"My cat is 8 months old, lately lethargic, with a fever that rises and falls, getting thinner, and his belly looks a bit bloated, the vet suspects FIP. What does that mean? Is there still hope?" A few years ago, the answer to this question was almost always heartbreaking. But since 2018-2019, there's been a major change in veterinary medicine regarding FIP, a disease once considered a "death sentence" now has treatment options with a significant recovery rate.
This article helps you recognise the signs of FIP, understand why it's hard to diagnose, and know the latest treatment options (including GS-441524, whose regulatory status in Indonesia is still a grey area). Important: FIP diagnosis and treatment must always be with an experienced vet, this article is educational guidance, not a substitute for a consultation.
What is FIP (Feline Infectious Peritonitis)?
FIP is a serious disease in cats caused by a mutation of the feline coronavirus (FCoV). What makes this disease tricky to understand:
- "Ordinary" FCoV is very common, many cats (especially those living with other cats) are infected with enteric FCoV without serious problems. What usually appears is just mild diarrhea or even no signs at all.
- In a small minority of cats, FCoV mutates inside the cat's own body into a form that can attack immune cells (macrophages) and spread to various organs. This mutation is what causes FIP.
- FIP is NOT transmitted from cat to cat directly as "FIP." What's transmitted is ordinary FCoV. The mutation to FIP happens individually per cat.
That's why you can have 5 cats that are all FCoV-antibody positive (because they've been exposed), but only 1 develops FIP.
Why are young and purebred cats more susceptible?
Based on the case patterns consistently reported in the literature (e.g. the classic Pedersen review in JFMS 2009 and ABCD/ISFM updates):
- Age, the majority of FIP cases appear in cats under 2 years of age, with a peak incidence around 4 months – 1.5 years. It can also appear in older cats, but the proportion is far smaller.
- Genetics / breed, some breeds are reported as more susceptible, including Persian, Ragdoll, Bengal, Burmese, Birman, Maine Coon, Abyssinian. It doesn't mean FIP is certain, only population statistics.
- Multi-cat / cattery environment, high density = high FCoV circulation = increased chance of mutation.
- Stress and immunosuppression, newly adopted cats, recently moved, recently spayed/neutered or had major surgery, or immunocompromised cats are at higher risk of triggering the mutation.
An important note: FIP can still occur in single-cat households, mixed breeds, and at any age. The pattern above is not an absolute rule.
The two clinical forms of FIP: wet (effusive) vs dry (non-effusive)
Classic FIP is divided into 2 clinical forms based on the presence or absence of abnormal fluid accumulation. Many cases actually overlap or move from one form to the other.
Wet / effusive FIP
Main feature: fluid accumulation in body cavities.
- Ascites (fluid in the abdomen), the belly looks enlarged, bloated, and on palpation feels fluid-filled. This is the most common form and the quickest to recognise.
- Pleural effusion (fluid in the chest cavity), the cat's breathing becomes rapid and laboured, it tends to sit with its chest upright (orthopnea), and appetite drops because it's hard to breathe.
- A combination of both can also appear.
Wet FIP usually progresses faster, weeks to months. The fluid is characteristic: straw-yellow, viscous, sometimes foamy, high in protein. This is a feature highly suggestive of FIP when a vet performs a fluid tap.
Dry / non-effusive FIP
Main feature: no fluid, but granulomatous lesions (chronic inflammation) in the organs.
- Neurological signs, ataxia (wobbly walking), seizures, behaviour changes, head tilt, nystagmus (abnormal eye movements). About a third of dry FIP cases involve the nervous system.
- Ocular (eye) signs, uveitis (inflammation inside the eye), iris colour change, small bleeds in the eye, asymmetric changes in pupil size.
- Granulomas in visceral organs, kidneys, liver, mesenteric lymph nodes, intestinal wall. May be palpable as a mass or seen on ultrasound.
Dry FIP usually progresses more slowly than the wet form (months), but is still fatal if untreated. Its diagnosis is also more difficult because the signs are non-specific and can resemble many other diseases.
Common clinical signs of FIP, the ones that often appear
Whatever the form, there are some common signs that often appear. If your young cat shows several of these persistently, FIP needs to be on the differential diagnosis list:
- Persistent fever unresponsive to antibiotics, usually rising and falling, often in the high range (above 39.5°C, sometimes over 40°C). Antibiotics don't lower this fever because the cause isn't bacterial.
- Gradual weight and muscle loss, even if it still eats a little early on, the cat looks increasingly thin, with the spine and pelvic bones becoming more prominent.
- Lethargy, a usually active cat becomes quiet, sleeps more, plays less, interacts less.
- Gradually declining appetite, eventually leading to complete anorexia.
- Dull coat and decreased grooming.
- Pale gums (anemia) in advanced cases.
- Jaundice (icterus), yellowing of the gums, the skin inside the ears, or the whites of the eyes. A sign of liver involvement.
What's characteristic and confusing for owners: a combination of non-specific signs + no response to usual treatment. