The treatment of feline infectious peritonitis (FIP) in the UK – an update FIP treatment protocols – what’s new?

Last updated: 02/2024

Sam Taylor BVetMed(Hons) CertSAM DipECVIM-CA MANZCVS FRCVS

Séverine Tasker BVSc BSc DSAM PhD DipECVIM-CA FHEA FRCVS

Danielle Gunn-Moore BSc(Hon), BVM&S, PhD, MANZCVS, FHEA, FRSB, FRCVS

Emi Barker BSc BVSc PhD PGCertTLHE DipECVIM-CA MRCVS

Stephanie Sorrell BVetMed(Hons) MANZCVS DipECVIM-CA MRCVS

Thank you to Richard Malik & Sally Coggins for their advice and assistance in producing this document.

The above specialists have come together to run the ‘FIP advice’ email address ([email protected]) answering queries on the new treatments on a voluntary basis and disseminating information to vets and vet nurses in the UK.

FIP treatment protocols – what’s new?
Use of oral GS-441524 for the whole treatment course, including at the start
Injectable remdesivir is reserved for cats that cannot be medicated orally
What dosages of GS-441524 and remdesivir should I use when treating FIP?
What should I do in the event of relapse of FIP?

If you are a cat owner, please watch our animal owner educational webinars.

FIP treatment protocols – what’s new?

(Updated February 2024)

Legally available antivirals in the UK and other countries via import now include remdesivir (injectable), GS-441524 (oral suspension and oral tablets), and EIDD-1931 (oral tablets). The following advice is based on published and unpublished data and experience. Treatment of individual cases remains the responsibility of the attending veterinary surgeon. The dosages below are based on experience using reputable preparations of known antiviral content. Extrapolation is not applicable to other oral preparations where the active component and/or its content are not known or provided by the manufacturer.

Use of oral GS-441524 for the whole treatment course, including at the start

Oral GS-441524 (available as a suspension of 50 mg/ml and tablets of 50 mg tablet) can be used from the start of FIP treatment for the entire (e.g., 12-week/84-days) course. It is important to support owners in medicating their cats, which can be challenging. Oral GS-441524 suspension or tablets can be given with a small treat (tablets can be crushed for this) or directly into the cat’s mouth. Further study is needed to review the effect of food on absorption, but it is recommended to give in a small treat or on an empty stomach, leaving a gap of an hour or more before feeding a larger meal.

Fasting cats overnight can increase their hunger to facilitate medicating in the morning, and similarly for an evening dose. However, starving kittens is never recommended as they cannot cope with this. Any withholding of food needs to be tailored to the age of the cat.

Injectable remdesivir is reserved for cats that cannot be medicated orally

Injectable remdesivir (10 mg/ml) is effective in the treatment of FIP but is associated with some side effects (see below), particularly pain on subcutaneous injection which is seen in 50% of cats. Previous FIP treatment protocols suggested this be used at the start of treatment before transitioning to oral GS-441254. However, we now know that FIP cats can be treated successfully with oral GS-441524 from their first day of treatment. This avoids pain on injections and reduces the costs of the treatment (the dose for the weight of the cat using GS-441524 is cheaper than remdesivir). Use of injectable remdesivir should be reserved for the following situations:

Severe neurological signs and inability to swallow or tolerate oral medication;
Extremely dehydrated/unwell cats;
Cats that cannot be orally medicated for other reasons.

In some circumstances, if a cat is hospitalised and has a poor appetite, which is affecting the ability to medicate it, 48 hours of remdesivir (given intravenously not subcutaneously) can result in significant clinical improvements which may facilitate subsequent oral medication with GS-441524. The remainder of the treatment course can then be given as an oral GS-441524.

The transition between remdesivir and oral GS-441524 can be immediate, i.e., from one treatment to the next.

The current recommendation is to treat for 84-days minimum. Some cats have been successfully treated with shorter courses, but large case studies have not yet been published. If cost constraints necessitate shorter courses, the dosage used should not be reduced and treatment sustained for as long as possible.

What dosages of GS-441524 and remdesivir should I use when treating FIP?

With experience, and as yet unpublished data on therapeutic drug monitoring (TDM), dosage recommendations have increased from previous FIP treatment protocols. However, evidence shows that over 85% of cats respond to the previously recommended drug dosages, which is still high. However, based on TDM studies, we now know that individual cats vary in their absorption of oral GS-441524, with those absorbing poorly requiring higher dosages to achieve clinical and biochemical remission. Ideally, dosage should be adjusted based on TDM, if available (see below), or response to treatment.

Compared to previous FIP treatment protocols, the important changes to dosage recommendations are:

Dosage of oral GS-441524 given as a divided dose, twice a day (every 12 hours), to optimise serum levels of GS-441524;
Higher dosages may overcome issues with poor absorption in some cats and have a better chance of crossing the blood brain barrier and the blood eye barrier;
Dosage should be adjusted according to response, and TDM if available.

