Feline Infectious Peritonitis in Cats: Symptoms, Diagnosis, Treatment and Survival

By Dr Duncan Houston

Feline infectious peritonitis used to be one of the most devastating diagnoses in feline medicine. Once clinical disease developed, treatment was largely supportive and most affected cats died within weeks.

That has changed.

Modern antivirals can now place most appropriately treated cats into long-term remission. The challenge is recognising FIP quickly, collecting the most useful diagnostic samples and starting a reliable antiviral before the cat becomes critically unstable.

Quick Answer

Feline infectious peritonitis, or FIP, is a systemic inflammatory disease caused by feline coronavirus variants that develop within a small proportion of infected cats.

Signs may include persistent fever, weight loss, reduced appetite, abdominal or chest fluid, jaundice, enlarged lymph nodes, eye inflammation, weakness, poor coordination or seizures. Oral GS-441524 is now the preferred first-line treatment, and survival rates commonly exceed 90% when treatment begins promptly and medication is administered reliably. (ABCD cats and vets)

FIP at a Glance

Question	Practical answer
What causes FIP?	Feline coronavirus variants capable of sustained replication in monocytes and macrophages, together with an abnormal inflammatory response
Is ordinary feline coronavirus common?	Yes, particularly in multi-cat environments
Do most coronavirus-positive cats develop FIP?	No. Most remain healthy or develop only mild intestinal signs
Is FIP directly contagious?	Usually no. Enteric feline coronavirus is contagious, but FIP-associated variants are rarely transmitted directly
Which cats are most affected?	Cats under two years, pedigree cats and cats from crowded or stressful multi-cat environments
What are the major presentations?	Effusive, non-effusive, ocular, neurological or mixed disease
Is there one definitive blood test?	No
What sample is most useful when fluid is present?	The effusion itself
What is the preferred treatment?	Quality-assured oral GS-441524
How long is treatment?	Commonly 42 to 84 days, depending on response and disease form
Can cats recover?	Yes. Long-term survival commonly exceeds 90% with prompt treatment
When is it an emergency?	Breathing difficulty, collapse, seizures, inability to stand or profound weakness

(ABCD cats and vets)

What Is Feline Infectious Peritonitis?

FIP is a multisystemic inflammatory disease associated with feline coronavirus, or FCoV.

FCoV normally infects intestinal cells. It spreads mainly through the faecal-oral route and is extremely common wherever cats share litter trays, bedding and living space. Most infections cause no obvious illness or only mild, temporary gastrointestinal signs. (ABCD cats and vets)

In a small proportion of cats, viral changes and host immune factors allow the virus to replicate effectively within monocytes and macrophages. These infected inflammatory cells travel through the body and produce severe inflammation around blood vessels, known as perivascular phlebitis.

Because blood vessels exist in every organ, FIP can affect almost any part of the body, including the abdomen, chest, eyes, brain, spinal cord, kidneys, liver, lymph nodes and heart. (ABCD cats and vets)

Is FIP Caused by One Specific Mutation?

Not exactly.

The traditional explanation is that harmless intestinal feline coronavirus mutates within an individual cat and becomes an FIP-causing virus. This remains broadly useful, but the biology is more complicated than one universal mutation.

Several spike-protein and other viral mutations may contribute, but no single mutation is present in every cat with FIP or absent from every cat without FIP. The cat’s immune response and other host factors also influence whether systemic viral replication becomes sustained and clinically destructive. (ABCD cats and vets)

For this reason, commercial “FIP mutation tests” should not be treated as definitive. Detecting a high viral load in an affected tissue or effusion, combined with compatible cytology and clinical findings, is usually more useful than searching for one particular mutation. (ABCD cats and vets)

Is FIP Contagious to Other Cats?

Ordinary feline coronavirus is contagious. FIP itself is usually not.

Cats commonly acquire enteric FCoV by ingesting microscopic faecal contamination from litter trays, paws, surfaces or shared equipment. Cats living together have often been exposed to the same intestinal virus long before one cat develops FIP. (ABCD cats and vets)

The FIP-associated virus usually develops inside the affected cat and is not readily shed in a form that directly causes FIP in another cat. Direct horizontal transmission of an FIP-associated variant is considered very uncommon, although rare outbreak situations have been reported in unusually dense and stressful populations. (ABCD cats and vets)

A cat with FIP therefore does not usually require isolation from familiar household cats solely because of the FIP diagnosis. Normal infection-control measures are generally sufficient in a veterinary hospital. Isolation may still be needed when the cat has another contagious disease, such as calicivirus or panleukopenia. (ABCD cats and vets)

Should Healthy Housemates Receive Antivirals?

No.

