Feline Infectious Peritonitis (FIP)

The majority of cats that develop FIP affected are young between 3 months and 2 years of age (80%), or elderly cats, and the disease often affects purebred cats, especially the following breeds: Ragdolls, Abyssinians, Bengals, British Short Hairs, Exotics, Devon Rex, Birmans.

Interesting, the purebreds that are less commonly represented are Siamese, Persian and Himalayan cats.

Cats that have been housed together in a rescue facility where multiple cats share a litter tray are also at a higher risk of developing FIP.

FIP is more common in male cats, with a male to female ratio of 2:1.

Any cat that has previously been exposed to enteric feline coronavirus can potentially develop FIP, but in the vast majority of cats, the enteric coronavirus is insignificant and does not require any treatment.

Laboratory changes on blood work typical of FIP may include:

• Hyperglobulinaemia (elevated globulins)
• Non-regenerative anaemia (low red blood cell count)
• Neutrophilic leukocytosis (elevated white blood cell count)
• Elevated liver parameters (ALT, ALP, elevated bilirubin)

An albumin to globulin ratio of < 0.8 raises the suspicion of FIP in a young cat. It’s important to note that no single laboratory abnormality will occur 100% of the time. If the albumin to globulin ratio is < 0.4 then there is a > 90% likelihood that the diagnosis is FIP.

Fluid analysis for cases of ‘wet’ FIP typical changes include:

• Specific gravity of 1.017-1.047
• Protein > 35g/L in all cases
• Low cellularity (< 20 000 cells/ml) containing mainly non-degenerate neutrophils and macrophages

For a definitive diagnosis of FIP the following tests can be used:

• Direct Immunofluorescence (IFA) for FIP antigen in body cavity fluid sample in ‘wet’ FIP cases (not blood) (available through Vetnostics or Veterinary Diagnostic Pathology Services (VDPS) which has a > 99% specificity and 75% sensitivity for FIP
• FIP PCR on fluid sample (‘wet’ FIP) (available through IDEXX laboratories)
• Immunohistochemistry or immunocytochemistry from an FNA or biopsy sample from an enlarged lymph node (‘wet’ or ‘dry’ forms)
• Brain MRI and/or CSF tap (‘dry’ FIP)

Co-morbidities such as toxoplasmosis and/or mycoplasmosis (if anaemic) could be present, so ruling out these diseases via toxoplasma titres (IgG and IgM) and a Mycoplasma PCR test is helpful. Testing for FIV and FeLV is recommended.

False negatives are common, so if a definitive diagnosis of FIP cannot be obtained, or the owner has financial constraints, then a remdesivir treatment trial can be considered. Usually, FIP cats respond rapidly to remdesivir, with fever resolving and significant improvement noticed within 3-5 days of starting treatment, sometimes as early as 24 hours after the first injection. ​

Faecal sample

Due to the fact that so many healthy cats carry feline coronavirus (FeCoV) with very little consequence (except for diarrhoea in some cases), obtaining a faecal PCR test that returns a positive coronavirus sample is unhelpful in supporting a diagnosis of FIP. Faecal testing is however helpful in ruling out parasites and concurrent infectious diseases in young kittens that may compromise recovery.

Blood serology

The feline coronavirus antibody test is generally unhelpful, as cats can have a high titre due to exposure to feline coronavirus but not necessarily have FIP, but more importantly, if negative, FIP still cannot be ruled out.

As a new graduate vet in the year 2000, Dr Gretta had to deliver very sad news to a young family with their beautiful new kitten who had developed FIP. The kitten had a very swollen abdomen full of fluid, a high fever and breathing problems. Unfortunately, at this time, there was no treatment available and as the cat was suffering, euthanasia was the only option.

18 years later, Dr Gretta first heard about GS-441524, a new anti-viral FIP treatment, when she attended a lecture about its discovery at an Australian and New Zealand College of Veterinary Scientists conference in Queensland in 2018.

Dr Gretta has always had an interest in infectious diseases and as a side-project during the first COVID-19 Sydney lockdown, Dr Gretta started volunteering as a veterinarian with the FIP Warriors Facebook group and via email, communicating with vets and owners located all over the world, including USA, Europe, Russia, South America, Middle East, Asia and home in Australia, to assist remotely in the management of their FIP cases. Through this rewarding, yet challenging experience, Gretta has collaborated with many other veterinarians and cat rescue groups who treat vast numbers of FIP cases.