Effects of Successful Treatment on Weight Gain and Growth in Kittens: Dose Adjustment Considerations — Table 1

Cats should be re-examined after 1-2 weeks (sooner if not improving or deteriorating) and dosage adjusted depending on monitoring at this point.

NOTE ON WEIGHING CATS: It is very important to weigh cats weekly during treatment, using accurate scales e.g., cat or baby scales. Weight gain and/or growth in kittens will occur with successful treatment necessitating an increase in dose to ensure that the dosage of antiviral administered is still appropriate for the type of FIP being treated as in Table 1. Not increasing the dose as the kitten grows appears to be one of the most common causes for a poor response to treatment, and treatment failure.

What should I do in the event of relapse of FIP?

e.g., recurrence or lack of resolution of effusion, pyrexia, development of new ocular or neurological signs, or persistent clinical pathology abnormalities:

Ensure that you are still confident that the cat has FIP; review the diagnosis, look for additional pathology, and consider repeat sampling (e.g., external laboratory analysis and culture of any fluid; cytology or biopsy of lymph nodes ± feline coronavirus antigen or RNA detection, but bear-inmind that finding the virus is more difficult when on treatment), AGP;
Consider TDM if available to check serum GS-441524 levels to inform dosing;
If relapse occurs during treatment; increase the dosage of GS-441524 (or remdesivir) by 2-3 mg/kg per dose and monitor as above, ensuring treatment is not stopped before the cat has been normal clinically and on clinical pathology for at least 2 weeks. The increased dosage used will depend on the dosage the cat is on at the time of the relapse, the nature of the relapse and finances, but can be up to that recommended for neurological FIP (see dosage table above) or even higher (please seek guidance when considering this);
If relapse occurs after completion of treatment; restart GS-441524 (or remdesivir) course at a higher dosage (minimum 2-3 mg/kg per dose higher than used previously) and ideally treat for another 12 weeks. The increased dosage used will depend on the dosage the cat was on before its relapse and the nature of the relapse, but can be up to that recommended for neurological FIP;
If the cat is already receiving a high dosage of GS-441524 and/or TDM serum levels are adequate, consider switching to EIDD-1931 (see below) and seeking guidance (FIP advice email or specialists), as adjunct treatments such as mefloquine, feline interferon or polyprenyl immunostimulant may be options (see below).

Treatment with EIDD-1931

This drug is another antiviral effective for the treatment of FIP in cats, although knowledge of its usage is much less than for GS-441524. The recommended dosage is 15 mg/kg every 12 hours, and it is available in 60 mg tablets for oral use. Potential adverse effects include cytopenia, especially neutropenia, rarely pancytopenia, reduced appetite/nausea, increased ALT enzyme activity and, potentially, renal compromise. Use of EIDD-1931 should be reserved for:

Cats failing to respond to treatment with GS441524 or remdesivir despite adequate dosage (ideally assessed with TDM);

Cats relapsing after treatment with GS-441524 or remdesivir at adequate dosages.

Treatment with feline interferon (IFN), polyprenyl immunostimulant, or mefloquine

Combinations of IFN omega, polyprenyl immunostimulant, and mefloquine have been used in the period following the end of treatment with GS-441524 (or remdesivir) in some cats; however, currently there is no evidence to suggest they are needed as high response rates of over 85% have been seen without these adjunct treatments;
Mefloquine has also been used to treat cats with FIP when cost constraints absolutely prohibit the use of a full course of, or increased dosage of, more effective antivirals such as GS-441524. Studies are needed to evaluate its effectiveness but it should only be used when absolutely no alternatives are available as GS-441524 is known to be very effective.

For information about what to monitor while treating FIP please read ‘What to look out for when treating FIP’. We also have some hints and tips based on common questions from vets which you can read FAQs about treating FIP with oral GS-441524 and/or injectable remdesivir. If you prefer webinar content, please watch ‘Feline Infectious Peritonitis – A new era in diagnosis and treatment ’.

Further reading

Tasker, S.; Addie, D.; Egberink, H.; Hartmann, K.; Hofmann-Lehmann, R.; Hosie, M. J.; Truyen, U.; Belak, S.; Boucraut-Baralon, C.; Frymus, T.; Lloret, A.; Marsilio, F.; Pennisi, M. G.; Thiry, E.; Mostl, K., ABCD Guidelines Feline Infectious Peritonitis – factsheets & diagnostic tools. 2022, accessed 16^th January 2023. https://www.abcdcatsvets.org/portfolio-item/factsheets-tools-for-feline-infectious-peritonitis-fip/

Thayer, V.; Gogolski, S.; Felten, S.; Hartmann, K.; Kennedy, M.; Olah, G. A., 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. Journal of Feline Medicine and Surgery 2022, 24, (9), 905-933. https://journals.sagepub.com/doi/full/10.1177/1098612X221118761