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Understanding Feline Viruses – Feline Infectious Peritonitis (FIP)

Understanding Feline Viruses – Feline Infectious Peritonitis (FIP)
by Dr Caity Venniker

Read time: 5 min

Previously we looked at two feline viruses which are encountered fairly commonly in veterinary practice, FIV and FELV. Today I want to focus on another virus, which is seen fortunately a little less frequently but also has very serious implications, Feline Infectious Peritonitis (FIP).

What is FIP?

Feline Infectious Peritonitis (FIP) is one of the most severe diseases caused by feline viruses encountered in veterinary practice. It can be difficult both to diagnose and initially to understand, as it follows an unusual path of development.

This year, “coronavirus” has been a word that has dominated global news; with COVID-19 spreading rapidly throughout the world. The disease FIP also originates from a coronavirus, but one that is specific to cats (so it cannot spread to humans) and also, most importantly, has undergone mutation within the individual cat.

Feline Coronavirus (FCoV) is common in cats and is found worldwide. Most infected cats show either no obvious signs of infection or mild respiratory or gastrointestinal symptoms. The development of FIP occurs in approximately 10% of cats infected with Feline Coronavirus(1). This is thought to happen when the individual cat has an inappropriate immune response to infection, combined with mutation of the virus. The resultant disease from the mutated virus is very different to the original infection and is much more dangerous.

Why and When Does FIP Occur?

FIP can occur in any cat infected with Feline Coronavirus; but there are certain predisposing factors. Cats less than three or older than ten years of age are most commonly infected, with approximately seventy percent of cases occurring in cats less than a year old(2). It is one of the leading causes of death in kittens and young cats(3).

It is estimated that FIP affects in the region of one percent of cats(7,3), although its incidence varies greatly between different environments, from zero to ten percent (3). The prevalence of disease is greater in environments of high cat density such as shelters and breeding facilities, especially where litter trays are shared. Feline Coronavirus is passed mainly via the faeces and saliva(4) through ingestion or inhalation(6). Purebred cats and male cats are more susceptible to the development of FIP, although the reasons for this are unknown(4).

Stress at the time of exposure to Feline Coronavirus appears to play a role, whereby stressed cats are at greater risk of having an abnormal immune response to infection, which in the face of viral mutation can lead to the subsequent development of FIP. Stressors can include weaning; rehoming; elective surgeries such as neutering; overcrowding; and concurrent infections(3); amongst others.

It is important to note that while Feline Coronavirus can spread between cats, FIP itself is not contagious, but develops via viral mutation in the individual cat.

What Does FIP Infection Look Like?

Cats infected with Feline Coronavirus usually have either mild or no obvious symptoms. The unfortunate development into FIP can occur weeks, months or even years after exposure(4) and the signs are initially varied and non-specific, such as weight loss, lethargy, fever or decreased appetite(4). Over time, the disease generally progresses into one of two major forms, referred to as the wet form and the dry form. These forms of disease are not mutually exclusive, and a combination of symptoms can occur.

The wet form usually progresses quite rapidly; and involves the accumulation of fluid into body cavities. This occurs commonly in the abdomen which can cause the appearance of a distended belly, as well as in the chest cavity, which can lead to difficulty breathing; but is not limited to these areas. This form occurs when the virus causes inflammation of blood vessels which then leak fluid. The dry form of FIP generally develops more slowly(4) and commonly involves neurological signs such as seizures and lack of coordination; or abnormalities of the eye; among others. Signs of this form occur from inflammatory cells accumulating in various organs.

How Do You Diagnose FIP?

Unfortunately, the diagnosis of FIP can be challenging, as there is no definitive method of testing. Different tests are useful, but each test has certain limitations which means that in certain cases a conclusive diagnosis cannot be obtained. Sometimes the diagnosis has to be made on a combination of tests, characteristic symptoms and lack of response to treatment.

How Do You Prevent FIP?

There is a vaccine for FIP available in the USA but its effectiveness is questionable and it is not routinely recommended(4). In environments of high cat density care must be taken to provide enough litter trays that are cleaned and disinfected on a regular basis. Managing stress levels, especially in young cats, may also be helpful.

How Do You Treat FIP?

Up until recently FIP was regarded as an untreatable disease, and is still largely accepted as such. At this stage there is no definitive, proven cure for FIP. However, there is a glimmer of hope on the horizon. An antiviral compound known as GC376 showed promising results in clinical trials(3) and is in the process of getting approval from the USA Food and Drug Administration (FDA) as a treatment option for FIP(5). Unfortunately, this process takes years so even if successful, being able to purchase a licensed, quality controlled, commercially available treatment is a long way off.

There is also some (largely anecdotal) evidence of new, unlicensed products from China slowing down the progression of FIP, with some cases achieving remission. These drugs are sold online on the black market(5); are very costly; and currently lack solid research backing their efficacy, as well as clear guidelines regarding their composition, shelf life, or side effects. Caught between the devastating effects of FIP; owners who are desperate for a cure; and above all, responsibility towards their patients; these drugs present a legal and ethical quandary which can be difficult for veterinarians to negotiate. Opinions vary; with little experience of the drugs being afforded to vets at this stage. Seeking the expertise of a specialist veterinarian in this circumstance can be beneficial as they may have greater experience with complicated illnesses and may be more up to date with current options.

Supportive care can extend the survival time of cats infected with FIP and make them more comfortable, especially those cats who are still in good physical condition. Polyprenyl immunostimulant (PPI) benefits some cats with FIP if started early on(5). Draining fluid and giving appetite stimulants can also help in the short term. However, in the absence of an effective treatment, the prognosis is grave, and quality of life must be considered.

FIP is a complicated and devastating disease, where diagnosis is difficult and comes with serious implications. Open, honest conversations with your vet about their recommendations are important in finding the best way to proceed. Exciting developments are under way, and treatments will hopefully become available and change the face of this disease in the future.

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  1. Addie, D.D. et al. (2020, June). Oral Mutian X stopped faecal feline coronavirus shedding by naturally infected cats. Retrieved from:

  1. Barrs, V. & Beatty, J. (2013). Infectious Diseases. In A. Harvey & S. Tasker (Eds.). BSAVA manual of feline practice: A foundation manual (pp. 446-448). Gloucester, UK: British Small Animal Veterinary Association.

  2. Burns,K. (2017, November 29). A glimmer of hope for a fatal feline disease. American Veterinary Medical Association. Retrieved from:

  3. Cornell Feline Health Center. (2020). Feline infectious peritonitis: What is FIP? Cornell University. Retrieved from:

  4. Dale, S. (2020, March 5). Is an FIP treatment close at hand? Veterinary Practice News. Retrieved from:

  5. Levy, J.K. (2018, August). Feline Infectious Peritonitis. MSD Veterinary Manual. Retrieved from:

  6. Wogan, L. (2019, August 22). Legal treatment for cat disease known as FIP still years away. ViN News Services. Retrieved from:

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