FIP in cats

FIP, or feline infectious peritonitis (from the English Feline Infectious Peritonitis), is one of the most serious and unpredictable diseases that can affect our cat. For many owners, simply hearing this abbreviation at the vet's practice brings enormous anxiety, and not without reason, because only a few years ago a diagnosis of FIP effectively meant a death sentence. Fortunately, the picture of this disease has changed dramatically in recent years. Thanks to modern antiviral therapies, more and more cats with FIP now have a real chance of a full return to health, provided the disease is caught early and treatment is carried out under continuous veterinary supervision. In this article we explain, step by step, what FIP is, where it comes from, how to recognise its symptoms, what today's diagnosis and treatment look like, and what we can do to reduce the risk of our cat becoming ill.

What is FIP, and where does this disease come from?

The abbreviation FIP comes from the English name Feline Infectious Peritonitis. This name refers to one of the most characteristic forms of the disease, in which fluid accumulates in the abdominal cavity. In reality, though, FIP is something far broader than a problem of the peritoneum alone; the disease can affect multiple organs, involving the kidneys, liver, lymph nodes, eyes, and even the central nervous system.

FIP originates from the feline coronavirus, FCoV for short, a virus that is widespread in the domestic cat population worldwide. It is estimated that around 25–40% of pet cats living alone or in small groups are FCoV carriers, while in environments with a higher density of animals, such as shelters, catteries or groups of stray cats, the carrier rate can reach as high as 80–90%. This is an important point: a cat simply coming into contact with the coronavirus is a very common occurrence in the population, and in the vast majority of cases it is completely harmless.

In most infected cats, FCoV is asymptomatic or causes only mild, transient gastrointestinal symptoms, such as slight diarrhoea or temporarily loose stools, particularly in kittens. Problems arise only when the virus mutates within a particular cat's body; FCoV then changes into a considerably more aggressive form, commonly referred to as the FIP virus, capable of multiplying within cells of the immune system, namely monocytes and macrophages, and of triggering generalised inflammation throughout the body.

How does FCoV infection occur, and why does FIP develop in some cats?

FCoV spreads mainly by the faecal-oral route; a cat becomes infected through contact with other cats' faeces, for example by sharing a litter tray, but also indirectly, through contaminated bowls, toys, bedding or an owner's hands. The virus is relatively resistant in the environment, which is why it circulates extremely easily in homes and shelters with multiple cats and shared litter trays.

Whether a mutation of the virus and the development of FIP occurs in a given cat depends on a combination of many factors, including:

• the individual immune response – in cats with strong cell-mediated immunity, the body usually controls viral replication effectively;
• age – the youngest cats, whose immune systems are still maturing, are statistically the most vulnerable;
• stress levels – moving house, adoption, a stay in a shelter, introducing a new animal into the home, or surgery can all weaken immunity and favour the development of the disease;
• the density of the cat population – the more cats living together, the higher the viral load in the environment and the greater the risk of repeated infection;
• genetic factors – FIP is recorded more often in some breeding lines, which suggests a role for genetic predisposition, although environmental factors and individual immunity still play the decisive role.

Importantly, FIP itself, meaning the already-mutated form of the virus, is in most cases not transmitted effectively between cats; rather, it is the original, unmutated FCoV that circulates in the population, while the mutation into a dangerous form occurs individually, within the body of a particular animal.

Clinical forms of FIP

The symptoms of FIP can be highly varied and are often non-specific, which makes the disease considerably harder to recognise at an early stage. Two main forms are classically distinguished, wet and dry, although in clinical practice the boundary between them can be blurred, and some cats show mixed signs.

Wet form (effusive)

The wet form is characterised by inflammatory fluid accumulating in body cavities:

• in the abdominal cavity, causing gradual, painless abdominal swelling;
• in the pleural cavity, leading to worsening breathing difficulties;
• less often in the pericardial sac, which can affect heart function.

Symptoms typically accompanying the wet form include:

• progressive weight loss, despite a normal or even increased appetite in the early stages,
• lethargy, listlessness and a marked drop in activity,
• gradual loss of appetite,
• breathlessness or rapid breathing when fluid is present in the chest,
• a fever that does not resolve despite standard anti-inflammatory treatment or antibiotics,
• a dull, unkempt coat and an overall decline in condition.

Dry form (non-effusive)

The dry form is harder to recognise, since it lacks the characteristic fluid buildup. Instead, granulomatous lesions develop in internal organs. In this form you may observe:

• a chronic, fluctuating fever resistant to treatment,
• enlarged mesenteric lymph nodes, sometimes palpable on abdominal examination,
• inflammatory changes in the liver, kidneys, pancreas or intestines,
• gradual weight loss and lethargy,
• a declining appetite.