Antiviral treatment is not recommended for healthy in-contact cats merely because they have shared a home with a cat diagnosed with FIP. Doing so adds cost, medication burden and a theoretical risk of antiviral resistance without proven benefit. (catvets.com)

Which Cats Are Most at Risk?

FIP can affect any cat, but it is particularly associated with:

Cats younger than two years
Kittens from shelters, rescues, breeding catteries or crowded homes
Pedigree cats
Cats closely related to another cat that developed FIP
Cats exposed to high environmental FCoV loads
Cats experiencing recent adoption, rehoming, surgery or significant social stress
Cats living in unstable or overcrowded groups
Cats with concurrent disease or impaired immune function

Young cats are overrepresented because their immune systems are still developing and they are commonly exposed to high FCoV loads during the months after maternal antibodies decline. (ABCD cats and vets)

A recent stressful event is common in the history, but stress does not independently prove that FIP is present. It is one part of the clinical picture rather than a diagnostic test.

Are Wet and Dry FIP Separate Diseases?

Not really.

“Wet” and “dry” remain useful descriptive terms, but FIP is better understood as a spectrum. A cat may have effusion, granulomatous lesions, eye disease and neurological disease at the same time, or move from one presentation to another.

Effusive FIP

Effusive FIP causes protein-rich inflammatory fluid to accumulate within body cavities.

Possible locations include:

Abdomen
Chest
Pericardial space around the heart
Scrotum in an entire male

The abdomen may become rounded or visibly enlarged. Chest fluid can compress the lungs and cause rapid or laboured breathing.

Non-Effusive FIP

Non-effusive FIP produces inflammatory nodules and organ infiltration without a large visible effusion.

Possible findings include:

Enlarged mesenteric lymph nodes
Irregular or enlarged kidneys
Liver lesions
Intestinal thickening
Pancreatic involvement
Chronic diarrhoea
Persistent fever
Weight loss

Ocular FIP

Eye involvement may cause:

Uveitis
Redness
Cloudiness
Unequal pupils
Colour change within the iris
Keratic precipitates
Retinal inflammation
Reduced vision
Sudden blindness

Neurological FIP

Neurological disease may affect the brain, spinal cord or both.

Possible signs include:

Wobbling or ataxia
Weakness
Head tilt
Nystagmus
Tremors
Behavioural change
Difficulty swallowing
Urinary or faecal incontinence
Seizures
Paralysis

Ocular and neurological involvement matter because higher antiviral doses are generally required to achieve adequate penetration across the blood-eye and blood-brain barriers.

What Are the Early Signs of FIP?

Early signs are often frustratingly vague.

Common initial changes include:

Fever that persists or repeatedly returns
Reduced appetite
Lethargy
Failure to gain weight
Progressive weight loss
Poor growth in a kitten
Reduced play or social interaction
Pale mucous membranes
Dehydration
Poor coat quality

A young cat with persistent fever, weight loss and inflammatory blood changes despite routine treatment deserves investigation for FIP even before fluid, eye or neurological abnormalities develop. (ABCD cats and vets)

What Are the More Specific Signs?

Signs Caused by Abdominal Effusion

Enlarging or pendulous abdomen
Reduced appetite
Difficulty moving comfortably
Shallow breathing from pressure against the diaphragm
A fluid wave on abdominal palpation
Weight gain despite muscle loss

The last point can be deceptive. A cat may appear to be gaining weight while actually losing muscle and accumulating litres of inflammatory fluid.

Signs Caused by Pleural Effusion

Rapid breathing
Laboured breathing
Abdominal respiratory effort
Neck extension
Reluctance to lie down
Open-mouth breathing
Blue or grey gums
Collapse

Pleural effusion can become immediately life-threatening because fluid physically prevents the lungs from expanding.

Signs of Abdominal Organ Involvement

Enlarged lymph nodes
Irregular kidneys
Jaundice
Vomiting
Diarrhoea
Abdominal pain
Increased thirst
Changes in urination
Elevated liver or kidney values

Eye Signs

Squinting
Redness
Cloudy eyes
Visible deposits behind the cornea
Abnormal pupil size
Iris colour change
Reduced vision

Neurological Signs

Progressive incoordination
Falling
Weakness
Tremors
Seizures
Personality change
Difficulty jumping
Incontinence
Paralysis

How Worried Should You Be?

Lower Risk

The cat:

Is bright and eating normally
Has no fever
Is maintaining weight
Has normal breathing
Has merely tested positive for feline coronavirus antibodies or faecal coronavirus RNA

What it likely means: Exposure to ordinary feline coronavirus is common and does not diagnose FIP.