Remdesivir (GS-5734 trade name Veklury, Gilead Sciences) was approved in 2020 as an anti-viral protease inhibitor for use in COVID-19 patients. Remdesivir is actually the prodrug for GS-441524 (which means remdesivir is metabolised into GS-441524 once in the body), but unfortunately, Gilead was not interested in pursuing the feline market for this drug.

In 2020, BOVA Animal Compounding Pharmacy (an Australian owned pharmacy) secured a supply of quality assured remdesivir, and towards then end of 2021, oral GS-441524 tablets, which means that vets can write a script to acquire it for their patients. In New Zealand, Optimus Healthcare pharmacy has been compounding FIP anti-viral medication since 2021 at www.optimushealth.co.nz/vetmed/.

If the pharmacy supplies the medication via a veterinary script, it is not the vet supplying the drug. This avoids cat owners attempting to illegally import an unlabelled, unregistered drug and deal with the consequences if caught by the authorities. Prior to FIP anti-viral medication being available via veterinary script, this was the only option for clients who wished to treat their FIP positive cats, and as it is illegal for vets to import unregistered medications, owners were left to source medication and learn how to medicate their cats themselves without veterinary guidance.

Dr Gretta Howard’s friend and colleague, one of the world’s most well-known and respected feline specialists, Dr Richard Malik, has been instrumental in volunteering his time and expertise helping vets treat sick cats world-wide and in conjunction with other FIP researchers, has put together a FIP remdesivir treatment protocol for veterinarians. This gives vets a much better way to effectively treat FIP patients while they remain under their care and is therefore a much better overall outcome for the welfare of cats with this horrible disease, which was previously a death sentence.

Since 2020, Dr Gretta Howard has treated over 50 cases in Australia with FIP successfully, with fewer than 5% of cases dying or relapsing.  Most of these cases were treated as outpatients, though some critically ill cases needed hospitalisation. Dr Gretta has also assisted other vets in managing cases when treatment was newly available and not widely known.

“Being able to help an owner save their beloved cat by offering treatment options for a previously fatal disease has been the single most satisfying part of my career so far”.

It is very sad to lose your cat to FIP, so the earlier treatment is implemented, the greater the chance of survival. For this reason, Dr Gretta is an advocate of starting treatment with anti-viral therapy as soon as possible to prevent further deterioration.

Dr Gretta Howard co-authored an article in The Veterinarian: “Treatment of FIP in cats with remdesivir” with Dr Richard Malik and Dr David Hughes, which discusses the protocol for FIP treatment with remdesivir. Since this article was published, dose recommendations for remdesivir have been increased. Dr David Hughes and Dr Rebecca Brady published an article with updated dose rates in Control & Therapy (Centre for Veterinary Education): “A feline infectious peritonitis treatment protocol”. These resources are available on this page.

As this is still in experimental stages, these treatment protocols will be subject to change and refined over time as more cats are treated successfully, so it is important to obtain the most current recommendations that have been based on research findings.

Since the oral GS-441524 tablets have become available, it is Dr Gretta Howard’s preference to treat FIP cats with the oral form in the majority of cases, particularly if they do not require immediate hospitalisation and are eating. See further information below regarding protocol used for the oral form. There is no one-size-fits-all protocol for treating FIP cases, as each case presents slightly differently and therefore has varying needs and in addition to this, each owner has a different budget, so treatment protocols are individually catered for at Turramurra Veterinary Hospital, depending on each case and the owner’s situation and preferences.

Dose recommendations continue to evolve over time as further research is pursued and more cases are treated with remdesivir.

Dose recommendations (updated July 2024)

Effusive ‘wet’ FIP                    10mg/kg once daily by subcutaneous injection

Non-effusive ‘dry’ FIP            15-20mg/kg once daily by subcutaneous injection

Ocular FIP                               15mg/kg once daily by subcutaneous injection

Note: any case that has neurological signs is considered a non-effusive ‘dry’ FIP case, as often cases can be ‘mixed’ FIP with signs of both wet and dry FIP. These should be treated at the higher dose of 15mg/kg.

Immunosuppressed cases: FIP cats that are also FIV positive as well as all Ragdoll cats (known to have reduced capacity to fight infectious disease) should be treated at the higher end of the dose at 20mg/kg.