Ocular and neurological forms

In some cats, regardless of whether the wet or dry form predominates, the eyes or the central nervous system become involved:

• ocular form – may present as inflammation of the uvea, clouding or a change in iris colour, unequal pupils, or visible inflammation in the front chamber of the eye;
• neurological form – may cause symptoms such as balance and coordination problems (ataxia), seizures, tremors, behavioural changes, increased sensitivity to touch along the spine, and, in advanced cases, paralysis.

The neurological and ocular forms can be particularly difficult to treat, because not every antiviral drug penetrates the blood-brain barrier and the blood-eye barrier equally well, something the treating vet has to take into account when choosing therapy and dosing.

Who is most at risk of FIP?

Age plays an important role in susceptibility to FIP; the disease most often develops in young cats between roughly 3 months and 2 years old, linked to the immaturity of their immune system at that stage of life. FIP can also appear in older cats, especially those weakened by other conditions or chronic stress, though this is considerably rarer.

Additional risk factors include:

• living in a large group of cats – shelters, catteries, groups of stray cats fed together;
• recent significant stress – adoption, moving house, surgery, introducing a new animal into the home;
• weakened immunity – concurrent chronic illness, malnutrition, infection with feline leukaemia virus (FeLV) or feline immunodeficiency virus (FIV);
• coming from a cattery – some breeding lines show a higher rate of disease, which may suggest a genetic predisposition, though this is not an absolute rule, and environmental conditions still matter most.

FIP in kittens, and the breeder's role

Kittens coming from catteries or foster homes where FIP cases have previously been recorded need particular attention. A responsible breeder should, among other things:

• limit the number of cats in a single room and provide an adequate number of litter trays and feeding and resting areas,
• minimise the stress associated with weaning, transport or a change of environment for young kittens,
• if FIP cases recur within a litter, consult a vet specialising in feline medicine about further breeding management,
• give new owners honest information about the health history of the litter and the breeding line.

For a future kitten owner, it is good practice to ask the breeder about the kittens' living conditions and whether FIP cases have occurred in the cattery before. Such information does not guarantee anything for certain about a particular kitten's future, but it does help you assess the situation more fully and know what to watch for especially closely in the animal's first few months.

FIP versus other feline diseases – why diagnosis can be difficult

One reason FIP has a reputation as a treacherous disease is that its symptoms can mimic many other, quite different conditions. Before a vet can diagnose FIP, they usually have to rule out, or weigh up in the differential diagnosis, among others:

• lymphoma and other cancers, which can likewise cause enlarged lymph nodes, abdominal fluid or weight loss;
• heart failure, leading to fluid accumulating in the chest or abdomen for entirely different reasons;
• chronic liver and bile duct disease, such as cholangitis;
• other bacterial or parasitic infections presenting with fever and general weakness;
• chronic inflammatory bowel disease, which can produce a similar picture of chronic weight loss and reduced appetite.

This is exactly why reliable FIP diagnosis takes time, patience, and usually several different tests that complement one another; a single, quick result rarely provides certainty, and a hasty diagnosis can lead to the wrong treatment being started, or to needless stress from a mistaken diagnosis.

What does FIP diagnosis involve?

FIP has long been considered one of the most diagnostically challenging feline diseases, because there is no single test that definitively confirms or rules out the disease in a living animal. A vet reaches a diagnosis by combining several elements:

1. History and clinical picture – the cat's age, how quickly symptoms are progressing, living conditions such as coming from a shelter or cattery, and the presence of a fever that does not respond to standard treatment.
2. Blood tests – FIP is often associated with an abnormal albumin-to-globulin ratio, elevated acute-phase proteins, lymphopenia, meaning a reduced lymphocyte count, and an elevated neutrophil count.
3. Testing fluid from body cavities – in the wet form, any fluid collected is assessed macroscopically (typically thick, slightly yellowish and viscous) and with tests such as the Rivalta test, along with cytology and protein concentration measurement.
4. Imaging – abdominal ultrasound and chest X-rays help assess the presence of fluid, enlarged lymph nodes, or organ changes.
5. PCR testing – detecting the coronavirus's genetic material in fluid, blood or tissue; a positive result confirms the presence of FCoV, but on its own does not always conclusively confirm FIP, so it is always interpreted together with the clinical picture.
6. Histopathology and immunohistochemistry – regarded as the most reliable confirmation of FIP, performed on tissue samples taken via biopsy or post-mortem.

Modern FIP treatment – a breakthrough thanks to GS-441524

Until recently, FIP was almost universally regarded as an incurable disease, and a diagnosis effectively meant palliative care. That changed with the emergence of antiviral drugs from the nucleoside analogue group, above all GS-441524, a substance that inhibits replication of the FIP virus in the cat's body. Clinical studies, together with a growing body of practical experience, show that with early diagnosis and consistently applied treatment, many cats achieve full clinical remission.