What to do: Continue routine care and monitor. Do not start FIP treatment based solely on coronavirus exposure testing. (ABCD cats and vets)

Moderate Concern

The cat has:

Persistent or fluctuating fever
Reduced appetite
Lethargy
Poor growth
Weight loss
Hyperglobulinaemia
Mild anaemia or lymphopenia
Enlarged abdominal lymph nodes

What it may mean: FIP is possible, but lymphoma, infection, inflammatory disease and other causes remain important.

What to do: Arrange veterinary investigation within 24 hours. Ultrasound, blood testing and targeted sampling may be indicated.

High Concern

The cat has:

Abdominal or chest effusion
Uveitis
Jaundice
Progressive neurological signs
Marked weight loss
Very low albumin-to-globulin ratio
Persistent fever despite previous treatment
Compatible effusion cytology or positive FCoV RT-PCR from an affected site

What it may mean: FIP is highly likely.

What to do: Seek same-day veterinary assessment and discuss immediate antiviral treatment while confirmatory testing is pursued.

Critical

The cat has:

Open-mouth breathing
Severe respiratory effort
Blue, grey or white gums
Collapse
Repeated seizures
Inability to stand
Profound weakness
Hypoglycaemia
Hypotension
Severe anaemia
Minimal responsiveness

What it means: The cat may die without immediate stabilisation.

What to do: Attend an emergency veterinary hospital immediately. Oxygen, fluid drainage, intravenous antiviral medication and intensive supportive care may be required.

When Is This an Emergency?

Seek immediate veterinary care if your cat develops:

Open-mouth breathing
Pronounced abdominal respiratory effort
A resting respiratory rate that is rapidly increasing
Blue, grey or very pale gums
Collapse
Repeated or prolonged seizures
Sudden inability to walk
Profound weakness
Inability to swallow medication
Severe eye pain or sudden blindness
Complete food refusal with rapid deterioration
A tense abdomen that is interfering with breathing
Minimal awareness of the surroundings

Keep the cat calm and handle them as little as possible. Stress increases oxygen demand and can worsen respiratory or neurological compromise.

Do not force food, water or oral medication into a cat that is struggling to breathe or cannot swallow safely.

What Else Can Look Like FIP?

FIP can mimic almost any systemic inflammatory disease.

FIP presentation	Important alternatives
Abdominal effusion	Septic peritonitis, lymphoma, liver disease, heart disease, severe hypoalbuminaemia, bleeding
Pleural effusion	Pyothorax, congestive heart failure, lymphoma, chylothorax, haemothorax
Persistent fever	Bacterial infection, toxoplasmosis, FeLV, FIV, lymphoma, immune-mediated disease
Enlarged abdominal lymph nodes	Lymphoma, mycobacterial disease, fungal infection, chronic enteropathy
Jaundice	Haemolysis, cholangitis, hepatic lipidosis, bile-duct obstruction
Uveitis	Toxoplasmosis, lymphoma, FeLV, FIV, fungal infection, trauma
Neurological disease	FIP, toxoplasmosis, cryptococcosis, brain tumour, congenital disease, toxin exposure
Kidney enlargement	Lymphoma, pyelonephritis, polycystic kidney disease
Chronic diarrhoea	Tritrichomonas, Giardia, diet-responsive disease, chronic enteropathy, lymphoma

A partial response to an antiviral does not completely rule out a second disease. Cats can have FIP alongside lymphoma, bacterial peritonitis, haemoplasmosis, myocarditis or immune-mediated haemolytic anaemia. (catvets.com)

What To Do Right Now if FIP Is Suspected

1. Arrange Examination Promptly

FIP is no longer a disease where waiting for absolute certainty is always reasonable.

Diagnostic testing should begin quickly, and antiviral treatment may need to start before every result is available when the cat is deteriorating or the probability of FIP is high. (ABCD cats and vets)

2. Monitor Breathing

Count the resting respiratory rate while the cat is asleep.

Record:

Breaths per minute
Abdominal effort
Open-mouth breathing
Neck extension
Ability to lie comfortably
Any coughing or gagging

A video can help the veterinary team assess progression.

3. Monitor Weight

Use the same accurate scale and weigh the cat at least weekly during treatment.

Kittens may gain weight rapidly once therapy begins. The antiviral dose must be adjusted upwards as the cat grows. Failure to increase the dose with weight gain is considered one of the common causes of poor response or treatment failure. (catvets.com)

4. Record Appetite, Temperature and Activity

A simple daily log should include:

Appetite
Food quantity
Body weight
Temperature if you have been trained to measure it
Energy
Breathing rate
Abdominal size
Eye changes
Neurological signs
Antiviral dose and administration time

5. Do Not Begin an Unverified Product Without Veterinary Input

Unregulated products may contain too little, too much or none of the stated antiviral. Injectable products may also be excessively acidic and cause severe pain or skin injury.

