Feline Infectious Peritonitis (FIP)
FIP, feline coronavirus disease, wet FIP / effusive FIP, dry FIP / non-effusive FIP, neurological FIP, ocular FIP, FCoV-associated pyogranulomatous perivasculitis
FIP stands for feline infectious peritonitis. It starts with a very common gut virus called feline coronavirus, which most cats carry without ever getting sick. In a small number of cats that virus changes inside the body and begins to inflame blood vessels all over, so the belly or the chest fills with fluid and the cat runs a fever, stops eating, and loses weight - or a kitten simply stops growing. FIP is fatal if it is not treated. Until about 2019 it was almost always fatal. That has changed. An antiviral medicine called GS-441524 now puts most treated cats into lasting remission: across 11 published studies covering 650 treated cats, about 85 in 100 were treated successfully - but those studies were mostly uncontrolled, some used antiviral combinations, and success was LOWER in cats with fluid build-up (the commonest form) and in cats whose disease reached the brain. The published literature reports remission, not a guaranteed cure, and long-term follow-up beyond six to twelve months is still thin. The single most important thing you can do is get a proper diagnosis fast and start real antiviral treatment. No supplement, immune booster, stem cell injection or exosome product substitutes for it, and using one instead will cost your cat time it does not have. FOUR SAFETY THINGS TO KNOW BEFORE YOU DO ANYTHING ELSE. (1) NEVER give your cat human pain or fever medicine. Not acetaminophen or paracetamol (Tylenol, Panadol) - that one can kill a cat outright. Not ibuprofen (Advil, Motrin), not naproxen (Aleve), not aspirin. FIP causes a fever that keeps coming back and owners reach for the bathroom cabinet; do not. The fever settles because the antiviral works. (2) NEVER give a cat a medicine that was prescribed for a dog, or for another cat, or for a person. Dog anti-inflammatories such as carprofen and dog-strength meloxicam are especially dangerous to cats. Species-specific dose, species-specific product, every time. (3) Do not buy this antiviral from an online seller or a social-media group. When researchers tested 127 samples from 30 of the most popular unlicensed brands, 95% of the 87 injectable samples contained MORE drug than the label claimed (on average 39% more) at an average pH of 1.30 - far more acidic than anything meant to go under a cat's skin - while 58% of the oral samples contained LESS than claimed (on average 39% less), and three samples also contained undisclosed remdesivir. Underdosing is how FIP comes back. (4) Once your cat is on treatment at home, CALL THE CLINIC BEFORE YOU CHANGE ANYTHING - before you change the dose, split or crush a tablet, stop early, restart after a gap, switch product or pharmacy, or add any other medicine, supplement or 'immune support'. Do not adjust a dose yourself because your cat looks better; cats look completely normal weeks before the course is finished, and stopping early is the commonest avoidable cause of relapse.
What you might notice
- A fever that comes and goes, keeps coming back, and does not get better on antibiotics - and never treat it with human fever medicine (see the safety note about acetaminophen, ibuprofen, naproxen and aspirin)
- Eating much less than usual, or refusing food altogether
- Losing weight - or, in a kitten, simply stopping growing while littermates keep getting bigger
- A belly that swells and feels like it is full of liquid, sometimes while the rest of the cat looks thin
- Fast, hard or open-mouth breathing, or breathing with the elbows pushed out from the body - fluid can collect around the lungs, and this can be easy to miss when everyone is watching the belly
- Very low energy: sleeping much more, hiding away, no interest in play
- Pale, white or grey gums, or sudden weakness - FIP causes anaemia, and it can become severe enough to need an emergency blood transfusion in the first days of treatment
- A change in one or both eyes - cloudiness, a colour change in the iris, an odd-looking pupil, visible redness, squinting or a cat that seems to bump into things. Eye disease in FIP is commoner than owners expect and can threaten sight: in a prospective study of 118 cats, 48 (40.1%) had ocular abnormalities on a proper eye examination, most often anterior uveitis, keratic precipitates and chorioretinitis - many of which an owner would never notice (PMID 41644507)
- Wobbliness, stumbling, a head tilt, tremors, circling or a seizure - FIP can reach the brain and spinal cord
- A yellow tinge to the gums, the whites of the eyes or the skin inside the ears (jaundice)
Standard of care
Make the diagnosis, and make it fast - but do not skip it
Approved / guideline-backedFIP treatment is a 6-12 week commitment with real cost, and several diseases mimic it. Work up the case properly: FIV/FeLV testing, bloods with an A:G ratio and glucose, AGP, imaging, cardiac and ophthalmic assessment, and above all sampling of any effusion for cytology plus FCoV RT-qPCR or antigen detection. The ABCD publishes FIP Diagnostic Approach Tools designed for practice use. At the same time, do not let a perfect diagnostic pathway delay treatment in a collapsing cat - FIP is fatal untreated, so a well-reasoned clinical diagnosis with supportive effusion findings is a legitimate basis to start while confirmatory results are pending.
Named products: No drug at this step - this is the diagnostic gate.
The ABCD guidelines state that making a diagnosis is complex, that sampling an effusion for cytology, biochemistry and FCoV RNA or antigen detection is 'very useful diagnostically', and that definitive diagnosis usually requires histopathology with FCoV antigen detection. Felten and Hartmann tabulate the sensitivity and specificity of every available direct and indirect test by sample type and publish diagnostic trees for practice use.
Start a nucleoside-analogue antiviral - this is the treatment, and nothing else replaces it
Approved / guideline-backedGS-441524 is the usual first choice, and that is guideline consensus plus depth of reporting rather than demonstrated superiority: ABCD recommends nucleoside analogues, and GS-441524 is by far the most extensively reported agent, but NO head-to-head randomised trial has compared it against oral remdesivir or molnupiravir, and the reported outcomes of those agents in their own uncontrolled cohorts are broadly comparable. Get the chemistry right, because the page must not contradict itself: GS-441524 is the PARENT NUCLEOSIDE of remdesivir; remdesivir (GS-5734) is its monophosphoramidate prodrug; both are phosphorylated inside cells to the same active nucleoside triphosphate. GS-441524 is therefore not itself 'the active form'. Be explicit with owners about what this drug is and is not: it is highly effective, and it is also not an approved veterinary medicine anywhere - there is no FDA NADA number and no EMA marketing authorisation for any GS-441524 product. In the United States it reaches patients only as a pharmacy-compounded, unapproved animal drug dispensed on a patient-specific veterinary prescription under FDA's enforcement-discretion position (see the regulatory step below).
Named products: GS-441524 (adenosine/nucleoside analogue). NO FDA APPROVAL (no NADA) AND NO EMA MARKETING AUTHORISATION EXISTS FOR ANY GS-441524 PRODUCT IN ANY SPECIES as of 12 September 2026 - do not imply one, and never write 'approved' without naming the regulator, species and indication. In the US, compounded oral dosage forms in general use are scored 50 mg tablets and an oil-based oral suspension supplied at 50 mg/mL. WE DELIBERATELY DO NOT NAME A BRAND OR A DISPENSING PHARMACY ANYWHERE ON THIS PAGE: naming one identifiable commercial compounded preparation next to an efficacy statement reads as promotion of an unapproved new animal drug for a specific indication, and FDA's 10 May 2024 enforcement discretion covers patient-specific veterinary prescribing, not third-party promotion. PETSMARTMEDS DOES NOT AND WILL NOT SELL, RESELL, IMPORT, DISTRIBUTE OR BROKER GS-441524, REMDESIVIR OR MOLNUPIRAVIR. Ask your veterinarian to write a patient-specific prescription and have it filled by a state-licensed compounding pharmacy.
