An update on treatment of FIP in the UK
Dr. Sam Taylor BVetMed(Hons) CertSAM DipECVIM-CA MANZCVS FRCVS
Prof. Séverine Tasker BVSc BSc DSAM PhD DipECVIM-CA FHEA FRCVS
Prof. Danielle Gunn-Moore BSc(Hon), BVM&S, PhD, MANZCVS, FHEA, FRSB, FRCVS
Dr. Emi Barker BSc BVSc PhD PGCertTLHE DipECVIM-CA MRCVS
Dr. Stephanie Sorrell BVetMed(Hons) MANZCVS DipECVIM-CA MRCVS
The above specialists have come together to run the ‘FIP advice’ email address (fipadvice@gmail.com)
answering queries on the new treatments on a voluntary basis and disseminating information to vets andvet nurses in the UK. So far, they have answered over 100 emails on the advice line

Introduction

In August 2021 remdesivir (Figure 1) became legallyavailable to UK vets for the treatment of FIP in cats.
Since that time many cats and kittens have beentreated and are still being treated successfully. As withany novel formulation, with experience, adjustments
are made in protocols and given the recent release(November 2021) of oral GS-441524 (50mg tablets)from a specials manufacturer in the UK (Figure 2), thisarticle has been created to support practitioners in theuse of remdesivir and GS-441524 in the managementof FIP. It is worth remembering that treatment may
need to be tailored to the individual cat based on
response, compliance and client finances. Specificprotocols are listed below to help vets make decisions
with their clients, but will not be appropriate for all.

daily dosage. This was ultimately more expensive thanif the treatment course had been started at a higher
dosage.

Figure 2: Oral GS-441524 tablets

Figure 1: Remdesivir for intravenous or
subcutaneous injection

Treatment protocols (updated November 2021)

Drug dosages have increased from previous
recommendations, based on the experience of ourAustralian colleagues who have treated over 600 cats.
Although some cats responded to the previously
recommended lower dosages, they found thatrecurrence at, or towards the end of, the 84-day(12-week) treatment period was possible, resulting in
the need for extension of the treatment at a higher

When using remdesivir and/or GS-441524, treatment
options now include a 12-week course of injectableremdesivir, transition from injectable remdesivir to oral
GS-441524, or an entirely oral GS-441524 protocol.
Suggested dosages, benefits and limitations of each
protocol are provided below. Remdesivir cannot begiven orally. Recommended drug dosages (Table 1)
depend upon clinical presentation – i.e. whether there
is an effusion present or not and whether there isocular and/or neurological involvement – this is due tovariation in the tissue penetration of the drugs. Where
there is doubt, use of the higher dosage is preferable.
Please note that these dosages of oral GS-441524 arehigher than quoted in some publications – this isbecause these publications have used ‘black market’
preparations of so-called GS-441524 in which theamount of active agent given to the cats was notconfirmed. The dosages provided in this article arebased on experience using an oral preparation ofNeed advice on the treatment of FIP?
If advice is needed on the diagnosis and treatment of a suspected case of FIP, please email
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known GS-441524 content legally available in the UKand Australia. Thus extrapolation is not applicable toother oral preparations where the active component
and/or its concentration are not known or given by themanufacturer.

started the next day.
The protocol chosen will depend on the severity of the
FIP disease in the cat. Dosages are shown in Table 1.

Combined injectable & oral treatment
protocols:

1. Initial treatment with once daily intravenous
remdesivir (Table 1) for 3-4 days i.e. on days 1, 2, 3
± 4. This provides a loading dose of the drug. On
each day, dilute the remdesivir dose required to a
total volume of 10ml with saline and administerslowly over 20-30 minutes manually or with asyringe driver.
2. Follow with once daily subcutaneous remdesivir at
the same dosage (Table 1) up to day 7-14
3. Change to once (or twice) daily oral GS-441524(Table 1) on day 8-15, and continue until at leastday 84.