Quality-assured, legally supplied preparations should be used whenever available. (catvets.com)

6. Do Not Routinely Treat Healthy Housemates

Healthy cats should not receive low-dose GS-441524 to “prevent” FIP. There is no evidence that this prevents disease, and unnecessary exposure could encourage antiviral resistance. (catvets.com)

How Is FIP Diagnosed?

There is no single routine test that identifies every cat with FIP or excludes the disease with certainty.

Diagnosis is based on combining:

Age and history
Clinical signs
Blood changes
Imaging
Effusion characteristics
Cytology
FCoV antigen or RNA detection in affected samples
Histopathology where possible
Response to antiviral treatment

(ABCD cats and vets)

Blood Count Changes

Common findings include:

Lymphopenia
Neutrophilia
Mild to moderate non-regenerative anaemia
Occasional thrombocytopenia
Microcytosis in some cats

None of these is specific to FIP. (ABCD cats and vets)

Biochemistry Changes

More useful patterns include:

Increased globulins
Low or low-normal albumin
Low albumin-to-globulin ratio
Increased bilirubin
Increased acute-phase proteins
Organ-specific enzyme changes

An albumin-to-globulin ratio below 0.4 substantially increases suspicion. A ratio above 0.8 makes FIP much less likely, although no ratio proves or completely excludes the diagnosis. (ABCD cats and vets)

Alpha-1 acid glycoprotein and serum amyloid A may support a diagnosis and can also help monitor response, but both can increase with other inflammatory diseases. (ABCD cats and vets)

Imaging

Ultrasound may identify:

Small amounts of abdominal fluid
Enlarged or irregular lymph nodes
Renal enlargement or nodules
Liver or splenic abnormalities
Intestinal lesions
Pancreatic changes
Pericardial effusion

Thoracic imaging may identify pleural fluid or pulmonary abnormalities.

Point-of-care ultrasound is especially useful because even a very small pocket of fluid may provide the best diagnostic sample. (ABCD cats and vets)

Why Is Effusion Sampling So Important?

When fluid is present, sampling it is usually the single most valuable diagnostic step.

Typical FIP fluid is:

Clear to slightly cloudy
Straw-yellow
Sticky or viscous
High in protein
Low to moderately cellular

FIP effusions commonly contain more than 35 g/L of protein and show pyogranulomatous inflammation with macrophages and non-degenerate neutrophils. (ABCD cats and vets)

The fluid can be used for:

Total protein
Cell count
Cytology
Rivalta testing
FCoV RT-PCR
FCoV antigen immunostaining
Bacterial culture when sepsis remains possible

What Is the Rivalta Test?

The Rivalta test is a simple in-house method for identifying highly proteinaceous inflammatory effusion.

A negative result makes FIP much less likely. A positive result only supports FIP because septic peritonitis and lymphoma may also produce positive results.

Interpretation is subjective, so the Rivalta test should never replace cytology, molecular testing and the rest of the clinical investigation. (ABCD cats and vets)

Which Coronavirus Tests Actually Help?

FCoV RT-PCR on Effusion

A high FCoV RNA load within an effusion that also has typical protein and cytological features makes FIP very likely.

Approximately 72% to 100% of FIP effusions test positive by RT-PCR in published studies. A negative result does not completely exclude FIP, particularly in a low-cellularity sample. (ABCD cats and vets)

FCoV RT-PCR on Fine-Needle Aspirates

Ultrasound-guided aspirates from enlarged lymph nodes, liver, spleen or other affected tissues may be useful when no effusion is present.

A positive result from an abnormal organ combined with pyogranulomatous cytology strongly supports FIP. A negative result may reflect a small or poorly cellular sample and does not rule it out. (ABCD cats and vets)

Immunocytochemistry on Effusion or Aspirates

Finding FCoV antigen within macrophages from a compatible effusion or lesion is highly specific when a validated technique is used.

A positive result can provide very strong evidence for FIP. A negative result does not exclude it because infected macrophages may be sparse or unevenly distributed. (ABCD cats and vets)

Histopathology and Immunohistochemistry

Histopathology showing typical pyogranulomatous or vasculitic lesions together with FCoV antigen immunostaining is regarded as the confirmatory diagnostic standard.

Biopsy may not be safe or practical in a very sick cat, which is why less invasive effusion and aspirate testing is frequently favoured. (ABCD cats and vets)

Which Tests Do Not Diagnose FIP?

Feline Coronavirus Antibody Tests

A positive antibody result means the cat has previously encountered feline coronavirus.

