FIP Treatment
Monitoring Guidelines
FIP Advocates and Champions Canada is an organization dedicated to providing help, support and hope for those facing this diagnosis. Though once a completely fatal disease, there is now a highly effective treatment and cure!
Treatment Guidelines:
The majority of cats can be treated with oral GS-441524 from day one. Critical cases presenting with severe neurological symptoms, inability to swallow, or severely dehydrated, or cats that cannot be safely orally medicated, will need to start treatment with injections. Remdesivir injections can be prescribed off label for these critical cases until they are stable enough to move to oral treatment (Taylor et al., 2025).
Dosage Guidelines for Oral GS-441524 adapted from ISFM Guidelines (Taylor et al., 2025).
- 15 mg/kg/day – Wet or Dry FIP that does NOT have ocular or neurological involvement
- 20 mg/kg/day for Ocular FIP
- 10 mg/kg q12hr (20 mg/kg/day) for neurological FIP
As explained by Taylor et al. (2025), the GS dose should be given on an empty stomach. A small treat can be given to facilitate medicating. Dividing the daily dose and administering q12h has been suggested to improve plasma concentration of GS-441524. Neurological FIP should be dosed q12h (Taylor et al., 2025).
Cook et al, (2024) explains that ocular and neurological symptoms pose additional challenges in terms of achieving therapeutic levels of GS-441524 in the central nervous system and eyes. According to Coggins (2024), when deciding the correct GS dosage, it is important to determine if the cat has ocular or neurological involvement as these cats require a higher dosage of GS-441524 than those with uncomplicated effusive or non effusive FIP.
Adjusting the dose throughout treatment as the cat gains weight is crucial. Not adjusting the dose in line with weight gain is one of the most common causes of treatment failure. Kittens in particular will typically gain a significant amount of weight throughout the duration of the treatment. Owners should weigh the cat weekly, and the dose should be adjusted as necessary (Taylor et al., 2025).
Some cats, particularly with the effusive form of FIP, will lose weight early on in treatment as the fluid is reabsorbed. The GS dose should remain consistent. It is not recommended to adjust the dose downwards with weight loss. Instead, maintain the starting dose until the cat has gained enough weight to warrant increasing the dose (Coggins, 2024).
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
*Cook, S., Wittenburg, L., Yan, V. C., Theil, J. H., Castillo, D., Reagan, K. L., Williams, S., Pham, C. D., Li, C., Muller, F. L., & Murphy, B. G. (2022). An Optimized Bioassay for Screening Combined Anticoronaviral Compounds for Efficacy against Feline Infectious Peritonitis Virus with Pharmacokinetic Analyses of GS-441524, Remdesivir, and Molnupiravir in Cats. Viruses, 14(11), 2429. https://doi.org/10.3390/v14112429
*Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf
Response to FIP Treatment:
The majority of cats respond quickly after initiating therapy with GS-441524. The cat’s appetite and energy level should be improving within the first 2-5 days and pyrexia resolved (Taylor et al., 2025).
It is not unusual for additional clinical symptoms to appear during the first 1-2 weeks of FIP treatment. These may involve a reoccurrence or new development of pleural fluid, neurological signs or uveitis (Taylor et al., 2022). It is important to note that if neurological or ocular symptoms develop, the GS dosage will need to be re-evaluated to meet the specific dosing guidelines for treating those presentations. According to Taylor et al., (2025), if abdominal or pleural effusion is still present 2 weeks into treatment, it is recommended to increase the dosage by 5-10 mg/kg/day and consider treating q12hr if currently treating q24hr.
It is typical for globulins to increase within the first few weeks as the effusion is absorbed. Anemia and lymphopenia may take up to 10 weeks to fully resolve. It is also common to see lymphocytosis at the end of treatment, and this is not a cause for concern (Taylor et al., 2025).
As Taylor (2024) explains, in some cases, treatment beyond 12 weeks is necessary. Cats that do not respond as expected throughout treatment, or relapse cases, may need an extended treatment. For cats with neurological FIP, residual symptoms, such as mild ataxia, may remain toward the end of treatment. In such cases, an increase in dosage and extension of treatment to monitor for progress can be considered.