It is worth understanding, in broad terms, what the treatment process looks like:

1. Confirming the diagnosis – starting an expensive, long antiviral therapy should always be preceded by the most reliable possible confirmation of FIP, based on a combination of the tests described above.
2. Choosing the drug and route of administration – GS-441524 is given as daily subcutaneous injections or, increasingly, as oral tablets. The choice of form and dosing is always up to the treating vet, and depends on the cat's body weight, the form of the disease, and its location, for example whether the eyes or the nervous system are affected.
3. Regular monitoring – treatment requires periodic check-up visits with blood tests, weight measurement and clinical assessment, to continuously evaluate the effectiveness of therapy and adjust the dose if needed.
4. An appropriately long treatment course – the course was classically described as lasting around 12 weeks, though more recent studies and clinical experience suggest that, in some cats with a good response to treatment confirmed by follow-up tests, somewhat shorter treatment schedules are also possible. The decision on treatment length, and any extension of it, always rests with the treating vet, made individually for each patient.
5. Observation after treatment ends – even once therapy is complete and improvement has been achieved, a period of observation is recommended, since some cats can experience a relapse of symptoms, which sometimes requires resuming or extending treatment.

Alongside GS-441524, other antiviral molecules also appear in the literature and in clinical practice, sometimes used as supportive or alternative treatment under specialist supervision. The decision on which specific treatment regimen to use should always belong to the vet, ideally one experienced in managing FIP therapy.

Availability of treatment in Poland

GS-441524 is still not formally registered as a veterinary medicine in Poland or in most European Union countries. This does not mean, however, that treatment is unavailable; cat owners can access it through the legal import of preparations under the so-called veterinary cascade, and through practices and specialists experienced in managing FIP therapy. The cost of a full course of treatment can be a significant burden on a household budget, and depends, among other things, on the cat's body weight, the form of the disease, and the length of treatment; it is best to discuss an accurate, current quote and the options available directly with the treating vet.

Supportive treatment and caring for a cat during therapy

Antiviral treatment is not everything; supportive care, which helps the cat's body return to balance, is equally important. This can include:

• fluid therapy – in more severe cases, intravenous or subcutaneous fluids may be necessary;
• nutritional support – cats with FIP often lose their appetite, so it is sometimes necessary to use energy-dense foods, appetite stimulants, or, in extreme cases, temporary feeding via a tube, always under veterinary supervision;
• pain relief and anti-inflammatory treatment – chosen individually, depending on the severity of symptoms;
• antibiotic treatment – used where there is a risk of, or confirmed, secondary bacterial infection;
• home care – providing the cat with a warm, calm place to rest, reducing stress, monitoring appetite, weight, temperature and behaviour, and keeping a simple symptom diary, which makes it much easier for the vet to assess treatment progress.

Prognosis – can FIP be cured?

For many years, FIP was synonymous with a death sentence; without antiviral treatment, the vast majority of cats died within days or weeks of the first clear symptoms appearing. The emergence of effective antiviral therapies has dramatically changed that picture: more and more cats in whom the disease was recognised sufficiently early, and who consistently completed a full course of treatment under veterinary care, return to good condition and a normal, active life.

The prognosis, however, depends on many factors and cannot be predicted in advance for every patient:

• the time elapsed between the first symptoms and the start of treatment – the earlier treatment begins, the better the results usually are;
• the form of the disease – the neurological and ocular forms can be harder to treat because of the limited penetration of some drugs into the brain and the eye;
• the cat's overall state of health – the presence of other conditions or infections, such as FeLV or FIV, can make recovery harder;
• consistency in carrying out treatment – giving the drug regularly and not missing check-up visits has a direct effect on how well the treatment works;
• the possibility of relapse – some cats experience a return of symptoms after finishing therapy, which calls for continued vigilance even after treatment formally ends.

No responsible vet will guarantee a hundred percent cure in every case; FIP remains a serious disease, and its course can be unpredictable. What can be said with confidence, though, is that the prognosis has improved enormously, and genuinely, compared with the situation just a few years ago.

Life after treatment

Finishing a course of antiviral therapy is an important moment, but it does not automatically mean the end of caring for your cat. It is worth planning the next stages with the treating vet:

• Follow-up visits after treatment ends – check-ups at set intervals after the last dose are usually recommended, to make sure the disease is not returning.
• Monitoring weight and appetite – these are among the simplest, yet most sensitive, indicators of a cat's wellbeing at home.
• A gradual return to normal activity – most cats return to full fitness and a normal lifestyle after successful treatment, though it is worth watching how the animal tolerates exercise and play.
• Continuing standard preventive care – after finishing FIP treatment, a cat should still be regularly treated for worms and vaccinated against other diseases as recommended by the vet.
• Vigilance without excessive worry – it is easy to fall into the extreme of constantly worrying about a relapse. It is worth finding a balance: watch your cat closely, but also let yourself, and your cat, enjoy the return to normal life.

