FIP in 2026: Treatment Guidelines – What Every Vet Needs to Know

Philip R Judge BVSc MVS PG Cert Vet Stud MACVSc (Vet Emergency and Critical Care; Medicine of Dogs)

Introduction

Feline infectious peritonitis (FIP) is a disease caused by feline coronavirus infection. For many years, it was regarded a fatal disease, with euthanasia commonly recommended following diagnosis due to its very poor prognosis.However, the advent of new and more effective treatments is changing the expectations of patient response to treatment. This article focuses on the latest updates on the treatment of FIP, presented by the European Advisory Boards on Cat Disease (ABCD), published in April 2026. 

Here are the top takeaways from the latest guidelines:

1. Oral GS-441524 is the New Gold Standard
   1. GS-441524 is a nucleoside analogue with published success rates in treatment of FIP ranging from 80-100% – including 5 studies reporting success rates of over 90%.
   2. Oral forms of GS-441524 are better tolerated than subcutaneous injections (which are painful), and are currently recommended as the first-line treatment.
   3. Dosage: 15 mg/kg every 24 hours.Twice daily doses of 10-15 mg/kg are tolerated well, and may be used in cases refractory to once-daily dosing, and in patients with neurological or ocular manifestations of FIP
   4. Duration: 42 days is now considered equally effective as the older 84-day – although 12 week courses may be used if clinical response is mild or delayed. protocols. This dramatically lowers costs and stress for the owner and cat.
   5. Success Rate: Studies consistently report success rates exceeding 90% .
   6. Treatment may begin prior to diagnosis confirmation if clinical signs and blood tests are highly suggestive
2. When to Use Remdesivir (Injectable)
   1. Remdesivir is a nucleoside analogue, and is the pro-drug of GS-441524
   2. Intravenous remdesivir is recommended in patients who are severely ill, who cannot tolerate oral GS-441524, who have severe intestinal dysfunction (vomiting, diarrhoea or decreased gut motility), or where GS-441524 is unavailable
3. What about other antiviral medications?
   1. Molnupiravir is another nucleoside analogue pro-drug, hydrolysed to EIDD-1931 with a success rate for treating FIP ranging from 72-91%. When used as a rescue agent, efficacy is reported to be 86-100%, if used with an 84-day treatment protocol. Side-effects are more severe than with Remdesivir or GS-441524, and response to treatment is slower than with the aforementioned agents also.
   2. EIDD-1931is also available as a treatment for FIP. Response rates are slower than with remdesivir and GS-441524, and side effects are more severe also. Both Molnupiravir and EIDD-1931 are potent mutagenic agents, which limits safety.
   3. Mefloquine, Recombinant Feline Interferon-Omega, Nirmatrelvir and GC376 all require further studies to confirm treatment potential.
4. What about immuno-modulation therapy?
   1. Glucocorticoids: glucocorticoids are not recommended as routine therapy for FIP, as they are not associated with improved outcome, when used alone. They may be considered in the following circumstances:
      1. Topical corticosteroids for management of uveitis
      2. FIP-associated neurological dysfunction (1 mg/kg/day)
      3. Non-haemotropic mycoplasma IMHA (1-2 mg/kg/day)
   2. Polyprenyl Immunostimulant: has been used in cats with non-effusive FIP, with abnormal albumin: globulin ratios.
5. Supportive Care
   1. Supportive care is crucial to achieving positive outcomes
   2. Recommended supportive care therapies include:
      1. Analgesia – opioid, pregabalin/gabapentin, NSAIDs (if no contraindications)
      2. Periodic pleural fluid drainage
      3. Antiemetics (metoclopramide, maropitant, ondansetron)
      4. Appetite stimulants (mirtazepine)
      5. Anticonvulsants for seizure management (+/- prednisolone)
      6. Vasopressors (for hypotension)
      7. Intravenous fluid support
      8. Nutritional support
      9. Management for cardiomyopathy or FIP-associated myocarditis when present
6. Monitoring, relapse and other concerns
   1. Effusion resolution: If effusion hasn’t decreased significantly by 2 weeks, consider increasing the dose of antiviral therapy
   2. Weight gain: weight gain is a common sign of treatment success – failure of weight gain, or weight loss indicates reassessment of treatment protocols, dosing and supportive care is required
   3. Bilirubin and globulin concentrations: should fall alongside clinical improvement. Sustained elevations require patient assessment
   4. Serum amyloid-A concentrations show rapid decreases with positive response to treatment, and may be used as a guide to treatment efficacy, or requirement for a review.

The Bottom Line

FIP has moved from “fatal” to “potentially curable.” The standard of care for FIP is now oral GS-441524 for 42 days, with survival rates of over 90% achievable. This is a major, and positive  development in the treatment of FIP.

Reference:

Tasker S, Spiri AM, Hartmann K, Addie DD, Belák S, Bergmann M, Egberink H, Frymus T, Hofmann-Lehmann R, Marsilio F, Pennisi MG. Update on Treatment of Feline Infectious Peritonitis: European Advisory Board on Cat Diseases (ABCD) Guidelines. Viruses. 2026 Apr 9;18(4):452. 

Check out the full article here: https://www.mdpi.com/1999-4915/18/4/452 

Based on: Tasker S, Spiri AM, Hartmann K, Addie DD, Belák S, Bergmann M, Egberink H, Frymus T, Hofmann-Lehmann R, Marsilio F, Pennisi MG. Update on Treatment of Feline Infectious Peritonitis: European Advisory Board on Cat Diseases (ABCD) Guidelines. Viruses. 2026 Apr 9;18(4):452.