Generally injectable anti-viral therapy with remdesivir is now reserved for cats in the following situations:

• Severe neurological signs, such as cluster seizures or depressed mentation, affecting the cat’s ability to swallow
• Marked dehydration
• Cats with gastrointestinal signs such as vomiting and/or diarrhoea that may affect tablet absorption
• Cats that are unable to be orally medicated due to their temperament or owner ability

If patients are critically ill, then an intravenous protocol can be used for the first 1-3 days using an initial dose of 10-20mg/kg once daily while the cat is hospitalised on intravenous fluids. It is critical to monitor these cases closely for the development or worsening of pleural effusion (fluid in the chest).

Instructions for BOVA sourced remdesivir reconstitution is attached as a PDF in the resources below.

Intravenous administration of remdesivir

Each vial of remdesivir contains 100mg so once reconstituted, the concentration is 10mg/ml (not 100mg/ml which is a common error in calculation).

The dose of remdesivir should be calculated and drawn up, then diluted to a total volume of 10ml with saline and administered slowly over 20-30 minutes manually or with a syringe driver.

Cats that start their FIP treatment with remdesivir injections generally improve within the first week of starting treatment. If this does not occur, then it is better to seek further diagnostics (if FIP has not been definitively diagnosed).

Generally, we recommend continuing treatment for 12 weeks (84 days) then stop if the cat is doing clinically well and the blood results look promising. The dose of remdesivir in kittens should be recalculated on a weekly basis upwards as they grow to prevent underdosing.

An increase in dose and/or extension of the course of treatment may be recommended based on the clinical response to therapy, blood parameters or signs of relapse.

(Cost updated July 2024)

BOVA sourced remdesivir is $110 per 100mg vial.

Treating a 2.5kg cat with effusive FIP at 10mg/kg for 12 weeks with remdesivir (25mg per day or 2.5ml) requires approximately 21 vials so costs approximately $2,350.

Note: costs do not include veterinary consultations, additional medications if required, blood test monitoring, script, postage and delivery fees.

The cost of treatment is expensive, and it is important to note that many cats with FIP have been cured at much lower doses, such as 5-6mg/kg once daily by subcutaneous injection, so if clients cannot afford the higher doses, a lower dose for a shorter duration is better than no dose.

I routinely recommend mefloquine to be used in conjunction with remdesivir to reduce the chance of a relapse (see below for information on timing, dose and duration).

Since October 2021, BOVA has been able to source GS-441524 tablets (50mg quarter-scored), which is a more humane option for cats that is both highly effective, well-tolerated and does not involve daily injections that can be difficult for the owner to administer and for cats to tolerate for a 12-week course.

In FIP cats that are fairly well-hydrated, Dr Gretta Howard prefers to start cases on oral GS-441524 tablets if they are eating and do not have dyspnoea (breathing difficulties).

Generally, both remdesivir and particularly the oral GS-441524 tablets have a high safety profile in cats.

Generally, we recommend continuing treatment for 12 weeks (84 days) then stop if the cat is doing clinically well and the blood results look promising. The dose of GS-441524 tablets in kittens should be recalculated on a weekly basis upwards as they grow to prevent underdosing.

Cats that start their FIP treatment with the oral GS-441524 tablets generally improve within the first week of starting treatment. If this does not occur, then it is better to seek further diagnostics (if FIP has not been definitively diagnosed) or switch to injections, in case the problem is relating to intestinal absorption.

Dose recommendations (updated July 2024)

Effusive ‘wet’ FIP                    10mg/kg once daily orally

Non-effusive ‘dry’ FIP            15-20mg/kg once daily orally

Ocular FIP                               15mg/kg once daily orally

Note: any case that has neurological signs is considered a non-effusive ‘dry’ FIP case, as often cases can be ‘mixed’ FIP with signs of both wet and dry FIP. These should be treated at the higher dose of 15mg/kg.

Immunosuppressed cases: FIP cats that are also FIV positive as well as all Ragdoll cats (known to have reduced capacity to fight infectious disease) should be treated at the higher end of the dose at 20mg/kg.

There is a general consensus that GS-441524 tablets should be given at least an hour before food, as they are absorbed better on an empty stomach, but if this is not possible, then administer with a small amount of food. It is best if tablets are given at approximately the same time each day so that the level of anti-viral medication in the system remains effective.

