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FIP in cats – is the cure here?

A year ago we wrote an article about Feline Infectious Peritonitis in cats, traditionally a largely incurable and fatal disease. However, we ended with some hope as new advanced antiviral drugs were starting to hit the market. Since, these drugs have become easier for veterinary surgeons treating cats with FIP to acquire, and this author has had one successful (so far) experience in using them. Today, we will briefly recap FIP in cats and the historical situation with treatment, and introduce you to the most up to date information on these possible cures to FIP.

What is FIP?

Feline infectious peritonitis (FIP) is a disease caused by a combination of Feline Enteric Coronavirus (FECV) and a poor immune response. FECV is a common virus found in many cats, especially within multi-cat households. Many cats carry the virus, spread between cats in faeces. If the virus causes disease, it causes mild vomiting, diarrhoea and upper respiratory tract issues, though cats can carry it asymptomatically. However, between 1 and 5% of cats with FECV go on to develop FIP.

We aren’t sure exactly why some cats with FECV will get FIP and others won’t. There’s likely multiple factors involved, including virus mutations and the cat’s health and genetics. Unfortunately, FIP is mostly seen in very young cats under 3, including kittens. 

FIP initially causes vague and mild symptoms, such as weight loss, depression, lethargy and inappetence. It can then progress to either wet FIP or dry FIP, or a combination of the two. Wet FIP causes fluid to build up in the abdomen and lungs, a yellow colour to the skin, a high temperature, and rarely brain and eye problems. Dry FIP leads to damage to multiple organs, as well as a higher risk of brain and eye issues. It often is slower to progress than wet FIP.

In either case, FIP is almost always fatal.

How Do We Diagnose FIP?

Diagnosis of FIP can be difficult. Any young cat that presents as severely unwell could have FIP, particularly if they are lethargic, have a bloated belly, and yellow skin and eyes. A basic blood test gives more clues, such as anaemia, changes to white blood cell counts, high protein levels, high bilirubin, liver damage and more. 

More definitive diagnosis requires identification of FECV. However, as mentioned above, many cats have FECV but do not develop FIP, so just finding FECV in blood or fluids doesn’t prove there is FIP, but it can raise the suspicion. If there is a significant amount of fluid in the abdomen with high amounts of FECV, then FIP is likely. The only definitive diagnosis requires either a biopsy, requiring a risky general anaesthetic, or a post-mortem examination. Thus, many cases of FIP are only presumptively diagnosed or done so after death.

What Were the Traditional Treatments?

Unfortunately, the traditional treatments for FIP were mostly palliative, and involved anti-inflammatory steroids and supportive symptomatic care, such as appetite stimulation, fluid therapy, draining excess fluid, and vitamin injections. This treatment generally kept the cat as comfortable as possible, but usually ended with euthanasia. Many vets recommend not even attempting treatment and instead advise euthanasia as soon as a diagnosis is made to prevent future suffering. Many other drugs have been tried with varying levels of success, but there is little evidence to back up their use. 

And the New “Cures”?

One class of drug has shown promise in treating FIP, these being human antiviral nucleotide analogues – they work by stopping the virus from replicating, slowing down and eventually stopping infection. The main drugs are GS-441524 and its pro-drug Remdesivir/GS-5734 (a drug that is broken down to GS-441524 in the body). These drugs can be given intravenously, via a subcutaneous injection or orally. There are multiple studies and anecdotal cases of cats surviving for far longer with FIP than normal, or even successfully surviving and being considered ‘cured’. 

The studies

One study of 10 cats found that all 10 were successfully ‘cured’ 8 months after they started treatment, though two required a second course of treatment. All eighteen of 18 cats were cured after oral medication was started in another study. The Royal Veterinary College documented a case where one cat responded very well to treatment at home, and reported 5 more cats were undergoing treatment. One article reported that its use to treat 500 cats by Australian vets was “highly effective”. Treatment appears to be rapid, with most cats showing significant improvement within only a few days. Anecdotal evidence appears to result in 80-95% of cats being successfully treated.

One of the most comprehensive studies involve 31 cats on long term treatment for FIP. Five were euthanised before treatment could be completed. Eighteen were healthy for the two years between treatment and the article being written after one course. Eight relapsed – of these, one was euthanised but the others recovered after two or three more courses of treatment (one cat was also euthanised for an unrelated heart problem). However, it is important to note that none of these cats had ‘severe neurological or ocular FIP’ before starting treatment. The drug was also very safe to give even at high doses. Ignoring the four that died within 2-5 days of starting treatment, this gives a cure rate of 25/26 cats that finished at least one course of treatment, or 96%. Including all cats, the success rate was still around 80%.

These cure rates are staggeringly high and seem a miracle for a disease that was once almost 100% fatal. However, it is important not to rush to conclusions and there is a lot to consider for pet owners wishing to pursue treatment.