In some cases, there may be permanent neurological symptoms and occasionally hyperglobulinemia persists post treatment. Small amounts of lingering effusion are also possible with some cats. In rare cases, lymphadenopathy may not entirely resolve. A lack of complete response is not necessarily indicative of treatment failure or potential relapse (Taylor et al., 2022).
References:
*Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf
*Taylor, S. (2024). FIP: so much has changed, where are we now? (Webinar transcript). Bova Scholars. https://bova.vet/2024/04/04/fip-changes/
*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
Monitoring Treatment and Side Effects:
Consistency and accurate dosing is crucial to a successful treatment outcome. Clients should be encouraged to obtain a digital scale (a baby or pet scale works well) to weigh their cat at least once a week. The GS dose should always be adjusted with weight gain (Taylor et al., 2025).
According to guidelines from International Cat Care (Taylor et al., 2025) the recommendations are:
48-72 hrs – Check-in with the client verbally to assess the patient’s response to treatment and whether any further symptoms have developed, potentially requiring a dosage increase.
2 weeks – In clinic or verbal check-in with client to assess response to treatment.
6 weeks – The patient should be examined and a CBC and Chemistry panel run to assess if the markers associated with FIP are resolving as expected. A dosage increase may be warranted if the labs and/or clinical exam are not within the parameters expected.
12 weeks – Clinical exam, CBC and Chemistry panel. This should be done BEFORE stopping treatment to assess if the cat is ready to end treatment.
4 weeks post-treatment – A clinical exam, CBC and Chemistry panel should be performed to ensure there are no concerns (Taylor et al., 2025).
Side Effects of FIP Treatments (Taylor et al., 2025).
– Remdesivir injections can cause pain when administered subcutaneously.
– ALT elevations have been seen in some cats during treatment; however, these cats do not require specific treatment such as hepatoprotectants.
– Mild lymphocytosis and eosinophilia have also been seen with treatment and do not require any intervention.
– There have been rare reports of uroliths associated with GS-441524. These have been linked to black market products that may have been used at excessively high dosages. To mitigate this risk when using high doses of antivirals, it is recommended to encourage water intake and monitor for urinary signs, as well as investigate promptly if they develop (Taylor et al., 2025).
Reference:
Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf
Supportive Care and Steroids:
Supportive Care:
– Abdominal effusions are typically not drained. Abdominocentesis is only indicated if it is interfering with the cat’s respiration. It is important to note that fluid drained from the abdomen will rapidly return. In contrast, pleural effusion should be drained by thoracocentesis (Roman, 2024).
– Cats with pleural effusion may require multiple fluid drainages during the initial two weeks of treatment. This is due to the time it takes for vasculitis to resolve and the antiviral medication to reach full effectiveness, rather than due to an adverse reaction to the treatment (Taylor, 2024).
Early into treatment, cats with FIP often require additional supportive treatment in addition to GS, including Subq or IV fluids, appetite stimulants, anti-nausea medication, anticonvulsants, nutritional support and pain management (Taylor et al., 2025).
Can Steroids be used with FIP treatment?
In certain cases, short-term steroid administration is necessary to reduce inflammation and stabilize FIP patients, particularly those with severe neurological symptoms. For cats with IMHA or other secondary conditions requiring steroid therapy, longer term use may be required. As steroids have an immune suppressing effect, their use should be minimized whenever possible. For cats with ocular FIP, topical steroids are often prescribed to manage uveitis. When FIP patients require anti-inflammatory treatment, NSAIDs can be considered as an alternative where appropriate (Taylor et al., 2025).
References:
*Roman, N. (2024). Feline infectious peritonitis. https://merckvetmanual.com/infectious-diseases/feline-infectious-peritonitis/feline-infectious-peritonitis
*Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf
*Taylor, S. (2024). FIP: so much has changed, where are we now? (Webinar transcript). Bova Scholars. https://bova.vet/2024/04/04/fip-changes/
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