Feline Infectious Peritonitis (FIP) in Cats

Written by Shula Berg BVSc CertAVP(GSAS) GPAdvCert(SASTS) MRCVS
Clinically reviewed by Elizabeth McLennan-Green BVM&S CertAVP(SAM) MRCVS

Table of Contents

- Overview
- Symptoms
- Diagnosis
- Treatment
- Outlook

What is Feline Infectious Peritonitis (FIP)?

Feline infectious peritonitis is caused by a type of coronavirus (very different to the one which causes COVID-19 in people). Up to 90% of pet cats carry this virus however it causes no symptoms. In a small number of cats, the virus mutates and causes clinical disease, known as feline infectious peritonitis (FIP). FIP is not contagious and doesn’t affect any other species. It can affect cats of any age, gender or breed; however, it is seen much more commonly in cats less than 2 years old, those from multi-cat households, and pedigree cats. 

What are the symptoms of Feline Infectious Peritonitis? 

FIP can cause a wide variety of symptoms and can affect several different body systems. Common symptoms include weight loss, fever, lethargy and inappetence. Most other symptoms of FIP can be divided into two variations, though some cats will experience a combination of both presentations: 

In “wet” FIP, inflammation causes fluid to accumulate in the body cavities such as the chest and abdomen. Cats often have a distended abdomen and may be jaundiced (yellow colour of the eyes and gums). Fluid in the chest may cause difficulty breathing. 

In “dry” FIP, inflammation does not cause fluid build-up. The most common site of inflammation is the central nervous system, so symptoms include a lack of coordination (known as ataxia), sometimes with paralysis or seizures. Inflammation can also affect the eyes causing vision disturbances, change in eye colour or pupil size, and discomfort. 

What tests are used to diagnose Feline Infectious Peritonitis? 

Due to the many ways in which FIP can present, and the fact that so many cats carry coronavirus, making a diagnosis of FIP can be extremely challenging. 

The first step in diagnosis is a full blood screen. In cases of FIP this will typically show very high protein levels, known as globulins. A compound called bilirubin may also be elevated, and cats may be anaemic (have reduced red blood cells) with a high white blood cell count. Blood tests do not give a definite diagnosis but are important to rule out other conditions causing the symptoms. 

Ultrasound imaging is required to look for evidence of fluid in the chest and abdomen, as well as examining the organs for other conditions. If fluid is present in the chest, this will need to be drained to help the cat breathe. If fluid is present in either the abdomen or the chest, a sample may be taken for testing at an external lab. 

Cases with “dry” FIP are much harder to diagnose. Sometimes a biopsy can be taken from lesions in the abdomen, or large lymph nodes. Cats with neurological symptoms may need MRI imaging, or sampling of cerebrospinal fluid (CSF). Cats with primary ocular symptoms may require fluid to be removed from the eye for testing. Specialist referral may be recommended as these procedures can be technically challenging, and the equipment may not be available in first opinion practice. 

Ultimately, if all other conditions have been ruled out then the suspicion of FIP may be high enough to trial treatment without a concrete diagnosis. Sometimes treatment is started while awaiting results, or definitive diagnosis is considered impossible (for example, if specialist referral isn’t an option). 

How is Feline Infectious Peritonitis treated? 

Many cats with FIP will generally be very unwell by the time they come to the vets. They are often dehydrated and have not been eating. Fluid therapy should be started, and supportive care given while tests are performed. This may include pain relief, appetite stimulants, placing a feeding tube, and anti-sickness medication. 

There is only one antiviral drug that is licenced for treatment of FIP in cats in the UK. This is available as an injection, called remdesivir, or as a tablet, called GS-441524, and must be purchased from a specific company that formulates it into cat-friendly versions. These treatments are relatively new, although data shows them to be very safe. Treatment is ideally given for a minimum of 84 days/12 weeks to reduce the risk of relapse. 

Neither remdesivir or GS-441524 formulations are available in many countries, including the USA, and as a result many black-market versions have become available on the internet. These are not regulated, and much like street drugs can contain a variety of substances as well or instead of the active compound. They are also illegal in the UK, and anyone caught purchasing or administering them is at risk of prosecution. Although it may be tempting to try and avoid the large costs associated with using the legal product, we strongly advise against this, as it puts both yourself and your cat at considerable risk. 

What is the outlook for cats with Feline Infectious Peritonitis? 

Historically, FIP was considered a virtually incurable disease and almost all affected cats were put to sleep at the time of diagnosis. Now that anti-viral treatments are available, this has changed. It is relatively early days in terms of evidence, however the studies available report that up to 80% of cats can make a complete recovery from FIP with anti-viral treatment. The remaining 20% may not respond or may respond but then relapse. 

Treatment of FIP is more possible than ever before, but with all treatment decisions, it is important to consider the time and cost implications. Anti-viral treatment has significant associated costs, often totalling thousands of pounds, and regular checks are required for several months including repeat blood tests and scans to monitor response. In cases where anti-viral treatment is not possible or appropriate, sadly euthanasia is recommended as the condition is otherwise incurable and patients are often very unwell. If FIP is suspected (or diagnosed) in your cat, we appreciate this can be a challenging time. Your vet will be happy to discuss the pros, cons and implications for you and your cat, for any diagnostic and treatment decisions you need to make. 


Please note that the content made available on this webpage is for general information purposes only. Whilst we try to ensure that at the time of writing all material is up to date and reflects industry standards, we make no representation, warranties or guarantees that the information made available is up to date, accurate or complete. Any reliance placed by yourselves is done so at your own risk.

Page last reviewed: 16th January 2024
Next review due: 16th January 2026