Feline infectious peritonitis (FIP): symptoms, diagnosis and the treatment that changed everything
Until a few years ago, an FIP diagnosis meant saying goodbye. Since the arrival of antivirals related to remdesivir, most treated cats are cured. But legal access, diagnosis and cost still raise many questions, and there is an unregulated market around it that you should know about.
What FIP is
Feline infectious peritonitis is caused by a variant of feline coronavirus, a very common virus that has nothing to do with the coronavirus that causes COVID-19 in people. In its usual form, feline coronavirus lives in the gut and at most causes mild diarrhoea; in multi-cat homes, catteries and shelters, most cats have had it. In a small proportion of infected cats, the virus mutates inside the cat itself, changes its behaviour, infects the immune cells called macrophages and spreads through the body, causing severe inflammation of the blood vessels. That is FIP. A cat with FIP does not usually pass on FIP as such: what circulates between cats is the intestinal coronavirus.
Which cats it affects
Mainly young cats, most of them under two, and more often pedigree cats and cats that live or have lived in places with many cats, such as catteries, shelters or colonies, where coronavirus circulates widely. Stress, such as adoption, neutering or moving house, is often linked to the onset of the disease. It can also appear in older cats, although it is less common. It is nobody's fault and not something that can be predicted in a particular cat: it is a combination of virus, the cat's genetics and circumstances.
The symptoms: wet and dry
Two forms are described, although they often overlap. In the effusive or wet form, fluid builds up in the belly, which gradually swells like a balloon, or in the chest, and then the cat struggles to breathe. In the non-effusive or dry form there is no fluid, and the symptoms are harder to recognise: a fever that comes and goes and does not respond to antibiotics, weight loss, lethargy, poor appetite, enlarged lymph nodes, jaundice, eye problems such as uveitis, with cloudy eyes or changes in iris colour, and neurological signs such as wobbliness, tremors, behaviour changes or seizures. In a kitten or young cat with a persistent fever and no clear cause, FIP must be considered.
A diagnosis that needs several pieces
There is no single, simple test that confirms FIP, and a positive coronavirus antibody test does not mean FIP, because a huge number of healthy cats have them. The diagnosis is built by putting together the history, the examination and several tests: in the blood, high total protein, high globulins with a low albumin to globulin ratio, mild anaemia, low lymphocytes and sometimes high bilirubin; in the fluid, if there is any, a yellowish, thick, protein-rich appearance, with specific tests; and detection of the virus in the fluid or tissues by PCR or specific staining. In the dry form, ultrasound, an eye examination and sometimes analysis of cerebrospinal fluid help. A sound diagnosis is essential before starting a long, costly treatment.
Treatment: GS-441524 and remdesivir
Since 2019, studies with GS-441524, the active metabolite of remdesivir, and with remdesivir itself have shown that most cats with FIP treated for about twelve weeks are cured, with success rates of around eighty per cent or more in published studies, including many ocular and neurological forms, which need adjustments. Treatment is given by injection, tablets or a combination, with regular follow-up blood tests, and must be directed by a vet. There are relapses in a minority of cases and some cats do not respond, but the change is enormous: a disease that was almost always fatal has become a treatable one.
The legal situation in Spain, with caution
For years these antivirals were not authorised for cats in Europe and many families bought them on an unregulated market, through social media and messaging groups, with no guarantee of content or quality. The situation has been changing. Commission Implementing Regulation (EU) 2024/1973 provides for the use of remdesivir to treat FIP in cats under the exceptional veterinary prescribing rules, applicable throughout the EU from August 2026, and some countries such as Italy and Belgium opened legal routes earlier, including compounded GS-441524 preparations. In Spain there is not, as of today, a veterinary medicine containing GS-441524 specifically authorised for FIP; any legal treatment goes through your vet's prescription and through medicines or preparations obtained via authorised channels. The specific conditions change, so ask your vet which route is available today.
Why the unauthorised market is a bad idea
Products sold online without oversight may work, and many cats have been cured with them, but there is no guarantee of what they contain or at what concentration, which directly affects the dose and the outcome, and they are often used without a confirmed diagnosis or follow-up blood tests. An insufficient dose encourages relapses and may increase the risk of resistance. What is more, giving an animal an unauthorised medicine of unknown origin can have legal consequences. If you find yourself in this situation, do not do it behind your vet's back: tell them and look for the safest option together.
Care during treatment
As well as the antiviral, the cat may initially need hospitalisation, drainage of the fluid if it is making breathing difficult, fluid therapy, assisted feeding, control of fever and pain and treatment for uveitis. At home, what helps most is a calm environment, tempting food, weighing them every week, because weight changes and the dose may change with it, and sticking to the blood test schedule. Symptoms usually improve within a few days; blood test abnormalities take longer. At the end, there is follow-up over several weeks to watch for relapse.
What it costs
The full treatment is expensive and depends mainly on the cat's weight, the form of the disease (ocular and neurological forms need more medication) and the drug and route used. As a guide, the medication for a twelve-week course usually comes to between 1,500 and 5,000 €, plus the diagnosis (200 to 600 € for blood tests, ultrasound and fluid tests), possible initial hospitalisation and follow-up blood tests at 60 to 150 € each. These are Spanish price ranges. Before starting, ask for the full estimate and the monitoring plan.
Prevention in multi-cat homes
You cannot completely eliminate feline coronavirus from a multi-cat home, but you can reduce the viral load and stress: few cats per space, one litter tray per cat plus one, daily cleaning of the trays, which is how coronavirus spreads, trays away from food and water bowls, and avoiding sudden changes. There is an intranasal FIP vaccine, but its efficacy is limited and guidelines do not recommend it generally. If a cat in your home has died of FIP, there is no need to put down or isolate the others: the sensible approach is to wait a while before bringing in a new kitten and to talk it over with your vet.
FIP is caused by a mutation of feline coronavirus within the cat itself, mainly in young cats. It used to be almost always fatal; today, with GS-441524 or remdesivir for about twelve weeks, most cats are cured. Look for a solid diagnosis and a legal treatment directed by your vet, not by a messaging group.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.