Feline Infectious Peritonitis, or FIP, is a serious disease of cats caused by a cat-specific Feline Coronavirus. It is related to the virus that causes COVID-19, but it does not affect people or other animals. Many cats who are infected with feline coronavirus will have mild or no symptoms and will clear the virus without an issue. Mild symptoms typically include upper respiratory symptoms (i.e. sneezing, eye or nose discharge) or gastrointestinal symptoms (most commonly, diarrhea). These symptoms can be caused by MANY different viruses, bacteria,and parasites, and we rarely test for a specific cause at this stage.    

In a small population of cats, the virus will mutate into a form that causes the severe illness called FIP. There seems to be a genetic predisposition in at least some cases, as siblings of kittens that have FIP have a higher risk of developing FIP if they’re also exposed to feline coronavirus. This new version of the virus is not considered contagious, and there’s generally little risk of it spreading to other cats. Stress is a very important trigger for the development of FIP, and I find that most cats who become sick have had a recent stressor such as adoption/rehoming, vaccines, surgery (including spay/neuter), a change in the household such as a new pet, crowded conditions such as a cattery, or other illness.    

Symptoms: There are 3 main types of FIP. Most cats will show general signs of not feeling well such as decreased energy, poor appetite, and weight loss. They may feel hot to the touch, as they typically have a fever. The 3 forms of FIP differ in the following ways:    

  • Wet: This form is characterized by a buildup of fluid in the abdomen and/or around the lungs, and occasionally around the heart. ○ Hallmark symptoms include a distended belly from fluid in the abdomen, and/or difficulty breathing from fluid accumulating around the lungs. The fluid from either of these locations can be tested for FIP with an FIP PCR test, and it also tends to have high protein levels and a specific straw-yellow color.
  • Dry: This form is much harder to diagnose, as the symptoms are very non-specific (poor appetite, decreased energy, weight loss, fever). Specific patterns of blood work changes are often helpful in pointing your vet in the right direction. An ultrasound of the abdomen might show suspicious areas such as enlarged lymph nodes or a (usually single) mass in any organ, which can be sampled and tested with an FIP PCR test. The spleen can also be a good location to sample if there are no specific abnormalities in the other organs.
  • Ocular/Neurologic: This is technically a kind of Dry FIP, but it is typically more serious and harder to treat. These cats need higher doses of the medicine used to treat FIP. They can also become very sick very quickly, particularly the neurologic form, so fairly rapid presumptive diagnosis and treatment is important. It is very hard to get a definitive diagnosis of these forms of FIP, since it is more invasive to obtain samples of the fluids that typically contain the virus. Fluid within the eyeball or spinal fluid are typical sites.

When a cat is being tested for FIP, a positive FIP PCR test gives us a definite diagnosis, but a negative PCR test does NOT mean we can rule out FIP (the virus isn’t always in the sample that you test, or you may miss it when getting the sample). Blood can be tested with an FIP PCR test for any form, but a false negative result can frequently occur.    

For decades, FIP has been considered an incurable, untreatable, invariably deadly diagnosis. Until very recently, all of the cats who were diagnosed with it were destined to die, and their parents were told that there was nothing they could do to stop it. But thanks to Dr. Niels Pedersen’s research at UC-Davis vet school, we have had an apparent cure for FIP since about 20181,2 . “GS-441524,” which becomes Remdesivir (one of the drugs used in the treatment of COVID-19) in the body, had incredible effects on cats diagnosed with FIP in Dr. Pedersen’s 2018 and 2019 studies, curing 25 of 31 cats (4 of the 6 who did not survive were euthanized or died within the first five days of treatment due to the severity of their FIP symptoms). In my experience, the only cats who have died or relapsed are the ones with the neurologic form of FIP (typically from seizures), and the ones who have gotten so sick while figuring out a diagnosis and finding treatment options that they die before starting treatment. As I’ll discuss later, there is currently no legal way for veterinarians in the US to access the drugs to treat FIP, but groups of volunteers are working tirelessly to get GS to the sick cats who need it. Our hope is that in the near future, veterinarians will gain legal access to GS once the FDA approves Remdesevir.    

In general, cats with FIP tend to improve within a few days of starting treatment. Treatment consists of 84 days of daily to twice daily treatment with either an injection or pill form of GS. During the course of treatment, we recheck cats every 4 weeks for an exam, weight, temperature, and blood work. There may be additional medications or supplements that are recommended based on a particular cat’s initial blood work and symptoms (e.g. liver involvement or anemia). Cats must also be weighed a few times a week at home so that the GS dose can be changed as needed as the cat gains weight - this is very important, as underdosing often leads to relapses. Injections of the drug can be painful, so a pain medication is often dispensed to be given an hour or two prior. Most cats need to start with the injectable drug, but some may be able to transition to the oral form if needed.    

After the 84 days of treatment, if blood work markers are all appropriate (based on Dr. Pederson’s recommendations), the drug can be discontinued and the cat is monitored for another 3 months. If the cat is clinically normal and blood work markers remain normal after the 3 month monitoring period, the cat is considered cured of FIP. It is very important not to stress the body during this time, so vaccines, anesthesia, and major stressors should be avoided. If a vaccine or spay/neuter are absolutely necessary, they should be done during the last month of the treatment period. Vaccines should be minimized after cure has been achieved in these patients. I personally recommend Feline panleukopenia titers every 3 years, and Rabies vaccination no more than every 3 years. The most common side effect of GS treatment is skin sores at the sites of injections. These are typically mild enough that they’re cured with specific treatment (topical and/or oral, depending on severity). Rare side effects include mild kidney toxicity in cats, and in people, mild kidney and liver toxicity have been noted in people being treated with Remdesivir. If a cat relapses or is resistant to GS, there are new protocols with another COVID drug called Molnupiravir (EIDD), or a drug called GC376 in combination with GS.    

Currently, there is no way for veterinarians to legally dispense GS to treat FIP in the United States. Our hope in the veterinary community is that we will soon have a legal treatment for FIP in the form of Remdesivir. Once it is fully legalized by the FDA, veterinarians should be able to prescribe it “off-label” for cats with FIP. In the meantime, there are dedicated groups of volunteers who have been able to facilitate access of the drug to cat parents in need. If you are concerned that your cat or kitten may have FIP, or they have been given an FIP diagnosis, you can schedule an in-person or virtual appointment with Dr. Lisa Fiorenza at Longevity Veterinary Center by calling us at (973) 606-1101.    



  1. The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Vet Microbiol. 2018 Jun; 219:226-233. Murphy BG, Perron M, Murakami E, Bauer K, Park Y, Eckstrand C, Liepnieks M, Pedersen NC. PMID: 29778200; PMCID: PMC7117434.
  2. Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. J Feline Med Surg. 2019 04; 21(4):271-281. Pedersen NC, Perron M, Bannasch M, Montgomery E, Murakami E, Liepnieks M, Liu H. PMID: 30755068; PMCID: PMC6435921.
  3. https://ccah.vetmed.ucdavis.edu/sites/g/files/dgvnsk4586/files/inline-files/Summary%20of%20GS %2C%20v8_0.pdf
  4. https://ccah.vetmed.ucdavis.edu/sites/g/files/dgvnsk4586/files/inline-files/Approaches-to-drug-re sistance-in-cats-treated-with-GS-441524-for-FIP-v3.pdf 5. https://ccah.vetmed.ucdavis.edu/cats/resources/general-feline-infectious-peritonitis-resources