Why These Drugs Aren’t the Catch-All Cure (Yet)

The biggest problem with the use of these drugs is the lack of evidence.

Certainly the anecdotal evidence is promising and even proper studies show good results, but these have all been limited in scope. There is a lot we do not know, particularly whether cats will relapse years later, as some vets have suggested. Doses for the drug, how precisely it is used, how often to monitor, what to monitor for, and more are all highly variable and clinician-dependent. This is particularly true for the oral tablets, which have even less studies on their use.

The other problem with the drugs is their availability and price

Until very recently, it was incredibly difficult to obtain these drugs in the UK legally – it generally required ordering in specially from abroad, which could take time the cat may not have. As such, some cat owners have been reported as having ordered the drug themselves from abroad. We want to emphasise that this is highly illegal drug importation and use, could result in fines or even prison, and no vet can condone it. It is also risky as there is no guarantee to the efficacy, quality or safety of the drugs. 

However, as of a few months ago, one drug company in the UK has started supplying the medications, as off-licence “special preparations” making ordering them significantly easier for vets (in my practice, we received both drugs within two days of ordering). But the cost is still highly prohibitive for many owners, with a full treatment course reaching £5000 or more, not including vet visits and monitoring, or further treatment if the cat relapses. This makes treatment for FIP simply unobtainable for many cat owners. Hopefully, as the drugs become more available, they may become more affordable. It is unknown if pet insurance will cover for these sorts of medications, and likely will be dependent on individual companies and policies.

One other aspect to consider is the drug’s use in human medicine, in particular Remdesivir

As an antiviral, these drugs have been used to treat human-viral infections, notably those suffering from COVID-19. In fact, it is partly due to Remdesivir’s use in humans and the increased production to treat COVID patients that the drug has become more widely available to UK vets for cats with FIP. Just like antibiotics, viruses can mutate and gain resistance to antiviral drugs. Some articles have questioned whether the use of these drugs in cats may inadvertently lead to resistance, meaning humans may not be able to be treated as effectively – there is certainly an argument that the drugs should be protected for human use only. Time will tell if this changes.

Considerations for Owners Using These Drugs

If you elect to pursue treatment for FIP, there are some considerations with regards to the drugs. Some of this information is anecdotal however. 

Firstly is the treatment protocol

As mentioned above, there is no set protocol, but numerous papers have submitted recommended dosages. It seems the consensus is that the more severe the FIP, the higher the dose. This means costs can rapidly skyrocket if the cat is more unwell. All protocols recommend a daily injection or tablet for 84 days, then a reassessment. If they are still unwell, a second 84-day course may be recommended. It is recommended to stop treatment only once the cat is well both clinically and after a blood test.

For the most severe cases, it is recommended the drug is given at a high dose intravenously in hospital before moving to daily subcutaneous injections. An owner can learn how to do this, allowing the cat to go home. For more stable cats, they can switch to daily oral tablets after two weeks, though remember that oral medication has been studied less than injections. 

The liquid forms of the drug are stable for a week in the fridge, meaning any product remaining at the end may not be effective, resulting in expensive wasted drug. 

The drugs appear to be currently very safe, but do have some mild side effects

anaesthetic cream on the injection site, and sedating the cat first have all been suggested to assist in administration. It can also result in swelling at the site of injection, depression, nausea, and a temporary worsening of fluid build-up in the first few days of use. It has also been reported that in cats with asymptomatic neurological FIP, treatment can unmask the symptoms and result in seizures. 

The drugs are human antiviral agents, but we have found little information regarding the consequences if humans accidentally inject themselves or swallow the tablet. As such, the highest safety must be enforced, keeping tablets well away from children and animals, and seeking a doctor’s assistance if accidentally self-injected. 

So is it worth it?

Without treatment, almost 100% of affected cats will die. But at the moment, that treatment is expensive and difficult to use, so if you decide it’s not right for your cat, that is a perfectly reasonable decision, and we won’t judge you for it.

Further Reading


The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies – ScienceDirect

Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis

Curing Cats with Feline Infectious Peritonitis with an Oral Multi-Component Drug Containing GS-441524 – PMC

Successful treatment of a South African cat with effusive feline infectious peritonitis with remdesivir 

Other Information:

FIP in cats – is there now hope? 

Successful FIP treatment with remdesivir – RVC 

Feline Infectious Peritonitis (FIP) Statement June 2022 – The London Cat Clinic 

Feline Infectious Peritonitis (FIP) – North Downs Specialist Referrals

Feline Infectious Peritonitis – ABCD Cats Vets 

ISFM Protocol: An update on a treatment of feline infectious peritonitis in the UK (Updated November 2021)

Treatment of FIP in cats with subcutaneous remdesivir followed by oral GS-441524 tablets – Zenby Cat

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