FIP Advocates and Champions

Relapse symptoms may be similar to the cat’s initial symptoms, or they can present very differently. As Taylor et al. (2022) points out, “Three-quarters of relapsing cats did so with clinical signs different from their initial predominant FIP type, and half were with neurological signs, as reported previously. Hence, vigilance for novel clinical signs during and after antiviral treatment is recommended.”  

In some cases, relapsing cats may have initially presented with neurological or ocular FIP and if the starting dosage was too low this can contribute to a relapse. The timeframe of highest risk for relapse was found to be within 60 days of stopping GS treatment. Symptoms to monitor for involve inappetence, lethargy, weight loss, pyrexia, ascites, jaundice, ocular symptoms and neurological involvement. Lab work may or may not show the common FIP markers (Taylor et al., 2022).

Protocols for Relapse Treatment:

– Restart GS-441524 at a higher dosage (by 5-10 mg/kg/day) than the cat ended treatment on. A relapse is typically treated for a full 12 weeks.

– Split the dose into twice daily doses if initial treatment was once daily (Taylor et al., 2025).

As stated in Taylor et al. (2022), “Given the lack of toxicity observed with remdesivir and GS-441524 treatment, the decision to use appropriate higher doses according to FIP type is appropriate.”

**Consider using an alternative antiviral when cats do not respond to treatment at adequate dosages with GS-441524 (resistance cases) and for situations when cats relapse after treatment with GS-441524 (Taylor et al., 2025).

Molnupiravir (EIDD 2801) is recommended to be used for:

• Cats showing insufficient response to GS-441524 despite being treated at a therapeutic dosage
• Cats exhibiting signs of regression or possible resistance to GS
• Relapse cases that have been treated with GS-441524 at a therapeutic dosage (Taylor et al., 2025)
• Situations where access to GS-441524 is not feasible (Evans & Jacque, 2024).

Paxlovid: 

Paxlovid is an important alternative antiviral when dealing with refractory FIP cases (Coggins, 2024). This drug  can be prescribed off label and has shown efficacy in cats where treatment with GS-441524 and Molnupiravir  has failed. 

Paxlovid is used in combination with either GS-441524 or Molnupiravir when previous antiviral treatment has  not been successful due to drug resistance or other factors (FIP treatment protocols: FIP treatment guide, n.d.). 

Current dosing recommendations (Coggins, 2024): 

Nirmatrelvir 1/2 tablet (75 mg) and Ritonavir 1/4 tablet (25 mg) 

A box contains 20 x 150 mg Nirmatrelvir tablets (protease inhibitor) 

 10 x 100 mg Ritonavir tablets (PK booster) 

(One box should last 20 days)

References:

Coggins, S. (2024). Cornell Feline Health Center: Feline Infectious Peritonitis (FIP) treatment  in the USA (Webinar) Cornell University College of Veterinary Medicine. 

https://www.youtube.com/watch?v=P4XlQbcCvj4

Evans, S. & Jacque, N. (2024). Compounded Molnupiravir: A new era in FIP management  (Webinar). Wedgwood Pharmacy. https://youtu.be/5bgiko0oRvM?si=7_mdynsfOhF-t0fh

FIP treatment protocols: FIP treatment guide (n.d.). https://fipvetguide.com/faq

Taylor, S. S., Coggins, S., Barker, E. N. & Gunn-moore, D., Jeevaratnam. K., Norris, J. M.,  Hughes, D., Stacey, E., Macfarlane, L., O’brien, C., Korman, R., Mclauchlan. G., Torres,  X. S., Taylor, A., Bongers, J., Espada Castro, L., Foreman, M., Mcmurrough, J., Thomas,  B., Royaux, E., Calvo Saiz, I., Bertoldi, G., Harlos, C., … Tasker, S. (2020–2022).  Retrospective study and outcome of 307 cats with feline infectious peritonitis treated with  legally sourced veterinary compounded preparations of remdesivir and GS-441524.  Journal of Feline Medicine and Surgery, 9 2023;25.  

https://doi.org/10.1177/1098612X231194460

Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S.

(2025). icatcare_fipupdate_July25.

Click to access icatcare_fip_update_july25.pdf