

There is no single simple “yes/no” blood test for FIP.

Instead, your vet has to put together several pieces of information, like a puzzle:

• Your cat’s age, lifestyle, history and symptoms

• Physical exam findings

• Blood tests – looking for typical patterns like:

  • Low lymphocytes (a type of white blood cell)

  • High total protein and globulins

  • Low albumin

  • High bilirubin

• Imaging (X-rays or ultrasound) to look for fluid and organ changes

• If there is fluid in the belly or chest, your vet may:

  • Take a sample with a fine needle

  • Examine it under a microscope

  • Sometimes run special tests (like PCR or immunostaining)

For a completely definite answer, the gold standard is to see the virus inside affected tissues under the microscope using special stains – but that often requires biopsies or post-mortem samples, which aren’t always possible in very sick cats.

Because of all this, FIP is often diagnosed as “very likely FIP” based on a combination of findings rather than one test.

Can FIP be treated?

For many years, FIP was considered untreatable and fatal. Supportive care could make cats comfortable for a while, but the disease almost always progressed.

This is changing.

Antiviral medications

Newer antiviral drugs that target the coronavirus directly have dramatically improved the outlook for many cats in countries where they are available and legal. These drugs work by stopping the virus from copying itself inside the body.

They are:

• Given for many weeks (often 12 weeks or more)

• Carefully dosed by weight

• Monitored by your vet with check-ups and follow-up tests

Important notes for owners:

• These medications are not available in every country, or may not be formally licensed yet.

• Black-market or unregulated products can be unsafe or fake – always work with a veterinarian.

• Even with antivirals, some cats – especially those with severe neurological disease – may not respond.

Supportive care

Alongside antivirals (or where they aren’t available), supportive treatment can include:

• Pain relief and anti-inflammatory drugs (used carefully under veterinary supervision)

• Fluid therapy for dehydration

• Nutritional support for cats that aren’t eating

• Draining chest fluid if breathing is difficult

• Treating secondary infections and eye problems

Your vet will tailor the plan to your cat’s condition and comfort.

What is the prognosis?

Without specific antiviral treatment, FIP is almost always fatal, often within days to weeks for the wet form and weeks to months for the dry form.

With modern antiviral treatment, however:

• Many cats can go into clinical remission

• Some treated cats are living normal, active lives after completing their treatment course

Because these drugs are relatively new in feline medicine, long-term data is still being gathered – but the outlook is now much more hopeful than it used to be.

Can FIP be prevented?

There is an intranasal vaccine against feline coronavirus, but it is not generally recommended by major feline vaccine guidelines, and its usefulness is debated. More importantly, prevention focuses on good husbandry and hygiene, especially in multi-cat environments:

• Keep litter trays very clean, and ideally have:

  • 1 tray per cat, plus one extra

• Scoop frequently and wash trays regularly

• Reduce stress: provide hiding places, vertical space, and a predictable routine

• Avoid overcrowding and unnecessary mixing of large numbers of cats

• In breeding or rescue situations, work with your vet on testing and management strategies

At home, for most pet owners, the key is:

• Keep your cat’s immune system strong – good nutrition, parasite control, regular vet checks

• Minimise stress and major upheavals where possible

• Monitor any ongoing tummy issues or unexplained illnesses promptly with your vet

Routine household disinfectants (like properly diluted bleach) easily kill feline coronavirus in the environment, so normal cleaning is effective.

When should I call my vet?

Contact your vet as soon as possible if you notice:

• A young cat (especially from a shelter, cattery or multi-cat household) with:

  • Persistent fever

  • Weight loss or failure to grow

  • Swollen belly

  • Breathing difficulties

  • Eye changes or neurological symptoms

FIP can’t be diagnosed at home, and early involvement of a veterinarian gives your cat the best chance, especially now that antiviral options exist in many places.

In summary

• FIP is a serious immune-mediated disease caused by a mutated form of feline coronavirus.

• Most cats with feline coronavirus never get FIP, but a small percentage do.

• There is no single simple test – diagnosis relies on a combination of signs, tests and imaging.

• Without treatment, FIP is usually fatal – but new antiviral medications are changing the outlook for many cats.

• Good hygiene, stress reduction and responsible multi-cat management can lower the risk, especially in group settings.

If you’re ever worried that your cat might have FIP, or you’ve just heard the diagnosis and feel overwhelmed, remember: you are not alone, and your veterinary team is there to guide you through every step – from testing decisions to treatment options, comfort care and support.