Feline Infectious Peritonitis (FIP) is a disease guaranteed to strike dread into any vet who diagnoses it, as the prognosis is extremely grave. A study in 2011 found that the average survival time after diagnosing FIP with effusions was only three weeks. However, there have been huge advances recently into some promising treatments, which offers great hope for the future. 

What is FIP?

Feline infectious peritonitis occurs after infection with a virus known as feline coronavirus. This is actually a common virus in cats, and usually causes mild illness only, such as a small gastrointestinal upset. However, in a few cases, the virus mutates to a strain which causes FIP. The virus then infects macrophages (a type of blood cell) and rapidly spreads through the cat’s body unless the cat can mount a sufficient immune response. 

What are the symptoms of FIP?

FIP usually causes a wide range of symptoms, with none being particularly specific, which can make it hard to reach a diagnosis. The disease usually starts with fairly mild and non-specific symptoms such as lethargy, not eating well and a fever. After some time, which can vary from days to weeks, or even months, the disease progresses. There are two recognised forms: wet and dry, although affected cats can have a mixture of both forms. 

Wet/Effusive FIP 

The virus causes damage and inflammation to blood vessels, causing fluid to leak out. Fluid can accumulate in the abdomen, causing distension of the belly area, or in the chest, causing breathing difficulties.

Dry/Non-effusive FIP 

The virus causes chronic inflammatory lesions inside various organs. The eyes and brain are most commonly affected, but the liver, kidneys, skin and lungs can also be affected. The symptoms will vary depending on where the lesions are found. For example neurological signs if the brain is targeted or bleeding in the eyes if they are affected.

Current treatments

FIP is an extremely challenging disease to treat, with a very poor prognosis attached. There are some options currently in use, but they have very limited success.

  • Supportive care – draining fluid, nutritional and appetite support and fluid therapy can all help maintain affected cats’ health as long as possible
  • Corticosteroids – as there is an immune component to this disease, it is thought that steroids may help control the response. There are no studies to prove any beneficial effect, but they are still widely used.
  • Feline interferon omega – again, there are no studies that conclusively show the benefit of this in cats with FIP, but it has been used to try and control the disease
  • Anti-malarial drugs – these small molecule compounds can stop viruses from replicating, and are potential treatment options, but this is still currently under investigation
  • Various other medications such as cyclosporins, itraconazole, TNF-A and propentofylline have elicited some interest, but currently none have been proven to have any benefit. 

New advances

Current research is focusing on new types of anti-viral agents. There are two types currently showing promise – protease inhibitors and nucleoside analogues. Neither of these options are currently licensed for use in cats against FIP, but the research is looking promising for the future.

Protease inhibitors

Protease inhibitors are molecules which stop the virus from making mature proteins. In a laboratory setting, they were shown to work well against feline coronavirus (FCoV), and so progressed to testing in cats infected with FIP. One compound, GC376, showed a huge positive effect in an experimental study where six out of eight cats with effusive FIP recovered following treatment. Further studies have also shown positive responses. A company based in the U.S. is currently pursuing a license for GC376 to treat FIP in cats.


Nucleoside analogues stop the virus from replicating its genetic material (RNA), a vital process for virus replication and spread. A 2018 study trialled the nucleoside analogue GS-441524 in an experimental study using ten cats with FIP. All ten cats recovered (although two relapsed and required further treatment) and were healthy at the 8-month post treatment mark when the study was published. Cats with neurological (brain) or ocular (eye) symptoms were excluded from the trial, but studies have since shown that these affected cats can also be successfully treated. These trials have all used the analogue GS-441524 injected under the skin. 

There is one published report of successful treatment of FIP using a nucleoside analogue given by mouth. It is thought to be a compound closely related to GS-441524, but the exact ingredients have not been made public. The treatment was only given for seven weeks (compared to twelve weeks in the injectable trial), with corticosteroids also given, and the cat appears to have been completely cured. 

Remdesivir (GS-5734)

Another potential option is Remdesivir (GS-5734), which is a pro-drug, which means it is altered once in the body to make the active ingredient of GS-441524. It shows huge promise for treating FIP. It is licensed in some countries (including the UK) for use in humans, and has been the source of some interest as to its potential to help those suffering from Covid-19. In countries where it is licensed for humans, it could then be used off-license for cats. 

So, can my cat be treated now?

Sadly, the situation is complicated. GC376 and GS-441524 are both neither licensed for use nor commercially available in the U.K. The process of licensing will likely take many months, if not years. 

There is a large online market for similar compounds which are being manufactured in Asia without any form of safety testing or regulation. One fairly common drug is marketed as a dietary supplement designed for cats with FIP, which goes against the Veterinary Medicine Directorate in the U.K., which states that nutritional supplements must not make medical claims. Those owners who are desperate to help their cats and are turning to these options are not only using a drug which has not had any safety or efficacy tests, cannot be sure what exactly they are buying, and are essentially importing drugs illegally. 

Remdesivir is a licensed drug for humans, and can therefore be used off-license in cats, but it can be difficult to obtain and is very costly. There are companies working on making a formulation that can be licensed and made available in the U.K., which will hopefully be a valid and more affordable option in the future.

These drugs are all also highly expensive currently, with a 12 week course costing up to £10,000 depending on the size of cat and the severity of symptoms, which makes the treatments simply unobtainable for many owners. 

Promise for the future

Although none of the investigated options are currently licensed in the U.K., the success of these drugs in ongoing research provides great hope for the future. For many years a diagnosis of FIP has been a death sentence, but it seems very possible that in the not-so-distant future this will no longer be the case. 


Tsai HY, Chueh LL, Lin CN, et al. Clinicopathological findings and disease staging of feline infectious peritonitis: 51 cases from 2003 to 2009 in Taiwan. J Feline Med Surg. 2011; 13: 74-80.

Kim Y, Liu H, Galasiti Kankanamalage AC, et al. Reversal of the Progression of Fatal Coronavirus Infection in Cats by a Broad-Spectrum Coronavirus Protease Inhibitor. PLoS Pathog. 2016; 12: e1005531. 

Murphy BG, Perron M, Murakami E, et al. The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Vet Microbiol. 2018; 219: 226-233 

Beigel JH, Tomashek KM and Dodd LE. Remdesivir for the Treatment of Covid-19 – Preliminary Report. Reply. N Engl J Med. 2020; 383: 994

You might also be interested in: