This is the second of a two-part series about FIP treatments.
Until recently, no treatment for FIP was possible, and cats with the disease died within days to weeks. Because of the COVID pandemic, interest in coronaviruses increased, resulting in the development and availability of several antiviral drugs. People commonly use four: GS-441524 and its precursor remdesivir, molnupiravir (Lagevrio), and Paxlovid (nirmatrelvir/ritonavir).
The mainstay of treatment is GS-441524. Its mechanism of action is premature RNA chain termination in the virus, preventing viral replication. Doctors give it by mouth once or twice a day. The dose is 15 mg/kg/day given once daily or 20 mg/kg/day divided into two doses if there is neurological or eye involvement. A kilogram (kg) is 2.2 lbs.
In cats that cannot swallow, the veterinarian can administer remdesivir subcutaneously or the medication can replace the first few IV doses. Remdesivir uses the same doses as GS-441524. The optimal duration of treatment is under investigation, but for now, 84 days (12 weeks) of therapy are standard.
There are ongoing studies investigating shorter treatment (6 weeks); the initial results on a group of twenty cats have been favorable. Medication presents substantial costs, and a twelve-week treatment might prove difficult to finance. Therefore, vets plan to assist owners in shortening treatment when they can.
If a cat is clinically doing well (alert, eating) and the lab results have tended towards normal by four weeks, discontinuing treatment at 6 weeks may be reasonable. Once the treatment ends, the veterinarian should examine the cat within one or two weeks, regardless of how long the therapy lasted.
If your cat’s symptoms come back, schedule an earlier check-up, as relapses may occur after 12 weeks of treatment. Research shows that 80% of cats go into remission following treatment. Table 1 shows key milestones for cats undergoing treatment with GS-441524.
| Abnormality | Expected time to resolution |
| Fever/inappetence | 2-7 days |
| Effusions | 1-2 weeks |
| Blood count abnormalities | 2-3 weeks |
| Elevated bilirubin | 2-3 weeks |
| Albumin: globulin > 0.6 | 6-10 weeks |
| Eye and neurologic abnormalities | Improvement by day 5, resolution within 2 weeks |
If cats respond poorly to GS-441524 (failing to reach clinical milestones on time), you can also add Paxlovid. It works differently from GS-441524 and may provide a synergistic effect.
Initially, veterinarians administered Molnupiravir as a rescue drug for cats not responding to GS-441524 (not reaching treatment milestones on time), but recent studies use it as the primary treatment. It works by causing multiple mutations in the replicating virus, resulting in its eradication. The dose is 10-15 mg/kg given twice a day.
Since it’s designed for humans, this dosage form is also impractical and can decrease appetite. As a result, it might be necessary to use appetite stimulants or antiemetics. It causes birth defects in rats and cartilage defects in growing humans. Treated cats exhibit folded ear tips, broken whiskers, and flaky skin.
Cats may also experience muscle wasting and elevation of ALT (Alanine Aminotransferase, present in the liver, muscle, kidneys, and other organs).
Paxlovid (nirmatrelvir/ritonavir) is two drugs in one. Nirmatrelvir inhibits RNA copying enzymes, and ritonavir slows the degradation of nirmatrelvir in the body. Doctors use it along with GS-441524 or molnupiravir if the response is incomplete. You must be careful if the cat is taking medications eliminated by the same pathway, and you may need to adjust their doses.
Supportive measures include appetite stimulants, pain meds and even a feeding tube if the cat is unable or unwilling to take medications by mouth.
With effective treatment for FIP, inappropriate use of the drugs has inevitably occurred. Reports show that whole litters of kittens received treatment because a vet diagnosed one kitten with FIP or even administered preventative treatment.
Though a Feline Enteric Coronavirus (FeCV) vaccine is available, the American Association of Feline Practitioners does not recommend it. You must vaccinate the cat before FeCV infects it, but this is impossible because many kittens contract the infection before they can receive the vaccine at 14 weeks old. Further, immunity wanes with time.