The decision on when to move from injectableremdesivir to oral GS-441524 may depend on the cat’s
tolerance of injections (or oral administration oftablets), formulation price differences (including cost ofneedles, syringes, sharps disposal, wastage), owner
preferences and finances.
Experience suggests this transition can be madebetween 7 and 14 days of starting remdesivir IV orsubcutaneous treatment. The change can be madedirectly; remdesivir is given one day and GS tablets

Severe disease (anorexic, dehydrated, cat
usually will be hospitalised)

Table 1: Summary of dosage recommendations for remdesivir and GS-441524

Clinical presenta�on
Cats with eﬀusions andwithout ocular or neurologicalsigns
No eﬀusion and without
ocular or neurological signs
Ocular signs present (eﬀusiveand non-eﬀusive)
Neurological signs present(eﬀusive and non-eﬀusive)

Remdesivir – by injec�on
10 mg/kg once daily

GS-441524 – oral
10-12 mg/kg once daily

12 mg/kg once daily

10-12 mg/kg once daily

15 mg/kg once daily

15 mg/kg once daily

20 mg/kg once daily

10 mg/kg twice daily (i.e. 20mg/kg given as a divided dose)Less severe disease (normal hydration, eating)
1. Initial treatment with once daily subcutaneous
remdesivir (Table 1) up to day 7-14
2. Change to once (or twice if very high neurologicaldosage needed) daily oral GS-441524 (Table 1) on
day 8-15, and continue until at least day 84.

Oral only treatment protocol:

An oral GS-441524 treatment only protocol isrecommended if injectable not tolerated/possible financially:
1. Once (or twice if very high neurological dosageneeded) daily oral GS-441524 (Table 1) until atleast day 84.

Potential adverse effects of remdesivir
Remdesivir seems well tolerated. However, the following adverse effects have been reported:• Transient local discomfort/stinging on injection (see later on prevention);• Development/worsening of a pleural effusion (not always proteinaceous) in the first 48 hours oftreatment, sometimes requiring drainage;• Cats may seem depressed or nauseated for a few hours after IV administration;• Increases in ALT enzyme activity have been reported (unclear if due to underlying FIP diseaseor an adverse drug effect);• Mild peripheral eosinophilia has been reported.
NOTE ON WEIGHING CATS: It is very important toweigh cats weekly during treatment, using
accurate scales – weight gain and/or growth inkittens will occur with successful treatment
necessitating an increase in dose to ensure thatthe dosage of antiviral administered is still
appropriate for the type of FIP being treated.

Options for cost limited clients – please note

that, ideally, therapy should be given using therecommended formulations and dosages for as longas possible (up to 84 days) to increase likelihood ofcure. Only take the options below if absolutely
necessary, as relapse may occur, which then requires
longer treatment, increasing costs;

Need advice on the treatment of FIP?
If advice is needed on the diagnosis and treatment of a suspected case of FIP, please email
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• Give oral GS-441524 treatment only for 84 days, asoutlined above;• Give injectable remdesivir or oral GS-441524 for as
many days as the owner can afford before switching
to oral mefloquine 62.5mg 2-3 times weekly (largecat, give 3 times a week) or 20-25mg orally once daily(if reformulation of tablets is possible e.g. PCCA Ltd)for completion of an 84-day treatment protocol;
mefloquine is cheaper than remdesivir andGS-441524 but more research is needed to judge itseffectiveness in this situation;• If an increase in remdesivir dosage is required (e.g.
due to neurological disease appearing duringtreatment) but cannot be afforded, mefloquine
treatment can be added as adjunct treatment, as thisis cheaper than remdesivir, although more research isneeded to judge the effect of this combination;• Feline interferon omega has also been used in theperiod following treatment with remdesivir/GS-441524
treatment, but further research is needed on thiscombination to judge if it is necessary.

Are oral treatments given with or withoutfood?

• GS-441524 is given on an empty stomach (washdown with a little water) – food can be given 30 mins
after treatment;• Mefloquine is given with food, otherwise vomiting
often results.
Do not forget to support clients giving oral medications,as this can also be challenging. Direct clients to theiCatCare website for information and videos:https://icatcare.org/advice/how-to-give-your-cat-a-tablet/