It does not prove:

FIP
Active systemic disease
Current faecal shedding
A dangerous mutation
That treatment is required

There is no genuine “FIP antibody test”. A high titre may increase suspicion within an already convincing clinical picture, but antibody testing alone has little value for diagnosing FIP in an individual cat. (ABCD cats and vets)

Faecal Coronavirus PCR

Faecal PCR identifies cats shedding enteric feline coronavirus.

Healthy cats commonly shed FCoV, as do cats with unrelated diseases. Faecal PCR therefore cannot diagnose FIP. (ABCD cats and vets)

Blood PCR Alone

A high FCoV load in blood can support FIP, but some cats without FIP also test positive and some cats with FIP test negative.

Blood PCR should be interpreted with clinical findings rather than used as a stand-alone answer. (ABCD cats and vets)

Spike-Mutation Testing

Mutation testing has not proved sufficiently reliable to distinguish every FIP-associated virus from ordinary FCoV.

A mutation result should not override the cat’s clinical findings, viral load, cytology or histopathology. (ABCD cats and vets)

Can Response to Treatment Support the Diagnosis?

Yes, when FIP is already considered highly likely.

Most correctly treated cats show improvement in demeanour, appetite and fever within 48 hours to five days. A rapid, sustained response adds weight to a presumptive diagnosis.

Failure to improve on an appropriate dose should trigger reassessment of:

The diagnosis
The antiviral dose
Medication administration
Product quality
Gastrointestinal absorption
Ocular or neurological involvement
Concurrent disease

A treatment response should support proper diagnostic reasoning, not replace it entirely. (catvets.com)

Can FIP Be Cured?

FIP can now be placed into durable clinical remission in most treated cats.

The term “cure” is increasingly used because relapse is uncommon following a complete, successful antiviral course. Current guidance reports relapse rates likely below 5%, with relapse extremely unusual once a cat has remained well for one month after treatment ends. (catvets.com)

Long-term monitoring remains sensible because:

Relapse can occasionally occur
A relapse may look different from the original presentation
Residual neurological abnormalities may remain despite viral control
Cats can still acquire ordinary enteric FCoV again
Concurrent diseases may emerge later

What Is the Best Treatment for FIP?

Oral GS-441524

Quality-assured oral GS-441524 is the preferred first-line antiviral for most cats.

It can usually be started from the first day of treatment, including in cats with effusive, non-effusive, ocular or neurological FIP. Oral treatment avoids the pain and skin injury associated with repeated subcutaneous injections. (catvets.com)

Current Veterinary GS-441524 Reference Doses

FIP presentation	Current oral GS-441524 guidance
Effusive or non-effusive FIP without eye or neurological signs	15 mg/kg every 24 hours, or divided every 12 hours
Ocular FIP	20 mg/kg every 24 hours, or divided every 12 hours
Neurological FIP	10 mg/kg every 12 hours

These recommendations apply to quality-assured preparations with a known antiviral concentration. They should not be extrapolated directly to unregulated products with uncertain content or bioavailability. Treatment must be prescribed and monitored by the attending veterinarian.

The daily dose may be divided into twice-daily administration to improve drug exposure in selected cats. Once-daily treatment remains effective and may produce better compliance when medication is difficult.

Remdesivir

Remdesivir is a prodrug that is converted into GS-441524.

Intravenous remdesivir may be useful when a cat:

Is too unwell to swallow
Has severe neurological dysfunction
Is profoundly dehydrated
Has vomiting or poor gastrointestinal absorption
Cannot be medicated orally

Once the cat stabilises, treatment can transition directly to oral GS-441524. Subcutaneous remdesivir is often painful and is generally avoided when intravenous or oral options are practical. (catvets.com)

Molnupiravir

Molnupiravir can treat FIP, but GS-441524 is generally preferred when available.

Molnupiravir is more commonly reserved for:

Failure to respond to adequate GS-441524
Relapse following GS-441524
Suspected antiviral resistance
Regions where it is the only accessible antiviral

Concerns include neutropenia, anaemia at high doses, nausea, gastrointestinal effects, teratogenicity, mutagenicity and the possibility of encouraging viral mutations. Gloves should be used when handling it, and pregnant people should avoid direct contact. (catvets.com)

GC376 and Other Protease Inhibitors

GC376 and newer protease inhibitors remain areas of active investigation. They may have a future role in combination treatment, refractory disease or antiviral resistance, but they are not currently the preferred stand-alone first treatment where GS-441524 is available. (catvets.com)

Is GS-441524 Legally Available?

Availability and prescribing rules vary by country.

