Feline infectious peritonitis: now a treatable disease

 

 

 

 

 

By Samantha Taylor BVetMed(Hons) CertSAM DipECVIM-CA FRCVS
EBVS® European Veterinary Specialist in Small Animal Internal Medicine
RCVS Recognised Specialist in Feline Medicine
Associate of the American College of Veterinary Internal Medicine
Veterinary Specialist Lead at The International Society of Feline Medicine (ISFM)
Honorary Lecturer in Small Animal Medicine at the University of Surrey

Introduction

Feline infectious peritonitis (FIP) is a serious disease of cats caused by feline coronavirus. In the past this disease was almost always fatal, but since 2020 when antiviral drugs became legally available, the majority of cats can be cured. 

What causes FIP? 

Coronaviruses are common viruses, and we are all very familiar with COVID-19, however, the coronavirus causing FIP only effects cats. Coronavirus is found in the faeces of many cats and most cats will encounter the virus in their life and not develop any illness. However, a small proportion of cats will go on to develop FIP due to changes (mutations of the virus) inside the cat. 

 It is important to remember that a positive for exposure to coronavirus doesn’t mean a cat has FIP, and more detailed tests will be needed for a diagnosis. In affected cats, the changed virus travels around the cat’s body and interacts with the immune system resulting in inflammation. This inflammation can cause leakage of fluid from blood vessels, and/or formation of collections of virus and immune cells called granulomas. 

Which cats are more likely to get FIP?

FIP more often affects young cats (under 2 years) but can be diagnosed in cats of any age. Pedigree breeds are more likely to develop FIP, particularly British Shorthairs for example, but non-pedigree cats can also develop the disease. It is not uncommon for cats with FIP to have a history of a stressful event, such as neutering or rehoming for example. 

Figure 1: FIP is more common in Pedigree cats and young cats

Signs a cat may have FIP

The signs a cat shows will depend on the part of the body affected and how the cat’s immune system reacts to the infection with coronavirus. Many signs such as eating less, lethargy/tiredness and a high temperature are not only seen with FIP, but also with other diseases. Importantly, signs tend to worsen over time and can change with new signs developing. Some signs that can suggest FIP include:

  • Neurological signs: a wobbly or unsteady gait, behaviour change, fits (seizures);
  • Swollen belly (abdomen) due to fluid build up (figure 2)
  • Breathing difficulties due to fluid in the chest;
  • Bleeding or cloudiness in the eyes (one or both) (figure 3) or change in iris (pupil) colouration;
  • Vomiting and/or diarrhoea;
  • Weight loss;
  • Poor growth in kittens;
  • A yellow-tinge to the skin and gums may be noted (jaundice);
  • Enlarged kidneys or mass in the abdomen.

Often FIP is described as ‘wet/effusive’ or ‘dry/non-effusive’ but most cats have a mix of both types of disease. 

Figure 2: Cats with effusive or wet FIP may have fluid in the abdomen making their belly look distended 

Figure 3: This cat with FIP has changes in both eyes with cloudiness and bleeding

How does a vet diagnose FIP?

A vet will not be able to make a diagnosis without conducting tests, and it can be a difficult disease to confirm as the cause of a cat’s illness. Other conditions can cause very similar signs and must be ruled out. Tests that are useful for diagnosing the disease are described here, but not all cats with FIP will have all changes: 

  • Blood tests: high protein levels, low red blood cells (anaemia), high white blood cells, higher inflammatory markers
  • Imaging studies (xrays/ultrasound): may show collections of fluid or changes in the organs
  • Sampling and analysis of any fluid (figure 4): fluid from cats with FIP has particular characteristics making it very useful making a diagnosis
  • Needle samples from any abnormalities
  • Advanced scans
  • Testing for the virus: it is possible to detect the virus in fluid or tissue samples

Figure 4: The fluid found in the chest or abdomen in cats with FIP is typically yellow and sticky

Testing for antibodies (exposure) to coronavirus (serology test) is not helpful for the diagnosis of FIP as many cats have been exposed but do not have FIP. Additionally, faecal (poo) testing is not helpful as many cats can encounter the virus and pass in their faeces without developing FIP.

On some occasions it is not possible to fully confirm the diagnosis but if other conditions have been excluded response to treatment may be used as evidence of the disease. However, it is important to monitor response closely as cats with FIP are expected to respond with improvement in 3-5 days (if not before). 