SPECIES: CAT, direct clinical evidence. CLAIM CEILING: in published case series and one randomised controlled trial, most cats with FIP treated with GS-441524 achieved clinical remission; no GS-441524 product is approved by FDA or EMA for any species. Pedersen's field study of 31 cats with naturally occurring FIP produced 25 long-term survivors, with the authors concluding GS-441524 'was shown to be a safe and effective treatment for FIP'. DISCLOSE THE LIMIT OF THAT STUDY: its own abstract states 'Cats with severe neurological and ocular FIP were not recruited', and its methods record that cats with obvious ocular or neurological disease were discouraged from entering because of concerns about blood-brain and blood-eye barrier penetration. Pedersen 2019 therefore cannot evidence efficacy in severe neurological or ocular FIP. For neurological FIP the citation is Dickinson et al. 2020: four cats with clinically diagnosed neurological FIP treated with GS-441524 at 5-10 mg/kg for at least 12 weeks, all with a positive response; three alive off treatment at 528, 516 and 354 days with normal physical and neurological examinations, and one euthanised 216 days after treatment initiation following relapses after primary and secondary treatment. The authors conclude that treatment 'shows clinical efficacy and may result in clearance and long-term resolution of neurological FIP' and that 'Dosages required for CNS disease may be higher than those used for nonneurological FIP'. The systematic review of 11 studies and 650 treated cases found an overall treatment success rate of 84.6%, higher with antiviral combinations and lower in wet FIP and in cases with neurological complications; publish its authors' own caveat alongside the number - 'randomized controlled trials are needed to establish evidence-based therapeutic protocols tailored to different FIP presentations'. The ABCD guidelines state that 'antiviral treatments now enable recovery in many cases from this previously fatal disease; nucleoside analogues (e.g., oral GS-441524) are very effective, although they are not available in all countries.'
Dose and duration: 15 mg/kg orally every 24 hours, and 42 days was enough in the randomised trial's population
Controlled trial in this speciesThe default taught for years was 84 days. A prospective randomised controlled study has now shown that 42 days of oral GS-441524 at 15 mg/kg every 24 hours is equally effective to 84 days in the population it enrolled. This matters enormously for owners: it roughly halves the cost and the pilling burden of the single most effective treatment in feline infectious disease. Be precise about who was studied, because the trial's actual inclusion criteria are narrower than 'cats with FIP' in ways that bear directly on dosing: (1) a diagnosis of FIP, (2) abdominal and/or pleural EFFUSION present, (3) body weight of AT LEAST 2 KG, (4) negative FIV-antibody and FeLV-antigen tests, and (5) no other severe unrelated disease. There was NO exclusion of neurological or ocular cases. The 2 kg floor matters on this page: FIP is overwhelmingly a disease of young cats and kittens that 'simply stop growing', and no cat under 2 kg was enrolled, so the 42-day/15 mg/kg evidence does not extend to a 1.4 kg kitten. Be equally precise about what the trial concluded, because it is the opposite of the cautious line often attributed to it. Two surviving cats - one in each arm - additionally had neurological or ocular signs that resolved on treatment and stayed in remission to day 168, and the authors write that 42 days 'is likely sufficient to achieve complete remission not only in FIP cats with effusion but also with neurological and/or ophthalmological symptoms', immediately adding 'Further studies are necessary to support this assumption'. OUR POSITION, ATTRIBUTED HONESTLY: we still advise against shortening the course in neurological or ocular disease, in non-effusive disease, in slow responders and in cats whose laboratory markers have not normalised. That is a deliberately conservative position resting on ABCD guidance, on the higher relapse rates reported in non-effusive and neurological disease, and on the published case of neurological re-emergence after a completed 42-day course - NOT on this trial, whose own authors reached the opposite conclusion while calling for further study. Say so on the page rather than misattributing our caution to them.
Named products: GS-441524, oral, 15 mg/kg every 24 hours in the randomised study. Pedersen's original SUBCUTANEOUS regimen was 2.0 mg/kg every 24 hours escalated to 4.0 mg/kg every 24 hours for at least 12 weeks; the higher oral figures reflect oral bioavailability, so NEVER transpose an oral number onto a subcutaneous route or vice versa. DOSE ROUNDING IS A REAL HAZARD, NOT A DETAIL. The common US oral solid is a 50 mg scored tablet breaking into 12.5 mg quarters. At 15 mg/kg a 2 kg cat needs 30 mg - not achievable with quarter-tablets, and rounding to 25 mg or 37.5 mg is a 17% underdose or a 25% overdose. Underdosing is a leading driver of relapse and of resistance selection; overdosing is not free either. The prescription must specify the dose in MILLIGRAMS PER DOSE and the veterinarian must state the rounding direction; small cats and kittens generally belong on the oral suspension, where an exact volume can be measured. Owners must never re-round a dose themselves, and must never assume a new tablet strength or a new pharmacy's product is interchangeable - call the clinic first.
SPECIES: CAT, prospective randomised controlled study. 40 cats with FIP with effusion, meeting all five inclusion criteria above (including a minimum body weight of 2 kg, imposed to comply with feline blood-sampling volume guidelines, and negative FIV/FeLV point-of-care tests), were randomised to oral GS-441524 15 mg/kg q24h for either 42 or 84 days and followed to day 168. Two cats died or were euthanised during treatment (days 3 and 31); the remaining 38 (19 per group) recovered with rapid improvement in clinical and laboratory parameters and marked reductions in viral load in blood and effusion. Clinical remission was achieved between days 14 and 84, median 28 days; 37 of 40 (92.5%) were in complete clinical remission within the first 42 days. Every cat that completed the short course successfully was still in complete remission on day 168. The authors conclude that 42 days at 15 mg/kg 'can be considered equally effective'. Separately, the pooled systematic review found the best outcomes at 5-10 mg/kg once daily or an equivalent subcutaneous dose, ADJUSTED FOR FIP TYPE, SEVERITY AND THE PRESENCE OF NEUROLOGICAL/OCULAR SIGNS, noting that higher doses may be split twice daily to avoid absorption problems.
Escalate for neurological and ocular FIP, and expect relapse to be a dosing problem rather than a treatment failure
Controlled trial in this speciesNeurological and ocular FIP need higher exposure because the drug must cross the blood-brain and blood-ocular barriers, and these are the cases that relapse. The evidence for escalation is Dickinson et al. 2020, whose authors state that dosages required for CNS disease may be higher than those used for non-neurological FIP, together with the systematic review, which endorses dosing adjusted for the presence of neurological and ocular signs and higher dosages for severe cases and relapse prevention. DO NOT CITE THE 42-DAY RANDOMISED TRIAL FOR THIS. That trial deliberately used a UNIFORM 15 mg/kg q24h oral dose for every cat regardless of neurological or ocular status, and gives its reasoning: 'in most cats, FCoV also invades the brain even without visible neurologic signs... Therefore, for cats both with and without neurological signs, a uniform dosage of 15 mg/kg q24h PO was chosen.' In that trial 15 mg/kg is already the neuro-adequate dose, not a baseline awaiting escalation. Handle relapse by revisiting dose and duration, not by abandoning the antiviral. In the original field study, every cat re-treated at the higher dose - including one with neurological disease - responded well and remained healthy, while cats re-treated at the original lower dose relapsed again.