There is currently work ongoing on an effective vaccine.
Finally, a word on the FIV on Cyprus. Reports conflict regarding the prevalence, though FIP is more widespread elsewhere. One source estimates that 30% of the feral cats in Cyprus have died of the disease. Twenty-eight percent of the affected cats have nervous system involvement, compared to 14% of FIP cats elsewhere. The malady infects felines of every age, not merely juvenile ones.
DNA sequencing has shown that the virus responsible is a recombination between the feline and canine coronaviruses.
The alarming property of this virus is that it is transmissible between cats. Folks from the United Kingdom and Europe have adopted feline companions while on holiday in Cyprus. At least one cat like that has developed FIP. It presents a risk regarding worldwide, European spread of the mutant virus.
Footnotes and further reading/listening
- Conference Proceedings: Am College Vet Internal Med Forum; ACVIM 2023. Sally J. Coggins
- Pedersen, N. et al., J, Feline Med Surg. 2019 Feb 13; 21(4):271-281. Efficacy and Safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis.
- Coggins, S.J. et al., J Vet Intern Med. 2023 Sep-Oct; 37(5): 1772-1783. Epub 2023 Jul 13. Outcomes of treatment of cats with feline infectious peritonitis using parenterally administered remdesivir, with or without transition to orally administered GS-441524.
- Taylor, S., Talk at the International Society for companion Animal Infectious Disease meeting 2024. An Update on Treatment of FIP Using Antiviral Drugs in 2024: Growing Experience but Still More to Learn.
- Coggins, S.J., Talk at the American College of Vet Int Med meeting 2023. Updates in Feline Infectious Peritonitis Treatment.
- Felten, S. et al., J Feline Med Surg. 2017 Apr; 19(4): 321-335. Detection of feline coronavirus spike gene mutations as a tool to diagnose FIP.
- Sorrell, S., Tasker, S., Taylor, S., Barker, E., Gunn-Moore, D.: Feline Infectious Peritonitis (FIP) Information for Cat owners. Stokes Pharmacy website: https://www.stokespharmacy.com/wp-content/uploads/2024/05/Pet-Owner-Brochure-Printable-Version.pdf
- Thayer, V. et al., J Feline Med Surg. 2022 Sep; 24(9):905-933. 2022 AAFP/Every Cat Feline Infectious Peritonitis Diagnosis Guidelines.
- https://bova.vet/
- https://www.stokespharmacy.com/stokes-bova-partner-for-feline-infectious-peritonitis-treatment/
- https://www.stokespharmacy.com/fip/cat-owner-resources/
- https://bova.vet/fip-resource-page/#FIP-Webinars
- https://vcahospitals.com/know-your-pet/feline-infectious-peritonitis
- https://veterinarypartner.vin.com/default.aspx?pid=19239&id=11618072
- https://www.merckvetmanual.com/cat-owners/disorders-affecting-multiple-body-systems-of-cats/feline-infectious-peritonitis-fip
16.Merck Veterinary Manual, 11th edition, 2016, pp. 782-790
17.https://www.wormsandgermsblog.com/tags/feline-infectious-peritonitis/ - https://ccah.vetmed.ucdavis.edu/sites/g/files/dgvnsk4586/files/inline-files/Inappropriate%20use%20of%20GS.pdf
- https://www.biorxiv.org/content/10.1101/2023.11.08.566182v3.full.pdf . Attipa, C., Warr, A. et al. BioRxiv preprint 3 Sep 2024: Emergence and spread of feline infectious peritonitis due to a highly pathogenic canine/feline recombinant coronavirus.
- https://thewebinarvet.com/videos/what-you-should-know-about-fcov-23-and-the-outbreak-of-fip-in-cyprus
- Gao, Y. et al: Virus Research V 326, March 2023: An updated review of feline coronavirus: mind the two biotypes
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