How to prevent FCoV infection and the development of FIP

Completely eliminating the risk of contact with the feline coronavirus is not realistically possible, especially if more than one cat lives in the home. It is possible, however, to significantly reduce the risk of infection and lower the viral load in the animal's environment:

1. Litter tray hygiene – litter trays should be cleaned and disinfected regularly, and in homes with several cats, the rule of thumb is: the number of litter trays should equal the number of cats plus one, placed in different locations.
2. Reducing cat density – the fewer cats sharing the same litter trays, bowls and spaces, the less the virus circulates in the environment.
3. Quarantining and testing new cats – a newly adopted animal is worth isolating from the rest of the household for a while, and, if in doubt, discussing with your vet whether screening tests are warranted.
4. Separating eating, drinking and toileting areas – bowls and litter trays should not be placed next to each other.
5. Good breeding practice – breeders who have recorded FIP cases in a genetic line or a particular litter should consider consulting a vet specialising in feline medicine about further breeding decisions.
6. Reducing stress – since stress favours viral mutation and disease development, it is worth providing a stable, predictable environment, especially during periods of higher risk, such as adoption, moving house, or introducing a new animal.

A limited number of vaccines against FCoV are currently available, but their effectiveness is debated within the veterinary community, and they are not widely recommended as a primary means of preventing FIP. The methods listed above, limiting exposure to the virus and reducing stress, remain considerably more effective.

How to support your cat's immunity day to day

A strong immune system does not guarantee that a cat will never become ill, but it genuinely increases its chances of successfully keeping an FCoV infection under control before the virus mutates. A few practical tips:

1. A balanced diet – high-quality food suited to the cat's age and state of health, rich in complete protein. It is best to talk to your vet about whether to add supplements such as omega-3 fatty acids to the diet.
2. Regular veterinary check-ups – annual check-ups, and more frequent ones for older cats, allow health problems that could weaken immunity to be caught early.
3. Stress reduction – a stable environment, access to hiding places and elevated spots, avoiding sudden changes to surroundings, and giving your cat plenty of play and activity.
4. Protective vaccination – although there is currently no fully effective vaccine against FCoV itself, regular vaccination against other infectious diseases, such as panleukopenia, feline herpesvirus or calicivirus, remains a key part of a cat's overall health prevention.
5. Maintaining a healthy body weight – both being underweight and being overweight can adversely affect how well the immune system functions.
6. A well-managed multi-cat environment – if several cats live in the home, it is worth making sure there are enough resources, such as litter trays, bowls, scratching posts and resting places, so the animals do not have to compete for them. Competition for resources, and the tension between cats that comes with it, is a common, if easily overlooked, source of chronic stress in multi-cat households.

Frequently asked questions

Is FIP contagious to other cats in the home?

The FIP virus itself, meaning the already-mutated form, is in most cases not transmitted effectively between cats. What is contagious is the original, unmutated FCoV coronavirus, which circulates in the cat population and, in the vast majority of cases, never mutates into a dangerous form. Even so, if one cat in the household is ill, it is worth asking your vet whether it makes sense to monitor or test the other cats.

Can a person catch FIP from a cat?

No. FCoV and FIP are diseases specific to cats and are not transmitted to people or to dogs.

Is there a vaccine that protects against FIP?

A limited number of products are available, but their effectiveness is considered insufficient within the veterinary community, so they are not currently a standard part of prevention. The most effective protection remains limiting exposure to the virus and reducing stress.

How much does FIP treatment cost?

The cost depends on the cat's body weight, the form of the disease, the length of therapy, and where the treatment is carried out, so it is not possible to give a single, universal figure. It is a real, significant expense, and one worth discussing honestly with the treating vet right at the start of the diagnostic and treatment process.

Can a young, apparently healthy cat be an FCoV carrier?

Yes, and this is very common; many cats carry FCoV without symptoms, or with only very mild, transient gastrointestinal signs, without ever developing FIP.

How long does GS-441524 treatment last?

It was classically described as lasting around 12 weeks, though more recent clinical experience indicates that other timeframes are possible for some cats, determined individually by the treating vet based on the response to treatment and the results of follow-up tests.

Can a cat that has been treated develop FIP again?

In some cats, symptoms can return after therapy ends, which is why a period of observation and follow-up visits are recommended even after treatment is formally complete. If anything seems concerning, it is best to contact your vet as quickly as possible.

FIP remains a serious, demanding disease, but thanks to advances in veterinary medicine it is no longer an automatic death sentence for a cat. The key to success is watching your pet closely, reacting quickly to worrying symptoms, and carrying out diagnosis and treatment always under the care of an experienced vet. If you notice worrying symptoms in your cat, or have questions about preventing or treating FIP, please get in touch with our practice. Book a visit — call +48 22 823 35 63.