Quality-assured compounded or legally supplied antiviral preparations are now available in many regions, including Australia, Hong Kong, the United Kingdom, parts of Europe, Canada and the United States. (catvets.com)

In the United States, no GS-441524 product is FDA-approved for FIP. However, the FDA has stated that it does not intend to enforce animal-drug approval requirements when GS-441524 is compounded under specified conditions for an individual cat with FIP under a veterinary prescription. The medication therefore remains an unapproved compounded drug supplied under enforcement discretion rather than a fully FDA-approved veterinary product. (U.S. Food and Drug Administration)

A veterinary prescription is required for reputable compounded products. (Cornell Vet School)

How Long Does FIP Treatment Last?

Most historical studies used an 84-day, or 12-week, course.

A prospective randomised study showed that 42 days of oral GS-441524 could be as effective as 84 days in cats with effusive FIP. However, those cats were diagnosed quickly, hospitalised during the first week and responded very rapidly to intensive treatment. (PubMed Central (PMC))

Current guidance therefore supports a response-based duration:

A 42-Day Course May Be Considered When:

The diagnosis was made promptly
The cat responds rapidly
Fever, appetite and activity normalise
Effusions resolve
Blood abnormalities normalise
Acute-phase proteins normalise where measured
The cat has been clinically and biochemically normal for approximately two weeks before treatment stops
Follow-up and relapse monitoring are reliable

An 84-Day Course May Be More Appropriate When:

Response is slower
Neurological or ocular disease is present
Effusions persist
Blood abnormalities remain
Medication administration has been inconsistent
Product absorption or quality is uncertain
Significant concurrent disease is present
Close biochemical monitoring is unavailable

Most published survival data still come from cats treated for 84 days, so treatment duration should be decided with the veterinarian rather than shortened automatically.

What Improvement Should You Expect?

Most responding cats show:

Within 48 Hours

Brighter demeanour
Reduced fever
Improved interaction
Early appetite improvement

Within Two to Five Days

Clear improvement in appetite
Increased activity
Reduced lethargy
Early reduction in effusion
Improved comfort

Within One to Two Weeks

Major reduction or resolution of pleural or abdominal fluid
Weight gain or improved growth
Reduced bilirubin
Improving albumin-to-globulin ratio
Improving inflammation

Anaemia and lymphopenia may take considerably longer to resolve. Enlarged lymph nodes and mild hyperglobulinaemia can also persist after otherwise successful treatment and do not automatically indicate relapse. (catvets.com)

How Should Treatment Be Monitored?

Weekly at Home

Record:

Body weight
Appetite
Activity
Temperature if practical
Resting respiratory rate
Abdominal size
Eye changes
Neurological changes
Every antiviral dose

The medication dose must be recalculated as the cat gains weight. If weight initially falls because abdominal fluid resolves, the antiviral dose should not automatically be reduced. (catvets.com)

At One to Two Weeks

The veterinary recheck may include:

Full clinical examination
Weight
Ultrasound or point-of-care ultrasound
Complete blood count
Albumin and globulin
Bilirubin
Liver and kidney values
Acute-phase proteins where available

An albumin-to-globulin ratio improving above 0.6 after two to three weeks is encouraging. (catvets.com)

Before Stopping Treatment

Ideally, the cat should be clinically normal and have normal or appropriately resolved laboratory abnormalities for approximately two weeks before treatment stops.

Persistent mild lymph-node enlargement, stable neurological deficits or mild hyperglobulinaemia may remain despite successful viral control. The entire clinical picture matters more than one imperfect number. (catvets.com)

After Treatment

A recheck at approximately four weeks after stopping therapy is useful when it can be performed without excessive stress.

Contact the veterinarian promptly for:

Appetite loss
Weight loss
Recurrent fever
New effusion
New eye abnormalities
New neurological signs
Reduced activity

Relapse is uncommon and becomes extremely rare once the cat has remained well for a month after treatment. (catvets.com)

What Supportive Care May Be Needed?

Antivirals stop viral replication, but a critically ill cat may still need intensive support while treatment begins working.

Oxygen and Respiratory Support

Cats with chest effusion or severe lung disease may require:

Oxygen
Minimal handling
Careful monitoring
Thoracocentesis
Assisted ventilation in extreme cases

Pleural effusion causing breathing difficulty should be drained to allow the lungs to expand. (ABCD cats and vets)

Should Abdominal Fluid Be Drained?

Not routinely.

Abdominal effusion generally returns quickly until the antiviral controls the disease. Repeated drainage may remove protein and destabilise fluid and electrolyte balance.

Abdominal fluid should usually be sampled for diagnosis. Therapeutic drainage is generally reserved for a very large effusion that is interfering with breathing, and only enough fluid should be removed to relieve that pressure. (ABCD cats and vets)

Nutrition

FIP cats are often underweight and have lost muscle.