What can I do to help the owners give the subcutaneous remdesivir?
Injection with remdesivir can cause transient local discomfort. The following may help reduce discomfort andimprove compliance:• Ensure owners use a new needle each time to withdraw the drug from the bottle (this will reduce the risk ofbacterial contamination of the bottle, as well as alcohol swabbing the reusable seal top of the bottle before
entry of the needle);• Ensure owners change the needle after withdrawing the drug from the bottle and before injection(puncturing the reusable seal will blunt the needle);• Needle size preference varies; some prefer a 21G needle to make injecting quicker, others find a finer
23G needle is better tolerated, so it may be worth trying both if problems arise;• Rotate the injection sites;• Have remdesivir at room temperature before administration;• Oral gabapentin (50 to 100 mg per cat) may be helpful and/or transmucosal or subcutaneous
buprenorphine given at least 30-60 mins before the remdesivir injection to induce mild
sedation/analgesia;• The area to be injected can also be clipped to help owners locate the appropriate site to inject and so that
topical EMLA cream can be applied 40 mins before injection, although surface desensitisation may not help
as it is usually the remdesivir under the skin that causes discomfort;• Ensure the full dose of injection is administered at each time-point and encourage owners toreport any mishaps as this may influence decisions if relapse occurs;• Cats will need several weeks of treatment. Encourage owners to make the injection experience
more positive by using treats (e.g. Lick-e-lix, Dreamies) around the time of injection, or stroking,brushing, or playing with the cat if they are less food motivated. Suggest owners spend time each daywith their cat positively engaged to avoid any damage to cat-owner relationships that can reducecompliance.

What should I expect during treatment?

• In the first 2-5 days you should see an improvement
in demeanour, appetite, resolution of pyrexia andreduction in abdominal (Figure 3) or pleural fluid if
an effusion is present (note that in some cases
pleural fluid can transiently worsen in the first
couple of days– if the cat is at home, advise owner
to measure resting respiratory rate, plus respiratoryeffort) – effusion typically resolves by 2 weeks;• If an effusion is still present at 2 weeks, consider
increasing dosage to one that is greater than thatbeing used e.g increasing the dosage from thatused for cats with effusions only;• Serum albumin increases and globulin decreases(i.e. they normalise) over 1-3 weeks, but note that
globulins can initially increase when a large volume
effusion is absorbed;• Lymphopenia and anaemia may take longer toresolve, up to 10 weeks;• Mild peripheral eosinophilia is a common finding
and may be a favourable marker for disease
resolution, as it is in COVID patients;

Need advice on the treatment of FIP?
If advice is needed on the diagnosis and treatment of a suspected case of FIP, please email
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• Lymph node size reduces over a few weeks;• If progress is not as expected, consider reviewing
the diagnosis (see below) and/or increasing dosage.

• Confirm resolution of previous abnormalities(clinically, POCUS, serum biochemistry, andhaematology);• Only stop treatment once cat has been normal(clinically and on serum biochemistry andhaematology) for at least 2 weeks (ideally 4 weeks).

If I am seeing no response or only a partial
response to treatment, what do I do?

• Ensure that you are still confident that the cat has FIP– review diagnosis, look for additional pathology,
consider repeat sampling (e.g. external laboratory
analysis of any fluid; cytology or biopsy of lymph
nodes);• If biochemical abnormalities (hyperglobulinaemia andthe albumin to globulin ratio in particular) remainpresent after 6-8 weeks, then increase dose as forrelapse (below) by 3-5 mg/kg per day and continue
the course, not stopping until parameters normalise
for at least 2 weeks as stated above for ‘when do I
stop treatment?’ – This may well include extending
the course over 12 weeks.

What do I monitor after treatment?
Figure 3: A cat with FIP and ascites. Effusions
should start to resolve over 3-5 days after starting
treatment

What do I need to monitor during treatment?

• Ideally, serum biochemistry and haematology after 2weeks and then monthly;• For cost limited clients, monitor
weight/demeanour/effusions (e.g. by in-housescanning)/neurological signs/key biochemical
abnormalities only (e.g. measuring just globulin,bilirubin or spinning microhaematocrit tube for
PCV/TP/colour of plasma);• NB. ALT enzyme activity may increase – it is notclear if this is due to FIP pathology vs. drug reaction,and it is not usually a reason to stop therapy. It is notknown if the addition of hepatoprotective therapy(e.g.SAMe) is helpful in these cases;• Point-of-care ultrasonography (POCUS) to monitor
for effusion resolution and/or lymph node size.

If I am seeing a positive response to treatment,
when do I stop treatment?
• Not before 84 days (12 weeks);• Advise the owner to monitor the cat closely for any
clinical relapse – this monitoring should continue for
12 weeks after completion of treatment;• Ideally, repeat serum biochemistry and haematology
two weeks and one month after stopping treatment(to detect any changes that could suggest earlyrelapse);• Note that relapse can occur with clinical signs but
without any significant biochemical/haematologicalabnormalities.