Named products: GS-441524 at escalated dose: 5-10 mg/kg for neurological disease in Dickinson 2020; 10 mg/kg orally every 12 hours in the published neurological re-emergence case; 4.0 mg/kg subcutaneously every 24 hours in Pedersen's re-treatment arm. Remdesivir 16.7 mg/kg intravenously every 24 hours in that same case while oral dosing was not tolerated. Adjunctive levetiracetam and prednisolone in that case - a case report, not a protocol. None of these agents holds an FDA approval or an EMA marketing authorisation for cats.
SPECIES: CAT. Of 26 cats completing at least 12 weeks in the Pedersen study, 8 relapsed within 3-84 days (mean 23 days; 6 non-neurological, 2 neurological); all 5 re-treated at 4.0 mg/kg SC q24h responded and stayed healthy, whereas one of 3 re-treated at the lower dose relapsed neurologically and was euthanised and the other two relapsed a second time before succeeding at the higher dose. Dickinson 2020 treated four cats with neurological FIP at 5-10 mg/kg for at least 12 weeks: all responded, three were alive off treatment at 528, 516 and 354 days with normal neurological examinations, and one was euthanised at 216 days after relapsing through primary and secondary treatment. A 2025 SINGLE CASE REPORT documents neurological re-emergence 17 days after a completed 42-day oral course in a 6-month-old British Longhair whose systemic signs had fully resolved. STATE THE ACTUAL OUTCOME, WHICH WAS NOT A CLEAN RECOVERY: after one day of resumed oral GS-441524 the cat developed severe hypoventilation requiring intubation and mechanical ventilation for 1.5 days; treatment switched to remdesivir 16.7 mg/kg IV q24h for 4 days, then oral GS-441524 10 mg/kg q12h to day 84; and 'Treatment resulted in partial recovery with moderate ataxia and reduced left-sided menace response remaining 181 days after starting the second treatment.' Adjunctive levetiracetam 20 mg/kg q8h and prednisolone 1 mg/kg q24h (to day 21) were part of the initial protocol, so this is not an antiviral-monotherapy result. Use it to set expectations honestly: severe neurological FIP can leave permanent deficits even when the cat survives.
Where GS-441524 cannot be obtained, oral remdesivir and molnupiravir are the evidence-supported alternatives
Controlled trial in this speciesDo not let supply dictate that a cat goes untreated. Remdesivir is the monophosphoramidate prodrug of GS-441524 and has prospective feline data; molnupiravir is a different-mechanism nucleoside analogue with retrospective feline data and is also widely used as rescue therapy after relapse. Both are unapproved for cats by FDA and unauthorised by EMA, and carry the same compounding and legal caveats as GS-441524. MOLNUPIRAVIR CARRIES SPECIFIC REPRODUCTIVE, PAEDIATRIC AND HANDLER WARNINGS AND MUST NOT BE PRESCRIBED WITHOUT THEM. See the safety list. In outline: it is contraindicated in pregnant and lactating queens and should not be used in breeding animals; the human product is not authorised for anyone under 18 years of age because it may affect bone and cartilage growth, which is directly relevant to a disease of kittens; and an owner giving capsules twice daily for 12 weeks needs handling instructions.
Named products: Remdesivir (GS-5734): oral 30 mg/kg every 24 hours; parenteral 15-30 mg/kg every 24 hours in the same study. Molnupiravir: median 14.3 mg/kg orally every 12 hours for at least 84 days. Neither holds an FDA approval nor an EMA marketing authorisation for cats; in humans, molnupiravir (LAGEVRIO) is not FDA-approved for any use and is available only under Emergency Use Authorization.
SPECIES: CAT. Oral remdesivir 30 mg/kg q24h (rounded up to the nearest capsule size) in 29 prospectively recruited cats in New Zealand, alone or after initial parenteral remdesivir 15-30 mg/kg q24h: 25 (86%) entered remission and survived beyond 6 months (range 6-27 months), 22 (75%) achieved remission within 84 days on oral remdesivir, and 3 needed molnupiravir rescue. Relapse was strongly form-dependent - 4 of 8 (50%) non-effusive cases relapsed versus 1 of 20 (5%) effusive cases - so the authors recommend an increased dose rate or frequency in non-effusive disease and note the survival rate in non-effusive cats was significantly lower. For molnupiravir as first-line therapy in effusive FIP, a 40-week retrospective study of 42 cats at a median 14.3 mg/kg twice daily for at least 84 days reported resolution of clinical signs and effusion at a median of 7 days, a transient ALT rise that normalised completely by week 7 without stopping treatment, and no significant deviation in haematology or creatinine between week 12 and week 40. It is retrospective and uncontrolled, and it carries no reproductive or paediatric safety data in cats at all.
Supportive care alongside the antiviral - and drain effusions only for a reason, with a consent conversation about the risk
Approved / guideline-backedMost cats improve within days of starting an effective antiviral, so supportive care is a bridge, not a plan. Drain a pleural effusion when it is compromising breathing. Be conservative with large-volume abdominal drainage: the fluid is protein-rich and removing it repeatedly removes albumin from a cat that is already hypoalbuminaemic. CENTESIS IS NOT RISK-FREE AND THE PAGE MUST SAY SO. In the randomised trial, one of only two deaths involved pulmonary changes that the authors state 'possibly... occurred secondarily due to bacterial infection caused by thoracentesis'. Use strict aseptic preparation, ultrasound guidance, the smallest adequate needle or catheter, and controlled drainage; counsel the owner about iatrogenic pyothorax, pneumothorax, haemorrhage and re-expansion pulmonary oedema; and do not repeat a tap that is not answering a question or relieving a symptom. Otherwise: warmth, quiet, assisted or highly palatable feeding, anti-nausea support, fluid therapy where indicated, glucose support where hypoglycaemia is found, and transfusion for severe anaemia - which in this disease can be needed in the first days, not weeks. Weigh the cat at every visit; in kittens, resumption of growth is one of the most honest signs the treatment is working. Before reaching for ANY analgesic or antipyretic, read the safety list. There is no human painkiller that is safe to give a cat at home.
Named products: No FIP-specific approved supportive drug. Use feline-appropriate agents at feline doses, and see the safety list before reaching for any analgesic or antipyretic. Human analgesics and antipyretics - acetaminophen/paracetamol, ibuprofen, naproxen, aspirin - are never appropriate for a cat, and canine NSAID products must never be given to a cat.
Guideline-level supportive-care framework in the ABCD guidelines. In the randomised 42-versus-84-day study, cats in both arms showed rapid improvement in clinical and laboratory parameters once oral GS-441524 was started, body temperature normalised soon after initiation and body weight rose - the practical basis for treating supportive care as short-term support rather than as therapy. The same trial supplies the hard supportive-care numbers this page must carry: 4 of 40 cats received whole blood transfusions for severe anaemia DURING THE FIRST DAYS of treatment, with haematocrit normalising in three of the four; one cat developed treatment-emergent immune-mediated haemolytic anaemia (diagnosed on autoagglutination with blood parasites excluded), was treated with prednisolone, and had a haematocrit back in range by day 42; the cat that died on day 3 had severe hypoglycaemia, recurrent seizures, sepsis and third-degree heart block; and the second death involved suspected thoracentesis-associated bacterial infection. In Pedersen's cohort, packed cell volume did not return to normal until after 6-8 weeks of treatment, so anaemia resolves more slowly than fever does.