Support may include:

Energy-dense complete food
Antiemetics
Appetite stimulants
Assisted feeding
Temporary oesophagostomy feeding tube
Correction of dehydration and electrolyte abnormalities

Effective antiviral treatment commonly produces a rapid return of appetite, but nutrition cannot be neglected during the first critical days. (catvets.com)

Pain Relief

FIP may cause pain from:

Peritonitis
Pleuritis
Liver or kidney capsule distension
Uveitis
Meningeal inflammation
Invasive procedures

Appropriate feline analgesia should be part of the treatment plan. (catvets.com)

Eye Treatment

Cats with uveitis may require:

Topical corticosteroids
Pupil-dilating medication
Intraocular pressure monitoring
Ophthalmic rechecks

Persistent ocular inflammation despite treatment can indicate that the systemic antiviral dose is inadequate for eye penetration. (catvets.com)

Seizure Management

Neurological FIP may require antiseizure medication such as levetiracetam or phenobarbital while the antiviral takes effect. (catvets.com)

Blood Transfusion

Severe anaemia, including occasional FIP-associated immune-mediated haemolytic anaemia, may require blood transfusion alongside antiviral therapy. (catvets.com)

Are Corticosteroids Needed?

Generally, systemic corticosteroids are not routinely required when effective antiviral treatment is being given.

They may still be appropriate for specific complications, including:

Severe uveitis, usually treated topically
Severe neurological inflammation or suspected increased intracranial pressure
Confirmed secondary immune-mediated haemolytic anaemia
A concurrent inflammatory disease that independently requires them

A short, clinically justified corticosteroid course does not appear to prevent cats from responding to GS-441524, but steroids should not be used as a substitute for antiviral treatment. (catvets.com)

What if the Cat Is Not Improving?

A poor or incomplete response should trigger a structured review.

Check:

Was the original diagnosis correct?
Is the product quality assured?
Is every dose being administered?
Has the cat gained weight without a dose increase?
Is the formulation being absorbed?
Have ocular or neurological signs developed?
Is a higher or divided dose required?
Is another disease present?
Is antiviral resistance possible?

Most cats should show at least some positive change within two to five days. Failure to improve on an adequate dose should prompt investigation rather than simply continuing the same plan and hoping for a late miracle. (catvets.com)

What Is the Prognosis?

The prognosis has changed dramatically.

With prompt diagnosis, quality-assured antiviral therapy, correct dosing and appropriate supportive care, survival rates commonly exceed 90%. (catvets.com)

The outlook becomes more guarded when the cat presents with:

Severe respiratory distress
Profound neurological dysfunction
Repeated seizures
Hypoglycaemia
Hypotension
Severe anaemia
Advanced organ failure
Inability to receive medication
Delayed treatment
Significant concurrent disease

Even critically ill cats may recover, but the first several days are the highest-risk period. Antiviral treatment cannot reverse irreversible brain, eye or organ damage that occurred before therapy began.

Can Neurological Signs Remain After Recovery?

Yes.

Some cats retain stable ataxia, weakness or incontinence after the infection is controlled. Persistent but non-progressive neurological signs do not automatically mean treatment has failed or that FIP remains active. (catvets.com)

Can FIP Be Prevented?

FIP cannot be prevented completely because FCoV is common and the development of systemic disease depends on viral, genetic, immune and environmental factors.

The practical goal is to reduce infection pressure and stress.

Improve Litter Hygiene

In multi-cat homes:

Provide one litter tray per cat, plus one extra
Position trays in different rooms
Keep trays away from food and water
Remove faeces at least daily, preferably twice daily
Clean trays thoroughly each week
Prevent litter tracking where possible

FCoV is maintained by repeated cycles of faecal contamination and reinfection. (ABCD cats and vets)

Reduce Crowding

Keep groups small, stable and socially compatible.

Provide multiple:

Feeding stations
Water stations
Sleeping areas
Hiding places
Scratching posts
Litter trays
Elevated resting areas

Reducing social conflict and abrupt group changes is particularly important for kittens and young genetically related cats after a household FIP diagnosis. (catvets.com)

Avoid Unnecessary Coronavirus Testing

A positive serum antibody result does not identify a cat destined to develop FIP.

Repeated testing and separating cats based on a single antibody or faecal PCR result may create considerable stress without reliably preventing disease. Shedding changes over time, and there is no universally accepted one-test protocol for identifying permanent shedders. (ABCD cats and vets)

Do Not Treat Healthy Shedders With GS-441524

Using antivirals in healthy cats to eliminate enteric coronavirus is controversial and not recommended routinely.

Reinfection is common, clearance may not be durable and unnecessary antiviral use could encourage resistant viral populations. (ABCD cats and vets)

Is the FIP Vaccine Recommended?

An intranasal FIP vaccine exists in some countries, but its effectiveness is questionable.