(Cost updated July 2024)

BOVA sourced GS-441524 tablets costs are as follows (cheaper if purchased in bulk):

10 x 50mg tablets       $508 ($50.80 per tablet)

20 x 50mg tablets       $837 ($41.85 per tablet)

50 x 50m tablets         $1695 ($33.90 per tablet)

Treating a 2.5kg cat with effusive FIP for 12 weeks with GS-441524 (25mg per day or HALF a 50mg tablet) requires approximately 42 tablets so costs approximately $2150.

BOVA sourced GS-441524 also comes in a 25ml liquid suspension at 50mg/ml for those kitties who are not easy to give tablets to.

Note: costs do not include veterinary consultations, additional medications if required, blood test monitoring, script, postage and delivery fees.

I usually recommend mefloquine to be used in conjunction with GS-441524 to reduce the chance of a relapse (see below for information on timing, dose and duration).

In NSW, while it is acceptable to charge a script fee, it is illegal for vets to place a mark-up on compounded medication from pharmacies. At Turramurra Veterinary Hospital, the script is emailed (and then the hard-copy posted) to BOVA at their priority email address, then once received, clients will receive a text message to then phone to make a direct payment to BOVA for the medication, which is then delivered to the vet hospital within 24-48 hours. Scripts for remdesivir are marked URGENT for remdesivir for priority post. It is recommended for critical cases, that the script is email and paid for prior to 3pm so that vials can be posted to the vet clinic on the same day.

If FIP cats have a reduced appetite, then mirtazapine is often started at the same time as anti-viral medication, to ensure that the patient is receiving sufficient calories to fight the disease. Dose of mirtazapine is typically 3.75mg (1/4 x 15mg tablet) every 72 hours orally for an adult cat weighing 4kg or above, though dose should be reduced for smaller cats or kittens. Caution with this medication should be used in cats with kidney issues or dehydration.

In addition to remdesivir, the human anti-malerial drug, mefloquine (Larium), can be used orally at a dose rate of 10-12mg/kg orally twice a week with food (eg Wednesdays and Sundays). For an average sized cat, this is approximately ¼ x 250mg tablet. For smaller cats < 5kg, I would recommend ordering this medication through BOVA animal compounding, to ensure the dose is correct.

Some cats do not tolerate mefloquine (vomiting, inappetence), so it is important that cats are eating well prior to starting mefloquine, so that it is obvious if they are having a side effect.

Mefloquine’s mechanism of action is suspected to be preventing the virus from entering macrophage cells (inhibition of endocytosis).

Dr Gretta recommends starting this once the cat is stable on remdesivir or oral GS-441524 therapy, then continuing to use mefloquine for 3 months beyond stopping remdesivir or oral GS-441524 therapy. Clinically stable refers to a resolution of overt signs of FIP and that the cat is eating well. There is some evidence to suggest that this protocol reduces the chance of a relapse as it buys the body some time to allow the cat’s immune system to figure out how to get rid any remaining virus.

Note: The life-span of a macrophage cell is 2 months, so it could be argued that mefloquine treatment should continue for a minimum of 2 months total if tolerated. I personally prefer a longer mefloquine treatment protocol.

Mefloquine is metabolised by the liver, so monitoring liver parameters is helpful if the owner’s finances allow for this.

If owners cannot afford remdesivir or oral GS-441524 tablets, then mefloquine alone can be used as a sole therapy and in some cases, treatment can be successful, so it is worthwhile trying mefloquine in these patients, or using it after an initial shorter course of remdesivir or oral GS-441524, as mefloquine can buy the cat some additional time so that their own immune system can work out how to fight FIP.

Molnupiravir is a newer oral anti-viral, manufactured by MSD for COVID-19 patients (tradename Lagevrio), that is being used as an alternative treatment option for FIP cats, particularly in cases where owners are unable to afford the BOVA compounded options. Dr Gretta Howard is able to advise owners on use of molnupiravir for FIP treatment, though her preference where possible, is to treat cases with oral GS-441524 due to the high safety margin and positive case outcomes.

Dose recommendations (updated July 2024)

All FIP cases (wet/dry/ocular):          15mg/kg twice daily orally

As with GS-441524, treatment duration is recommended for 12 weeks (84 days).

An article by Dr Niels Pedersen is available on this page: “The long history of Beta-D-N4-Hydroxycytidine and its modern application to treatment of COVID-19 in people and FIP in cats”.

EIDD-1931 is another BOVA-sourced FIP anti-viral therapy and is the metabolised active ingredient for molnupiravir, which is known as the prodrug.