Corticosteroids are palliation, not treatment - and never a reason to delay the antiviral
Early evidenceBefore antivirals existed, steroids were most of what we had, and they can still transiently make a cat look better. They do not cure FIP, and a cat on steroids alone is a cat losing time. They also confound the diagnosis: they blunt fever, shrink lymph nodes, alter cytology and lymphocyte counts, and can make a subsequent work-up harder to interpret, so a cat that has been on steroids before referral needs that fact recorded. This is not a blanket prohibition and the page should not pretend otherwise - prednisolone was used adjunctively in two of the sources cited on this page: 1 mg/kg q24h to day 21 alongside antiviral therapy in the published neurological case, and to treat a cat that developed immune-mediated haemolytic anaemia during the randomised trial. Where steroids are used for a specific problem - marked ocular inflammation, treatment-emergent immune-mediated haemolysis, bridging an unstable neurological case - that should be a deliberate, time-limited decision made ALONGSIDE antiviral therapy by the treating veterinarian, never instead of it, and never a reason to postpone starting the antiviral. Owners must not start, stop or adjust a steroid dose on their own.
Named products: Prednisolone 1 mg/kg every 24 hours to day 21 alongside antiviral therapy in one published neurological case (PMID 40980462), and prednisolone for treatment-emergent immune-mediated haemolytic anaemia in the randomised trial (PMID 39066306). Those are case-level uses, not a protocol, and not a treatment for FIP.
SPECIES: CAT, indirect and suggestive rather than definitive. In the 60-cat Polyprenyl Immunostimulant field study in non-effusive FIP, survival times were significantly longer in cats that were NOT treated with corticosteroids concurrently. That is an uncontrolled field study and confounding by disease severity cannot be excluded, so it is a signal to weigh, not proof of harm. The stronger argument is opportunity cost: an effective antiviral now exists and delay is the main modifiable driver of a bad outcome. For the legitimate adjunctive uses, see the single neurological case report (prednisolone 1 mg/kg q24h to day 21) and the randomised trial (prednisolone for treatment-emergent immune-mediated haemolytic anaemia).
Obtain the drug lawfully-as-possible: an unapproved compounded drug, from a licensed compounding pharmacy, on a patient-specific prescription
Approved / guideline-backedThis is the part of FIP care that owners find most confusing, and the part the internet gets wrong most often. Between roughly 2019 and 2024, US owners bought GS-441524 from an unregulated grey market because no other route existed. That has partly changed - but not into legality, and the difference matters. State it precisely. Compounded GS-441524 is an UNAPPROVED animal drug. FDA has stated (10 May 2024) that it does not intend to enforce new animal drug approval requirements for products compounded from bulk GS-441524 and prescribed by a veterinarian for a specific cat patient for the treatment of FIP, under the conditions listed in Guidance for Industry #256. FDA states in the same announcement that animal drugs compounded from bulk drug substances 'are unapproved drugs and are not, in fact, legal', and that 'As of May 10, 2024, the FDA has not received a nomination for GS-441524' - so patient-specific prescribing is the only route inside the discretion, and clinic office stock is not covered. This is enforcement discretion, not approval, and FDA publicly pushed back on social-media claims that the drug had been 'legalised'. Do not write 'lawfully dispensed', 'legal' or 'approved' anywhere on this page. WHAT TO ACTUALLY DO, without naming a brand or a pharmacy: ask your veterinarian to write a prescription for your cat by name, for a stated milligram dose per administration, and to send it to a state-licensed compounding pharmacy; ask that pharmacy for its state licence status and for a certificate of analysis or potency assay for the batch; and keep the labelled container. Outside the US, availability varies and, per ABCD, nucleoside analogues 'are not available in all countries'; UK and EU access runs through veterinary special-order manufacturers and national cascade or import rules, so check local law before relying on any of this.
Named products: Compounded GS-441524 - an unapproved animal drug - in generic dosage forms: scored 50 mg oral tablets and an oil-based oral suspension at 50 mg/mL are in general US use. No brand and no pharmacy is named here, by policy. PETSMARTMEDS DOES NOT AND WILL NOT SELL, RESELL, IMPORT, DISTRIBUTE OR BROKER GS-441524, REMDESIVIR OR MOLNUPIRAVIR, AND WILL NOT CONNECT OWNERS TO UNLICENSED SOURCES. Product bought from an unlicensed online seller or a social-media group has no assured identity, purity, potency or sterility - see the safety list for what testing of those products actually found - and is an unapproved new animal drug.
REGULATORY, not clinical. FDA CVM Update, 10 May 2024 (content current as of 05/10/2024), verified directly: the enforcement statement, the explicit statement that compounded animal drugs from bulk substances remain unapproved and not, in fact, legal, the statement that no FDA approved drug is available to treat FIP in cats, and the statement that no nomination for GS-441524 had been received as of that date. FDA CVM GFI #256, 'Compounding Animal Drugs from Bulk Drug Substances' (August 2022), sets the conditions. We deliberately name no commercial compounded product and no dispensing pharmacy on this page, because a named product sitting next to an efficacy statement is promotion of an unapproved new animal drug for a specific indication.
Monitor on a schedule, and treat the 12 weeks after the last dose as an active observation window
Controlled trial in this speciesThe page must not commit an owner to 6-12 weeks of therapy without a recheck plan, and must not let 'the course is finished' be mistaken for 'discharged'. Stopping early is the commonest avoidable cause of relapse, and cats usually look completely normal weeks before the course is done. DURING TREATMENT: examine and re-test on a fixed schedule rather than on demand, including neurological and ophthalmic examination and point-of-care ultrasound at each visit, with haematology and biochemistry at intervals. A cat that is NOT clearly improving by week 2 needs the diagnosis or the dose re-examined, not more time. AFTER TREATMENT: recheck at 2 weeks and 12 weeks off treatment with bloods including the A:G ratio and AGP. Relapses in the original field study occurred 3 to 84 days after stopping, mean 23 days. A rising AGP in a cat that has finished treatment is a reason to be seen, not to wait. CHECK THE KIDNEYS AFTERWARDS, AND DO NOT OVERSTATE WHY. In 80 cats given oral GS-441524 for 42 days and followed to day 183, FIP-associated renal abnormalities largely REVERSED on treatment - urinary cystatin B was raised in 83.8% at day 1 and declined, and proteinuria fell from 91.3% of cats at day 1 to 3.8% at day 183. Over the same period, however, the proportion with creatinine above 140 micromol/L rose from 2.5% at baseline to 23.8% at day 183, and 8.8% met the authors' composite criteria for 'potential renal dysfunction' versus 2.5% at baseline. The authors state plainly that whether this reflects delayed FIP-related renal injury or an adverse antiviral effect 'remains unknown', while concluding that treatment should not be extended unnecessarily. Publish it exactly that way: as a reason to monitor renal parameters and to avoid unnecessary course extension, NOT as established drug-induced kidney damage. A cat that is clinically well with a normalised A:G ratio and AGP at the end of its post-treatment observation period, and stays well, is considered to be in durable remission - and that is now the expected outcome, not the exception.
Named products: No drug at this step. Owners must not stop, restart, shorten or re-round a dose on their own at any point in this schedule - call the clinic first.
