What Is Feline Infectious Peritonitis (FIP)?
FIP was long described as a disease no cat survived once the signs appeared. Antiviral treatment has changed that. Here is the whole picture — cause, signs, diagnosis and treatment — in plain language.
FIP starts with feline coronavirus (FCoV), a virus most cats carry at some point and which causes little or no harm on its own.
In a small number of cats the virus changes inside the body and begins multiplying inside immune cells, which leads to inflammation of blood vessels throughout the body. That is FIP.
Treatment centres on the antiviral GS-441524. Starting early matters, and completing the full course matters just as much.
What FIP is
The virus behind FIP is feline coronavirus (FCoV). Despite the name it is a different virus from the one that causes COVID-19 in people, and it does not infect humans.
FCoV spreads through faeces: virus shed by one cat is picked up by another through grooming or shared litter. Wherever several cats live together — multi-cat households, shelters, breeders, pet shops — most cats become infected at some point.
Infection, however, is not the same as illness. Most cats show nothing at all, or a bout of mild diarrhoea that settles by itself. The problem arises only in the minority of cats in which the virus changes its behaviour. Reported figures vary, but only a few per cent up to roughly 10% of infected cats go on to develop FIP.
Figure 1: FCoV is a common virus and most infected cats never become ill. Reported proportions vary between studies; the risk is higher in cats under two years old and in multi-cat environments.
What happens inside the cat
As FCoV replicates in the gut, copying errors — mutations — occur from time to time. Occasionally a mutation gives the virus the ability to replicate inside macrophages, the immune cells that patrol the body, rather than staying in the cells lining the intestine.
Once that happens, the immune system itself carries the virus everywhere. Wherever it lands, the immune response overreacts and intense inflammation develops around the blood vessels. That state is FIP.
Inflamed vessels leak fluid and protein, and the fluid collects in the abdomen or the chest. In other cats the inflammation instead forms small lumps, or granulomas, inside organs. One disease, two very different appearances.
Figure 2: FIP is not usually caught from outside. It develops when a virus already present in the cat changes its behaviour.
A cat with FIP does not normally pass the disease to its housemates. What moves between cats is the ordinary enteric coronavirus.
There are exceptions. FCoV-23, which spread through Cyprus in 2023, is a recombinant virus carrying spike sequence from a pantropic canine coronavirus. It spread directly from cat to cat and caused FIP at an unusually high rate (Attipa et al. 2023), and it reached the UK in an imported cat (Warr et al. 2023). Such strains are rare, but “FIP is never contagious” is too strong a statement.
Four ways it shows itself
FIP is named by where the inflammation does the most damage. The categories overlap, and more than one can be present in the same cat.
Figure 3: The four presentations. Wet and dry forms can shift into one another, and eye or neurological signs can appear alongside either.
Signs that should prompt a vet visit
Taken one at a time, the signs of FIP look like any minor illness. What matters is that several appear together and persist over days to weeks.
- Fever that will not settle — no response to antibiotics, or it falls and then returns
- Lethargy and reduced appetite that continue rather than pass
- Weight loss, or a kitten that fails to gain weight
- A swelling belly while the spine and legs become thinner
- Fast, shallow breathing and reluctance to move — a sign of fluid in the chest
- Yellow tinge to the gums or the whites of the eyes (jaundice)
- A change in eye colour or clarity, or pupils of different sizes
- Unsteady walking, loss of balance, seizures
When a visit is delayed, the situation deteriorates in stages.
-
Fever and poor appetite continue
Weight and strength fall, leaving less reserve to get through treatment -
Fluid collects in the belly or chest
Breathing becomes difficult and the cat can no longer rest lying down -
Inflammation reaches the eyes, brain and spinal cord
Uveitis, wobbliness, seizures — and lesions in sites the drug reaches poorly -
Treatment starts in a debilitated cat
Response is slower, remission takes longer, and relapse remains a concern
A fever that does not respond to antibiotics, together with a young cat that is not gaining weight. When both are present, ask your vet to consider FIP early. The disease moves quickly and a delay of a few days can change the outcome.
How the diagnosis is made
The most important thing to understand is that no single test says “positive, therefore FIP”. Vets build the picture from clinical signs, blood work, imaging and the nature of any fluid present.
Figure 4: Diagnosis is built in three layers. When an effusion is present, sampling it for cytology, biochemistry and FCoV antigen or RNA is the single most useful step (ABCD 2023). In practice, treatment is often started at stage 2 rather than waiting for confirmation.