In the event of relapse

e.g. recurrence of effusion, pyrexia, development ofocular or neurological signs, or return ofhyperglobulinaemia:• Ensure that you are still confident that the cat has FIP– review diagnosis, look for additional pathology,
consider repeat sampling (e.g. external laboratory
analysis of any fluid; cytology or biopsy of lymph
nodes);• If relapse occurs after completion of treatment –restart treatment with remdesivir or GS-441524 at ahigher dosage (typically 3-5 mg/kg higher per day
than dosage used previously) and treat for another 12weeks. The increased dosage used depends on thedosage the cat is on at the time of the relapse and theNeutering & routine treatments during therapy for FIP• Neutering is ideally performed a month after treatment is completed if the cat has responded. However, if
leaving unneutered is causing much stress e.g. attempts to escape or distress when queens are on heat,neutering during the treatment course may be preferred. If the latter is needed, neutering should ideallybe performed at a time when the cat is doing well on treatment and still has 2 weeks of treatment
remaining following the date of neutering (so antiviral treatment ongoing at a potential time of ‘stress’
after neutering);• There is no contraindication to routine worming and flea treatment for cats on remdesivir or GS-441524;• No information is available on vaccination of cats receiving treatment for FIP. Vaccines should beadministered as normal if the cat is well during treatment, as still likely to be protective. For cats that havereceived an initial course, consider providing a third dose of vaccine after completion of FIP treatment(see WSAVA Vaccination Guidelines);• If veterinary procedures are necessary, clinic stays should be minimised, and Cat Friendly Clinic(www.catfriendlyclinic) protocols and handling implemented to avoid stress to the cat.

nature (e.g. severity and/or development ofneurological signs) of the relapse, but can be up tothat recommended for neurological FIP (20 mg/kg- see Table 1 earlier). It is possible some cats will
respond to a shorter course but ideally treatment
for relapse after completion of a course oftreatment is continued for the full 12 weeks to limit
repeat relapse;• If it is not possible to increase the dose ofremdesivir or GS-441524 (e.g. the highest
neurological dosage of 20 mg/kg is already in use),consider use of mefloquine as adjunct treatment(see above) whilst continuing remdesivir or
GS-441524 treatment at the same dosage.

Adjunctive treatments• If the cat is on prednisolone treatment, this should
be stopped whilst giving remdesivir or GS-441524,
unless it is required for short term management ofspecific immune-mediated disease arising as aresult of FIP e.g. haemolytic anaemia;• Supportive therapies such as antiemetics, appetite
stimulants, fluid therapy and analgesics can begiven with remdesivir or GS-442415 as required.

Potential future updates

We are constantly learning about treatment withthese drugs and advice may change in time. Otheragents, e.g. protease inhibitors (e.g. GC374) andother nucleoside analogues (e.g. molpurinavir) havealso been trialled in cats, but are not commerciallyavailable at this time. How these agents and otherimmunomodulatory agents (e.g. polyprenylimmunostimulant) will fit into a future protocols is
unknown at this time.

Updated 10th March 2022

Thank you to Richard Malik and Sally Coggins fortheir advice and assistance producing thisdocument.
Dr. Richard Malik DVSc MVetClinStud PhD FASM
graduated from the University of Sydney in 1981. He is a
specialist in small animal internal medicine and has a
special interest in infectious diseases of dogs and cats.
He works for the Centre for Veterinary Education helping
to organise CPD.
Dr. Sally Coggins BVSc (hons I) MANZCVS (FelineMedicine) graduated with first-class honours from The
University of Sydney in 2007. Sally is currently
investigating novel antiviral therapeutics for Feline
Infectious Peritonitis and is currently conducting clinical
trials open for national recruitment.

ISFM supports the creation of evidence-based documents to support vets with
their clinical decision making and encourages the collaboration of experts tohelp cats. For further information on ISFM's work and becoming a member visit
https://icatcare.org/veterinary/isfm/vet-membership/.

Need advice on the treatment of FIP?
If advice is needed on the diagnosis and treatment of a suspected case of FIP, please email
fipadvice@gmail.com