SPECIES: CAT. The randomised trial monitored on day 1 and at scheduled follow-up visits on days 14, 28, 42, 56 and 84, with a final recheck at day 168, using physical examination, abdominal and thoracic ultrasonography, detailed cardiologic examination (electrocardiogram, echocardiography, cardiac troponin I where myocarditis was suspected), neurological examination, haematology and biochemistry including SDMA, SAA and AGP, urinalysis where cystocentesis was possible, and FCoV RNA loads in blood, effusion and faeces - and it followed cats to day 168, that is 126 days beyond the end of the short course, reporting all short-course completers still in complete remission at that point. The 29-cat prospective remdesivir study built a practice-scale schedule: rechecks at weeks 1, 2, 4, 8 and 12 during treatment and at 2 and 12 weeks AFTER treatment, with full physical examination including neurological and ophthalmic examination and point-of-care ultrasound at each recheck, and complete blood count and biochemistry at weeks 4, 8, 12 and 24. In the Pedersen field study, relapse occurred within 3-84 days of stopping (mean 23 days), packed cell volume took 6-8 weeks to normalise, and albumin and the A:G ratio reached normal at around week 8, making those the most reliable markers of progress. Renal follow-up findings are from the 80-cat longitudinal renal cohort.
Reduce feline coronavirus pressure in the household - and tell the owner FIP itself is not contagious cat-to-cat
Early evidenceFIP is not passed cat-to-cat as FIP; what circulates is feline coronavirus, which spreads faeco-orally, overwhelmingly through shared litter trays, with litter dust acting as a fomite. Say this plainly and early, because it is the single biggest anxiety in a multi-cat home and owners get it wrong in both directions - some isolate a dying cat unnecessarily, others ignore tray hygiene entirely. Practical measures: more litter trays than cats, sited away from food and water, scooped at least daily and disinfected regularly; damp-wipe around trays to limit litter dust; short-clip long coats around the perineum; avoid crowding; quarantine, faecal-test and where appropriate treat new arrivals BEFORE they share a tray; and for breeders, early weaning and separation protocols. If a household has cleared FCoV, keeping it out matters. Expect re-exposure to happen even during treatment: in the randomised trial, faecal shedding stopped in all 21 cats that were shedding at day 1, but 8 of 40 cats later tested positive again, 'presumably due to reinfection by a partner cat'.
Named products: No approved product. Household FCoV control is husbandry, not pharmacology. Do not dose healthy in-contact cats.
SPECIES: CAT, retrospective with a historical control group - promising but not a randomised trial. In 27 households containing 147 cats from which FCoV had been eliminated at least 6 months earlier, follow-up over 6 months to 3.5 years found 11 deaths but no cases of FIP, versus 37 FIP cases among 820 FCoV-exposed cats in a previous field study used as a retrospective control (P = 0.0062). A 4-7 day course of oral GS-441524 stopped faecal FCoV shedding. The authors themselves note that if FCoV is re-introduced into a household, FIP can still result, and describe one excluded cattery where that happened (reported as data not shown). Treat this as a strong rationale for hygiene and quarantine. It does NOT establish prophylactic antiviral dosing of healthy cats as standard of care, and PetSmartMeds must not promote that use. The randomised trial's authors put the counter-argument directly: because cats can shed FCoV even while on GS-441524, 'cats without FIP should not be treated with GS-441524 only to terminate FCoV shedding, as this will not be effective in the long term and might lead to resistance development.'
Polyprenyl Immunostimulant: historical interest only, and superseded
Early evidencePI is the one immunomodulator with a published feline FIP dataset, and in the pre-antiviral era it represented a genuine step forward for dry FIP. It is not a cure and it is not comparable to an antiviral. Listing it here is for completeness and for owners who encounter it online - it must not displace or delay a nucleoside-analogue antiviral.
Named products: Polyprenyl Immunostimulant. Do not present it as an alternative to antiviral therapy.
SPECIES: CAT, uncontrolled field study. 60 cats with non-effusive FIP diagnosed by primary-care and specialist veterinarians, treated without intentional selection of milder cases: 8 of 60 survived beyond 200 days and 4 of 60 beyond 300 days, against a literature search of 59 previously reported non-effusive FIP cats in which no cat with dry FIP alone had lived longer than 200 days. The authors' own conclusion: 'While not a cure, PI shows promise in the treatment of dry form FIP, but a controlled study will be needed to verify the benefit.' Set 8 of 60 surviving past 200 days beside a pooled 84.6% antiviral success rate and the comparison makes itself.
Feline Infectious Peritonitis: European Advisory Board on Cat Diseases (ABCD) Guidelines (2023) — European Advisory Board on Cat Diseases (ABCD)
How it is diagnosed
- Characterise the presentation first: full physical examination including rectal temperature, body weight and growth trajectory in kittens, abdominal palpation, thoracic auscultation, plus a deliberate ophthalmic and neurological examination. Ocular and neurological involvement changes both the prognosis and the treatment plan, so it must be looked for actively rather than assumed absent (ABCD 2023, PMID 37766254). A standardised ophthalmic examination found abnormalities in 48 of 118 cats with FIP (40.1%), including potentially vision-threatening posterior-segment change; the authors' conclusion is that ophthalmic examination belongs in the routine evaluation of every suspected FIP case (PMID 41644507).
- Test for feline immunodeficiency virus antibody and feline leukaemia virus antigen BEFORE committing to 6-12 weeks of antiviral therapy. FIV-antibody and FeLV-antigen negativity were inclusion criteria in the pivotal randomised trial, so the dosing and duration evidence on this page comes from a retrovirus-negative population; and FeLV/FIV-associated disease sits in the differential list below (PMID 39066306).
- Complete blood count and serum biochemistry with total protein, albumin and globulin, and calculate the albumin:globulin ratio. A low A:G is one of the most useful routine pointers - it was below 0.8 in 86.8% of tested cats in the Cyprus FIP cohort (PMID 42198625). Expect non-regenerative anaemia, lymphopenia, neutrophilia, hyperglobulinaemia, hyperbilirubinaemia. Include BLOOD GLUCOSE: severe hypoglycaemia featured in the fatal case in the randomised trial alongside recurrent seizures and sepsis (PMID 39066306). None of these findings is specific on its own.
- Serum alpha-1-acid glycoprotein (AGP), with serum amyloid A where available, as acute-phase markers. These support the diagnosis and give you non-invasive numbers to follow during and after treatment. The evidence base for serial monitoring is the 40-cat randomised trial, in which all 40 cats had increased AGP on day 1 and every cat's SAA was back within the reference interval by day 28 at the latest (PMID 39066306), supported by the 80-cat renal cohort in which elevated AGP and SAA normalised during treatment (PMID 42590897). A single case report additionally describes AGP rising again before neurological relapse and calls it a 'promising' non-invasive approach for early detection of relapse - that is n=1 and is labelled as such (PMID 40980462).
- Point-of-care ultrasound of abdomen and thorax to find effusion, lymphadenomegaly and organ changes - and to guide sampling. If there is an effusion, sample it. Effusion is the single highest-yield specimen in FIP.
- Screen the heart, not just the lungs. The randomised trial assessed every cat with electrocardiography and echocardiography, plus cardiac troponin I where myocarditis was suspected: myocarditis was diagnosed in 4 of 40 cats, one cat still had signs of myocarditis at day 42 and remained on cardiovascular treatment, and the cat that died on day 3 had third-degree heart block (PMID 39066306). Auscultation alone will not find this.
- On the effusion: cytology and biochemistry (protein-rich, low cellularity, pyogranulomatous) plus FCoV RNA detection by RT-qPCR or FCoV antigen detection by immunocytochemistry. This combination is the practical route to a confident clinical diagnosis (PMID 37766254; PMID 31731711). Sample and drain with strict aseptic technique and consent the owner for the procedure risk - see the safety list on centesis complications.