It is intended for coronavirus-naïve cats aged at least 16 weeks. By that age, many cats in at-risk environments have already encountered FCoV. ABCD does not recommend routine use. (ABCD cats and vets)

Common FIP Mistakes

Assuming a Coronavirus-Positive Cat Has FIP

FCoV exposure and faecal shedding are common. Neither diagnoses systemic FIP.

Waiting for a Definitive Test While the Cat Deteriorates

Confirmatory testing is ideal, but a highly likely case may need antiviral treatment while results are pending.

Using an Unverified Product When a Quality-Assured Option Exists

Unknown drug content creates risks of underdosing, toxicity and treatment failure.

Failing To Increase the Dose as a Kitten Gains Weight

This is one of the most preventable causes of inadequate treatment.

Using the Same Dose for Neurological or Ocular Disease

Eye and central nervous system involvement generally require higher drug exposure.

Stopping Treatment Because the Cat Looks Better After a Week

Clinical improvement is expected quickly. It does not mean the prescribed treatment course is complete.

Draining Abdominal Fluid Repeatedly

Routine drainage can remove protein and disrupt circulation while offering only temporary cosmetic improvement.

Treating Healthy Housemates Prophylactically

There is no evidence that this prevents FIP and it may contribute to antiviral resistance.

Assuming Persistent Lymph Nodes or Mild Hyperglobulinaemia Mean Relapse

Some abnormalities remain after successful treatment and must be interpreted with the complete examination and laboratory trend.

Frequently Asked Questions

Is FIP curable now?

Yes. Most appropriately treated cats enter long-term remission, with success rates commonly above 90%. Relapse is uncommon after a complete and well-monitored antiviral course. (catvets.com)

How quickly should GS-441524 work?

Most cats show improved appetite, energy and temperature within 48 hours to five days. Effusions commonly reduce substantially within one to two weeks. (catvets.com)

How long does FIP treatment last?

Treatment commonly lasts between 42 and 84 days. Six weeks may be suitable for selected rapidly responding cats, particularly those with effusive FIP, while slower, ocular, neurological or complicated cases may need longer treatment.

Can my other cats catch FIP?

They can catch ordinary feline coronavirus, but direct transmission of FIP-associated virus is considered very uncommon. Most housemates have already shared exposure to enteric FCoV. (ABCD cats and vets)

Does a positive feline coronavirus test mean my cat will develop FIP?

No. Most FCoV-positive cats never develop FIP. Antibody or faecal PCR results identify exposure or shedding, not future disease. (ABCD cats and vets)

Final Thoughts

FIP remains a serious disease, but it is no longer a hopeless one.

The most important points are:

FIP develops in only a small proportion of cats infected with feline coronavirus.
Viral changes and the cat’s immune response both contribute.
FIP is a systemic blood-vessel and inflammatory disease, not simply abdominal fluid.
Effusive, non-effusive, ocular and neurological presentations overlap.
Enteric FCoV is contagious, but FIP itself is usually not directly transmitted.
A positive coronavirus antibody or faecal PCR test does not diagnose FIP.
Effusion is the most useful diagnostic sample when present.
FCoV antigen within macrophages or high viral loads in affected tissues strongly support the diagnosis.
Histopathology with antigen immunostaining remains the confirmatory standard.
Oral GS-441524 is the preferred first-line antiviral.
Ocular and neurological disease generally require greater drug exposure.
Treatment commonly lasts 42 to 84 days and should be guided by response.
Most cats improve noticeably within two to five days.
Weight must be checked weekly and doses adjusted as the cat grows.
Pleural fluid causing breathing difficulty should be drained.
Abdominal fluid should not be repeatedly drained unless it compromises breathing.
Survival commonly exceeds 90% with prompt, reliable treatment.
Relapse is uncommon after a complete successful course.
Healthy housemates should not receive preventive antiviral treatment.
Breathing difficulty, collapse, seizures or inability to stand is an emergency.

The mistake that now causes the greatest harm is continuing to think of FIP as automatically fatal.

It is still urgent. It is still diagnostically complicated. But with rapid investigation, quality-assured antivirals and close monitoring, most affected cats now have a realistic chance of recovery.

ASK A VET™ can help you organise effusion results, imaging, bloodwork trends, weekly weight changes and antiviral treatment records while you continue working directly with your cat’s veterinary team. A cat with open-mouth breathing, collapse, seizures, severe weakness or inability to swallow medication still requires immediate hands-on emergency veterinary care.

Know how they are, wherever they are

The ASK A VET™ Tracker logs live location and activity trends, so a change in routine never goes unnoticed.

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About the author

Dr Duncan Houston

Veterinarian · Founder of ASK A VET

Every ASK A VET article is written and reviewed by qualified veterinarians.

About Dr. Duncan →drduncanthevet