In some cases, FIP relapses (also known as FIP recrudescence) can occur and in these cases, or cases where there is poor response to GS-441524, we either opt to repeat the initial GS-441524 course at a higher dose (maximum 20mg/kg) or switch to EIDD-1931, which come in 60mg tablets.

Dose recommendations (updated July 2024)

All FIP cases (wet/dry/ocular):          15mg/kg twice daily orally

As with GS-441524, treatment duration is recommended for 12 weeks (84 days).

There have been less cases treated with EIDD-1931 and this medication is not as well-tolerated as remdesivir or GS-441524, so is generally not used as a first-line therapy for FIP.

Monitoring is recommended every 4-6 weeks until the end of the 12-week treatment period, however, if the cat is doing well on anti-viral therapy, and the owner has financial constraints, then less frequent reassessments may be recommended. Monitoring recommendations vary from patient to patient.

A steady improvement in the albumin to globulin ratio is one of the markers to look for when determining response to therapy. The only problem with this approach, is that we rarely have a pre-FIP albumin to globulin ratio to compare to, so we never really know what a specific individual’s normal levels are. Another positive indicator is seeing the lymphocytes at > 30% on a haematology sample.

It’s vital to treat the patient and not just the laboratory values, because there is no one individual marker that determines whether a patient is cured.

Treating veterinarians must use their best judgement based on clinical response to treatment, laboratory values and the owner’s financial constraints to make decisions about dose increases and extensions and when to discontinue treatment.

If your cat was healthy at the time of taking out pet insurance cover, then your insurance company may cover the cost for FIP treatment. Approval is on a case-by-case basis and subject to review. If you have pet insurance cover for your cat but your claim has been declined, then ask your vet to phone the insurance company’s vet help line to have the claim reviewed. This has sometimes resulted in an overturn on a decision to deny a claim, resulting in a client being able to claim the costs for FIP treatment.

Many drugs used in veterinary medicine, including remdesivir, are not APVMA approved, but are approved by the Australian Register of Therapeutic Goods (ARTG). Given research and journal articles supporting remdesivir as a proven FIP treatment, if your vet prescribes remdesivir for FIP treatment, then it is recommended to submit an insurance claim for consultations and treatment costs.

Dr Gretta Howard is available for face-to-face or telehealth consultations for cat owners based in NSW who wish to seek her opinion on FIP case management. In addition to a consultation, this involves reviewing any prior veterinary records, laboratory results and management recommendations. Consultations (face-to-face or telehealth) are $169 and subsequent reassessment is $99, payable over the phone at the time of booking your appointment. Written prescriptions are an additional $39.50 per medicine prescribed.

If you would like to go ahead with a consultation, please phone our customer care team on (02) 9988-0198 so they can book you in with Dr Gretta Howard at a suitable time. Please ensure to include an update on whether your cat is critically ill or stable (eating, comfortable) so that priority can be given to very sick cats. 

Once booked in, please ask your vet to forward any previous veterinary records, laboratory results and a summary of your own via email to our reception team, including your cat’s current weight if known. Where possible, these records can then be reviewed prior to the appointment time. 

Unfortunately, due to NSW veterinary board regulations, Dr Gretta Howard is unable to provide consultations outside of NSW but may be able to provide you with the names of vets that have experience treating FIP in your Australian state so that you can arrange an appointment with them. Your vet is welcome to email questions regarding case management to Dr Gretta Howard for further assistance on FIP case management if required. ​

 Please click on the following links to reference the PDF article or website.

• Control & Therapy article: “An infectious peritonitis treatment protocol” co-authored by Dr David Hughes and Dr Rebecca Brady (PDF)
• The Veterinarian article: “Treatment of FIP in cats with remdesivir” co-authored by Dr Richard Malik, Dr David Hughes and Dr Gretta Howard (PDF – 6MB)
• Dr Niels Pedersen’s dedicated FIP information page: https://sockfip.org/
• Dr Niels Pedersen’s research article (PDF)
• Dr Jacqui Norris’ update on FIP treatment article (PDF)
• Dr Severine Tasker’s on FIP diagnosis (PDF – 6MB)
• Remdesivir drug profile in Australian Prescriber (PDF)
• BOVA remdesivir reconstitution instructions (PDF)
• Mefloquine pharmacokinetic profile in cats research article (PDF)
• Dr Niels Pedersen’s article on molnupivavir (PDF)
• BOVA FIP updates 2024