What the common tests actually tell you
| Test | What it shows | Limitation |
|---|---|---|
| Biochemistry | Prolonged inflammation raises globulin and lowers albumin; a low albumin-to-globulin ratio is typical of FIP | Other chronic inflammatory diseases behave the same way; not diagnostic alone |
| SAA / AGP | A fast, numerical measure of how much inflammation is present | Measures inflammation, not FIP; most useful for tracking response |
| Haematology | Anaemia, low lymphocyte counts, and jaundice from raised bilirubin | Indicates severity, not cause |
| Imaging | Small volumes of fluid, enlarged lymph nodes, nodules in organs | Findings can be absent early; a clear scan does not rule FIP out |
| Effusion | Colour, viscosity, protein content and cell types; FIP fluid is straw-coloured, viscous and protein-rich | Other diseases can produce similar fluid, though the contribution is large |
| RT-PCR | Detects viral RNA in fluid or tissue | A positive result on faeces or blood only shows FCoV is present |
| Immunostaining | Shows viral antigen directly inside cells; this is the confirmatory method | Requires a sample and is not available at every practice |
First, a high coronavirus antibody titre does not mean FIP. Perfectly healthy cats can have high titres, and antibody levels cannot make the diagnosis.
Second, a positive spike (S) gene mutation PCR is not confirmation either. It is often presented as definitive, but it is a supporting test; confirmation rests on antigen detection in tissue or cells.
Treatment
Treatment now centres on the antiviral GS-441524. It slips into the viral genome as the virus copies itself and halts the copy partway through. Remdesivir, the human drug, converts into this same molecule inside the body.
The first field trial, published in 2019, treated 31 cats with naturally occurring FIP using daily subcutaneous injections for 12 weeks and found the approach safe and effective (Pedersen et al. 2019). Prospective oral studies and long-term follow-up have followed (Zwicklbauer et al. 2023). A disease once considered uniformly fatal is now one in which remission is a realistic goal.
Figure 5: The course of treatment. A randomised controlled study found 42 days as effective as 84 days in cats with effusive FIP (Zuzzi-Krebitz et al. 2024), so duration is a clinical decision case by case.
What matters during treatment
- Earlier is better. Starting before the cat is severely debilitated has a large effect on the outcome.
- Injectable and oral forms both exist. Injections are often used while the cat is unwell because absorption is more reliable, with a switch to tablets once things stabilise.
- Neurological and ocular cases need higher doses, because the brain and the eye are shielded by barriers that limit drug penetration. These forms are harder to treat and relapse more often.
- Response is judged by the cat and the numbers together: fever resolving, appetite and weight returning, A:G ratio and SAA normalising.
- Do not stop partway. Cats look well long before treatment is complete, and stopping there raises the risk of relapse.
In many countries, including Japan, products containing GS-441524 are not licensed as veterinary medicines. Where that is the case, treatment should be carried out under veterinary supervision, and owners should not set dose, duration or stopping points on their own.
Reducing risk in multi-cat homes
There is no established way to prevent FIP directly. What can be done is to reduce how much FCoV cats are exposed to, and to reduce stress.
- One litter tray per cat, plus one spare; too few trays means more faecal-oral exchange
- Scoop often, and use trays and placement that limit litter scatter
- Keep trays away from food and water
- Avoid overcrowding, which raises both viral load and stress
- Introduce new cats gradually, in a separate room at first
- FCoV vaccination is generally not recommended
Common questions
Can people catch it?
No. Feline coronavirus infects cats and is a different virus from the one that causes COVID-19 in humans.
Will my other cats catch FIP?
FIP itself is not normally transmitted between cats; what spreads is ordinary FCoV. The exception is unusual strains such as FCoV-23, which did spread directly between cats and caused FIP at a high rate, so it is sensible to keep an eye on housemates.
My vet says FIP is likely. Should we wait for confirmation?
Confirmatory testing is invasive and takes time, and FIP progresses fast. When the clinical picture and blood work line up, starting treatment before confirmation is a common and reasonable decision. Discuss the timing with your vet.
Once a cat recovers, can it relapse?
Yes. Most relapses happen within a few weeks of finishing treatment, and they are more frequent in neurological cases. Keep up weight checks and blood tests through the monitoring period.
Does draining the fluid help?
When there is enough fluid to make breathing difficult, draining brings immediate relief. It treats the symptom rather than the virus, so it belongs alongside antiviral treatment rather than instead of it.
References
- Tasker S, Addie DD, Egberink H, et al. Feline Infectious Peritonitis: European Advisory Board on Cat Diseases Guidelines. Viruses. 2023;15(9):1847. doi:10.3390/v15091847
- Pedersen NC, Perron M, Bannasch M, et al. Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. J Feline Med Surg. 2019;21(4):271–281. doi:10.1177/1098612X19825701
- Zwicklbauer K, Krentz D, Bergmann M, et al. Long-term follow-up of cats in complete remission after treatment of feline infectious peritonitis with oral GS-441524. J Feline Med Surg. 2023;25(8). doi:10.1177/1098612X231183250
- Zuzzi-Krebitz AM, Buchta K, Bergmann M, et al. 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. doi:10.3390/v16071144
- Attipa C, Gunn-Moore D, Mazeri S, et al. Concerning feline infectious peritonitis outbreak in Cyprus. Vet Rec. 2023;192(11):449–450. doi:10.1002/vetr.3143
- Warr A, Attipa C, Gunn-Moore D, Tait-Burkard C. FCoV-23 causing FIP in a cat imported to the UK from Cyprus. Vet Rec. 2023;193(10):414–415. doi:10.1002/vetr.3696