- When there is no effusion: fine-needle aspirates of affected organs for cytology with FCoV RNA or antigen detection; CSF and/or aqueous humour sampling with FCoV RT-qPCR where neurological or ocular signs are present; advanced imaging (MRI) where neurolocalisation is needed. Definitive diagnosis usually still requires histopathology with FCoV antigen detection (PMID 37766254).
- Do not diagnose FIP on a serum FCoV antibody titre. FCoV seropositivity is widespread in healthy cats and the enteric and FIP-causing biotypes are indistinguishable by several routine methods; a titre neither confirms nor excludes FIP (PMID 31731711; PMID 37766254). The ABCD publishes FIP Diagnostic Approach Tools for exactly this reason - use them.
- Work the differentials honestly before committing to 6-12 weeks of antiviral therapy: lymphoma, other neoplasia, cardiac and hepatic causes of effusion, bacterial septic peritonitis or pyothorax, toxoplasmosis, FeLV/FIV-associated disease, and congenital portosystemic shunt in a poorly growing kitten.
Where a biologic fits
Our own product, graded by the same rule
There is no legitimate role for stem cell, exosome or peptide therapy in the treatment of feline infectious peritonitis: FIP is now treatable with a high remission rate when an effective nucleoside-analogue antiviral is started promptly, and the only feline stem cell study in FIP - a randomised, double-blinded trial in which ten cats received GS-441524 alone or GS-441524 plus two intravenous infusions of allogeneic MSCs - measured immune-cell recovery rather than cure rates, showed no clinical advantage over the antiviral alone, and had authors who stated outright that their design could not assess clinical impact.
What exists
One real feline study exists, and we are going to describe it accurately rather than hide it or inflate it. Wanakumjorn et al. (2025) ran a RANDOMISED, DOUBLE-BLINDED trial at UC Davis in cats with naturally occurring effusive FIP. Eleven cats were enrolled and ten completed, five per arm. Every cat - both arms - received oral GS-441524 at 16-25 mg/kg daily for 12 weeks. The MSC arm additionally received two intravenous infusions of 20 million allogeneic SPF feline adipose-derived MSCs, two weeks apart. The endpoints were immunological and transcriptomic, not clinical: antiviral therapy alone reduced cytotoxic T-cell exhaustion by downregulating PD-1, TIM-3 and LAG-3, and the MSC-treated cats additionally showed reduced expression of exhaustion-related transcription factors (IKZF2, ZEB2, PRDM1), increased regulatory T-cell populations, and single-cell RNA-seq shifts in mesenteric lymph node consistent with immune rejuvenation. Residual cytokine elevations persisted at the study's end. Safety was acceptable: no severe adverse effects were recorded, with intermittent diarrhoea in three antiviral-only and one combination cat. SPECIES LABEL: this is cat data in cats with the actual disease - which is more than can be said for most veterinary biologic marketing - and the paper is explicitly framed as a translational model for severe human coronavirus infection (PMID 40659357). The only other FIP-and-vesicles work we located is diagnostic, not therapeutic: a serum exosomal miRNA signature (miR-21-5p, miR-155-5p, miR-423-5p) with a reported combined-model AUC of 0.93, sensitivity 88.6% and specificity 91.2% for early FIP diagnosis (PMID 42643253). That is biomarker discovery in a single unreplicated study, not a validated test and not a treatment.
What has not been shown
1. NO CLINICAL BENEFIT HAS BEEN SHOWN. The MSC trial reported no comparative clinical outcome advantage: in both arms viral loads became undetectable by week 1, fever resolved, body weight normalised and haematologic abnormalities corrected. There is no published difference in survival, remission rate, time to resolution of effusion, or relapse between GS-441524 plus MSCs and GS-441524 alone. The authors state their own limit directly: 'we enrolled in the study cats that were not critically ill, which limited our ability to observe the clinical impact of MSC therapy.' Ten cats in a mechanistic study is not an efficacy trial, and the antiviral cured both arms. 2. NO EXOSOME OR EXTRACELLULAR-VESICLE PRODUCT has any in-vivo clinical efficacy evidence for FIP - or for any condition in dogs or cats. The FIP exosome literature is one diagnostic biomarker paper. 3. NO PEPTIDE has any evidence in FIP. A PubMed search for BPC-157 and thymosin beta-4/TB-500 crossed with feline coronavirus, peritonitis and cats returned ZERO records (searched 12 September 2026). There is no peptide in the MicroAmino catalogue with controlled efficacy evidence in cats for anything, let alone for FIP. 4. NO APPROVED PRODUCT. In the United States, no animal cell, exosome or extracellular-vesicle product is FDA-approved; they are unapproved new animal drugs. For contrast, name the regulator where one IS authorised and note what it is not: DogStem is an EQUINE umbilical cord MSC product authorised by the EUROPEAN COMMISSION on EMA assessment (procedure EMEA/V/C/005829) for DOGS with osteoarthritis - a different species of donor, a different species of patient, and a different disease. It says nothing about cats and nothing about FIP. No FIP indication is authorised by any regulator for any cell or exosome product. 5. 'IMMUNE-PRIVILEGED' AND 'ZERO-FLARE' ARE BANNED CLAIMS ON THIS SITE. Punzon et al. (Front Vet Sci 2023;10:1098029, PMID 37266387) found that BOTH equine and canine MSCs RAISED ANTIBODY TITRES in dogs; safety was nonetheless acceptable, and the authors' own framing is that MSCs are immune-TOLERANT rather than immune-PRIVILEGED. The pivotal canine efficacy dataset for that product is Punzon et al. 2022 (PMID 36198051), an 80-dog randomised placebo-controlled trial in canine osteoarthritis. All of that is DOG data, not cat data - which makes the position more conservative, not less: for cats we do not even have the immunogenicity dataset that dogs have. Never write that these products are immune-invisible or flare-free. 6. AND THE DECISIVE POINT: FIP is now a treatable disease with a high remission rate. Pooled antiviral success is 84.6% across 650 cases, and a randomised controlled trial saw every cat that completed 42 days of oral GS-441524 still in complete remission at day 168. Selling an unproven biologic into a disease with an effective antiviral is not merely unsupported - it is actively harmful, because the harm is the delay.