Treatment of FIP

Prior to 2020 there was no legal effective treatment for FIP. Thankfully antiviral drugs have become available, and are much more affordable since then. In many countries vets can easily and legally prescribe these drugs. Antivirals are generally safe and very effective at treating FIP. Available drugs may vary between countries due to drug licensing rules but include:

  • GS-441524: available as tablets and a suspension (liquid) and can be used from day 1 of treatment
  • Remdesivir: injectable medication available in some countries and given in hospital to cats unable to take tablets/liquids orally
  • Molnupiravir/EIDD-1931: tablets generally used as a second choice of treatment in countries where it is available for example if cats relapse or don’t respond to GS-441524

Most cats are treated for 12 weeks to ensure the disease has completely resolved. However, some cats can be successfully treated in 6 weeks. This should be decided by the veterinarian based on the type of FIP (effusive) and is only suitable for cats that respond rapidly and completely within 2 weeks for example. If there is any doubt over the response, or the eyes/brain are involved in the disease, a longer course is currently recommended.

Note that some medications to treat FIP are sold online but it is not legal to source such drugs without a veterinary prescription and studies have shown the illegal drugs have variable content of antivirals and can cause significant pain and injury if injected. Always seek veterinary advice. 

Practicalities of treating cats with FIP

Giving tablets or liquids to cats can be difficult, especially for long courses, but giving medication successfully is vital for cats with FIP. Tips include:

  • Put the medication in a treat (liquid paste or putty-type treats for example) or a small amount of favourite strongly flavoured food (Figure 5)

Figure 5: GS-441524 suspension given on a small amount of tasty food

  • Remove food overnight to ensure a cat is hungry in the morning and may accept medication (check suitable for kittens or cats in very poor body condition)
  • Associate medicating with positive experiences such as grooming, stroking or playing (whatever preferred by the cat) and give a second treat without medication
  • If a cat will not take medication in food, the veterinary team can demonstrate how to put tablets directly into the mouth

Any caregiver having trouble giving FIP medication must contact their vet as it is vital doses are not missed. 

Side effects of treatment

Antiviral medications are generally well tolerated but any changes in a cat’s health or behaviour should be discussed with the veterinarian. Remdesivir injections can be painful and hence are now generally only given in hospital to very sick FIP patients.  Other side effects may include changes in blood test results (mild elevations in liver enzymes and changes in white blood cells) that generally have no effect on a cat’s health but should be monitored. Side effects should be discussed with the veterinarian supervising the FIP treatment.

Other supportive treatments

Various other treatments may help cats with FIP during their antiviral treatment. Very unwell cats may need to be hospitalised for management of dehydration and lack of nutrition and to start their antiviral treatment. Many cats with FIP will benefit from painkillers and anti-nausea drugs at diagnosis and cats with changes in their brain or eyes may need additional medications for example to control seizures. It is very important cats with FIP eat well during treatment as often they have lost weight, so occasionally feeding tubes and appetite stimulants are required. Fluid is generally only drained if in the chest and affecting breathing, as with treatment the fluid will be reabsorbed. 

Monitoring cats on treatment of FIP

Cats with FIP treated with antiviral medications should improve rapidly, within 3-5 days appetite should increase, and tiredness should be resolving. High temperatures tend to resolve quickly (within 48 hours) and neurological and ocular signs start to improve in the first week of therapy, but may take longer to completely resolve.  Importantly, caregivers should monitor their cats at home for appetite, and energy levels and weighing cats with scales for babies weekly can provide very useful information (Figure 6).

Figure 6: Recording body weight at home can help monitoring the success of FIP treatment

How successful is treatment for FIP?

Research has shown that about 85% of cats will respond to antiviral treatment. Many of these cats appear to be cured and long-term follow-up over 4 years has shown they remain healthy in most cases. Some cats with FIP are so unwell that even with intensive treatment they will sadly not recover. If a cat is not responding as expected, the diagnosis should be reviewed by the veterinarian and the medication may need to be adjusted.

It is early days in the experience of treating FIP. Further research will help improve understanding, but it is hoped that treated cats will be cured. Some cats may ‘relapse’ and become unwell with FIP again during or after treatment so all treated cats should see their veterinarian if they show signs of illness again. Cats suffering relapse may be treated with repeat courses of antivirals with adjustment of dosage or choice of antiviral.

Vaccination, parasite prevention and neutering (desexing) cats with FIP

Cats treated for, or recovered from FIP, are often young and may not have had full vaccination courses. It is important they are protected from other infectious diseases and should be vaccinated after completing treatment as recommended by the veterinary team. Neutering may reduce stress and this is should be performed towards the end of treatment, or at least a month after completion of therapy. Choosing a veterinary clinic that is a Cat Friendly Clinic or Cat Friendly Practice can help to reduce the stress of veterinary procedures. 

Conclusion

FIP was once a disease that killed young cats, and now the majority can be successfully treated which is a great success story. We are learning more and more about treatment as research forges ahead and there is room for even more optimism for cats with FIP. 

June 2025

Feline Ailments

"I believe cats to be spirits come to earth.  A cat, I am sure, could walk on a cloud without coming through"

Jules Verne