Sources
- THE ONE REAL FELINE STUDY - randomised, double-blinded, n=10 completing, MSC added on top of an effective antiviral; immunologic endpoints only, no clinical advantage shown, authors concede the design could not measure clinical impact — Wanakumjorn P, Kimura K, Castillo D, McLarty E, Formaker R, Qiao R, Farrell K, Brostoff T, Ramarapu R, Pires J, Cohen-Davidyan T, Cassano J, Murphy B, Reagan K, Kol A. Mesenchymal stem/stromal cell therapy improves immune recovery in a feline model of severe coronavirus infection. Stem Cells Transl Med. 2025;14(7):szaf025. PMID 40659357. DOI 10.1093/stcltm/szaf025. https://doi.org/10.1093/stcltm/szaf025
- THE ONLY EXOSOME WORK IN FIP IS DIAGNOSTIC, NOT THERAPEUTIC - single unreplicated serum exosomal miRNA biomarker study, not a validated test and not a treatment — Gu H, Wu H. The value of serum exosomal miRNA signature profiles for the early diagnosis of feline infectious peritonitis. Exp Biol Med (Maywood). 2026;251:10915. PMID 42643253. DOI 10.3389/ebm.2026.10915. https://doi.org/10.3389/ebm.2026.10915
- WHY A BIOLOGIC IS THE WRONG ANSWER HERE - pooled antiviral success 84.6% across 11 studies and 650 cases, higher with combinations and lower in wet FIP and neurological disease, from a literature whose authors say randomised trials are still needed; every unproven product offered instead of this costs the cat time — Gokalsing E, Ferrolho J, Gibson MS, Vilhena H, Anastacio S. Efficacy of GS-441524 for Feline Infectious Peritonitis: A Systematic Review (2018-2024). Pathogens. 2025;14(7):717. PMID 40732763. DOI 10.3390/pathogens14070717. https://doi.org/10.3390/pathogens14070717
- THE EFFECTIVENESS IS ESTABLISHED IN A RANDOMISED CONTROLLED DESIGN - 42 days of oral GS-441524 equalled 84 days in 40 cats with effusive FIP (at least 2 kg, FIV/FeLV-negative); all short-course completers still in remission at day 168 — Zuzzi-Krebitz AM, Buchta K, Bergmann M, Krentz D, Zwicklbauer K, Dorsch R, Wess G, Fischer A, Matiasek K, Honl A, Fiedler S, Kolberg L, Hofmann-Lehmann R, Meli ML, Spiri AM, Helfer-Hungerbuehler AK, Felten S, Zablotski Y, Alberer M, von Both U, Hartmann K. Short Treatment of 42 Days with Oral GS-441524 Results in Equal Efficacy as the Recommended 84-Day Treatment in Cats Suffering from Feline Infectious Peritonitis with Effusion - A Prospective Randomized Controlled Study. Viruses. 2024;16(7):1144. PMID 39066306. DOI 10.3390/v16071144. https://doi.org/10.3390/v16071144
- GUIDELINE POSITION ON WHAT ACTUALLY WORKS - 'nucleoside analogues (e.g., oral GS-441524) are very effective, although they are not available in all countries' — Tasker S, Addie DD, Egberink H, Hofmann-Lehmann R, Hosie MJ, Truyen U, Belak S, Boucraut-Baralon C, Frymus T, Lloret A, Marsilio F, Pennisi MG, Thiry E, Mostl K, Hartmann K. Feline Infectious Peritonitis: European Advisory Board on Cat Diseases Guidelines. Viruses. 2023;15(9):1847. PMID 37766254. DOI 10.3390/v15091847. https://doi.org/10.3390/v15091847
- 'IMMUNE-PRIVILEGED' IS NOT DEFENSIBLE - both equine and canine MSCs raised antibody titres in DOGS; safety acceptable, and the authors' framing is immune-TOLERANT not immune-privileged. Dog data, not cat data, so the feline immunogenicity picture is even less characterised — Punzon E, Garcia-Castillo M, Rico MA, Padilla L, Pradera A. Local, systemic and immunologic safety comparison between xenogeneic equine umbilical cord mesenchymal stem cells, allogeneic canine adipose mesenchymal stem cells and placebo: a randomized controlled trial. Front Vet Sci. 2023;10:1098029. PMID 37266387. DOI 10.3389/fvets.2023.1098029. (24 police working dogs, 1:1:1 randomised; no adverse events detected 'despite both equine and canine MSC generate antibody titres in the dogs'. DOG data - there is no equivalent feline immunogenicity dataset.) https://doi.org/10.3389/fvets.2023.1098029
- THE PIVOTAL CANINE MSC EFFICACY TRIAL, FOR SCOPE - equine UC-MSC in 80 dogs with osteoarthritis; an EU/European Commission authorisation for DOGS (EMEA/V/C/005829) using EQUINE cells, with no feline and no FIP indication anywhere — Punzon E, Salguero R, Totusaus X, Mesa-Sanchez C, Badiella L, Garcia-Castillo M, Pradera A. Equine umbilical cord mesenchymal stem cells demonstrate safety and efficacy in the treatment of canine osteoarthritis: a randomized placebo-controlled trial. J Am Vet Med Assoc. 2022;260(15):1947-1955. PMID 36198051. DOI 10.2460/javma.22.06.0237. https://doi.org/10.2460/javma.22.06.0237
- THE HISTORICAL IMMUNOMODULATOR, FOR HONEST COMPARISON - Polyprenyl Immunostimulant in 60 cats with dry FIP: 8 survived past 200 days, 4 past 300; authors' own words, 'while not a cure' — Legendre AM, Kuritz T, Galyon G, Baylor VM, Heidel RE. Polyprenyl Immunostimulant Treatment of Cats with Presumptive Non-Effusive Feline Infectious Peritonitis In a Field Study. Front Vet Sci. 2017;4:7. PMID 28261584. DOI 10.3389/fvets.2017.00007. https://doi.org/10.3389/fvets.2017.00007
- REGULATORY STATUS OF EVERY UNAPPROVED PRODUCT IN THIS SPACE - FDA states drugs compounded from bulk drug substances 'are unapproved drugs and are not, in fact, legal'; enforcement discretion is not approval — US FDA Center for Veterinary Medicine. FDA Announces Position on the Use of Compounded GS-441524 to Treat FIP. CVM Update, 10 May 2024 (page content current as of 05/10/2024). | US FDA. CVM GFI #256 - Compounding Animal Drugs from Bulk Drug Substances. Guidance for Industry, August 2022. https://www.fda.gov/animal-veterinary/cvm-updates/fda-announces-
Tracking response
How you know whether it is working
THERE IS NO VALIDATED OWNER-REPORTED OR CLINICIAN-SCORED OUTCOME INSTRUMENT FOR FELINE INFECTIOUS PERITONITIS. We searched and did not locate one. The validated feline clinical metrology instruments that do exist - FMPI, FMPI-sf, CSOMf, MiPSC, SNoRE - are musculoskeletal-pain instruments and have no applicability here; do not repurpose one and do not imply a validated FIP score exists. Monitoring in FIP is objective and laboratory-anchored, and the published trials tell us exactly which numbers to follow: rectal temperature; body weight, and in kittens the resumption of growth; appetite and activity (the randomised trial used the Karnofsky score modified for cats, which is a quality-of-life classification rather than a validated FIP index); packed cell volume/haematocrit and lymphocyte count; total protein with albumin, globulin and the albumin:globulin ratio; serum alpha-1-acid glycoprotein (AGP) with serum amyloid A as the acute-phase markers and the most practical early signal of re-emergence; blood glucose in any collapsing or seizuring cat; creatinine, urea and SDMA with urine specific gravity and urine protein:creatinine ratio for renal follow-up; ALT for drug tolerance; gum colour daily at home in the first fortnight; and point-of-care ultrasound for effusion volume. Cardiac assessment (electrocardiogram, echocardiography, cardiac troponin I where myocarditis is suspected) and a standardised ophthalmic examination belong in the baseline work-up, not just in referral cases. Where available, FCoV viral load in blood and effusion fell markedly on treatment in the randomised study and is the most direct evidence the antiviral is working.
Zuzzi-Krebitz AM, Buchta K, Bergmann M, Krentz D, Zwicklbauer K, Dorsch R, Wess G, Fischer A, Matiasek K, Honl A, Fiedler S, Kolberg L, Hofmann-Lehmann R, Meli ML, Spiri AM, Helfer-Hungerbuehler AK, Felten S, Zablotski Y, Alberer M, von Both U, Hartmann K. Short Treatment of 42 Days with Oral GS-441524 Results in Equal Efficacy as the Recommended 84-Day Treatment in Cats Suffering from Feline Infectious Peritonitis with Effusion - A Prospective Randomized Controlled Study. Viruses. 2024;16(7):1144. PMID 39066306. DOI 10.3390/v16071144. | Renner KA, Cattin R, Kimble B, Munday J, White A, Coggins S. Efficacy of oral remdesivir in treating feline infectious peritonitis: a prospective observational study of 29 cats. J Feline Med Surg. 2025;27(5):1098612X251335189. PMID 40424099. DOI 10.1177/1098612X251335189. | de Witt Curtius CC, Rodary M, Hofmann-Lehmann R, Spiri AM, Meli ML, Bouzon AC, Wenk J, Cerchiaro I, Pineroli B, Pot SA, Beckmann K, Chan T, Wieser M, Unterer S, Felten S, Meunier SM. Navigating neurological re-emergence in feline infectious peritonitis: challenges and insights from GS-441524 and remdesivir treatment. JFMS Open Rep. 2025;11(2):20551169251360625. PMID 40980462. DOI 10.1177/20551169251360625. (SINGLE CASE REPORT, n=1.) | de Witt Curtius CC, Meli ML, Crespo Bouzon A, Wenk J, Stachowski JM, Cerchiaro I, Pineroli B, Unterer S, Felten S, Major A, Meunier SM, Howard J, Zwicklbauer K, Dorsch R, Hartmann K, Hofmann-Lehmann R, Spiri AMM. Renal Health in Cats with Feline Infectious Peritonitis treated with GS-441524: Baseline Findings and Longitudinal Effects. J Feline Med Surg. 2026. PMID 42590897. DOI 10.1177/1098612X261480355. | Bell M, Pot S, Meunier S, Spiri A, Felten S, Hofmann-Lehmann R, Rampazzo A, Suter A. Ocular findings in cats with feline infectious peritonitis treated with GS-441524: a preliminary prospective study of 118 cases. J Feline Med Surg. 2026;28(4):1098612X261424314. PMID 41644507. DOI 10.1177/1098612X261424314. | Tasker S, Addie DD, Egberink H, Hofmann-Lehmann R, Hosie MJ, Truyen U, Belak S, Boucraut-Baralon C, Frymus T, Lloret A, Marsilio F, Pennisi MG, Thiry E, Mostl K, Hartmann K. Feline Infectious Peritonitis: European Advisory Board on Cat Diseases Guidelines. Viruses. 2023;15(9):1847. PMID 37766254. DOI 10.3390/v15091847.
Suggested cadence: DURING TREATMENT: rechecks at weeks 1, 2, 4, 8 and 12 with full physical examination including neurological and ophthalmic assessment plus point-of-care ultrasound, and complete blood count with biochemistry at weeks 4, 8, 12 and 24 - the schedule used in the 29-cat prospective remdesivir study. The randomised trial's own schedule was day 1 plus days 14, 28, 42, 56 and 84, with a final recheck at day 168, and included baseline electrocardiography, echocardiography, cardiac troponin I where myocarditis was suspected, neurological examination, SDMA, SAA, AGP, urinalysis and viral loads. Check packed cell volume early and repeatedly in the first fortnight: 4 of 40 cats needed a whole blood transfusion in the first days, and packed cell volume takes 6-8 weeks to normalise. Weight at every single visit; in kittens, plot it, and re-calculate the milligram dose as the kitten grows. Temperature daily at home in the first fortnight if the owner can do it calmly, plus a daily look at gum colour. Expect fever and effusion to resolve fast on an effective antiviral - median 7 days to resolution of clinical signs and effusion in the 42-cat molnupiravir cohort, and clinical remission at a median of 28 days in the randomised trial - so a cat that is NOT improving by week 2 needs the diagnosis or the dose re-examined, not more time. COURSE LENGTH: 42 days at 15 mg/kg oral GS-441524 every 24 hours was equally effective to 84 days in the randomised trial's population (effusion present, at least 2 kg, FIV/FeLV-negative; neurological and ocular cases were NOT excluded, and the trial's authors judged 42 days likely sufficient for them too while calling for further study). We nonetheless keep 84 days as our default for non-effusive, neurological and ocular disease and for any slow responder, and we attribute that caution to ABCD guidance, Dickinson 2020 and the published re-emergence case rather than to the trial. AFTER TREATMENT: this is the part owners skip. Recheck at 2 weeks and 12 weeks post-treatment with bloods, including A:G ratio and AGP, plus renal parameters - creatinine, SDMA, urine specific gravity and urine protein:creatinine ratio - because in the 80-cat renal cohort the proportion with creatinine above 140 micromol/L rose from 2.5% at baseline to 23.8% by day 183 while proteinuria resolved, with causation unresolved and the authors' conclusion being not to extend treatment unnecessarily. Relapses in the original field study occurred 3 to 84 days after stopping, mean 23 days. The randomised study followed cats to day 168 - 126 days past the end of the short course - before calling remission durable. A rising AGP in a cat that has finished treatment is a reason to be seen, not to wait and watch. LONG TERM: once through the observation window and well, a recovered cat needs ordinary preventive care plus periodic renal screening, not indefinite FIP surveillance. Say that out loud - owners of recovered FIP cats live with more fear than the evidence warrants.
Questions you can answer at home
- Has your cat's appetite gone up, stayed the same, or gone down since the last check-in? Roughly how much of a normal meal is she eating - all of it, most, half, a few bites, or nothing?
- Has your cat's weight gone up, down, or stayed the same? If she is a kitten, is she visibly growing again - collar looser, longer legs, catching up with littermates? Tell us her current weight, because the dose is calculated from it and a growing kitten outgrows her dose.
- Have you taken her temperature, and if so what was it? If you are not taking a temperature: does she feel hot to the touch over the ears and belly, and is she shivering or seeking heat?
- Lift her lip and look at her gums. Are they a healthy pink, or pale, white, grey, yellow or muddy? Pale or white gums, or any yellow tinge, means phone us today - do not wait for this check-in to be answered.
- How is her energy and behaviour compared with a healthy cat her age - playing and exploring, quiet but engaged, sleeping most of the day, or hiding away?
- Has her breathing changed at all - faster, harder, noisier, or breathing with her elbows held out? Count her breaths for 30 seconds while she is resting and double it, and tell us the number.
- Have you noticed anything new with her eyes (cloudiness, colour change, redness, squinting, bumping into things) or her balance and movement (wobbling, stumbling, head tilt, tremors, circling)? Eye changes are common in FIP and easy to miss, so look at both eyes in good light.
- Have you been able to give every single dose, at the right number of milligrams, at the right time? If you have missed doses, how many, and why - is she fighting the pill, vomiting it, spitting it out, or is cost or supply the problem? Tell us honestly; missed and self-adjusted doses are the commonest reason FIP comes back, and there are ways we can help.
- Has anything about the medicine itself changed - a different pharmacy, a different tablet strength, a different liquid, a different colour or smell, a dose you rounded up or down, or a gap while you waited for a refill? And have you given her anything else at all, including any supplement, herbal product, immune booster, or any medicine belonging to a person, a dog or another cat? Tell us even if you think it was harmless - and please call us BEFORE making any change like that, not after.
Last reviewed: 2026-09-12 · Every claim on this page carries a citation you can check. If one does not hold up, tell us and we will correct it. science@azzamedical.com
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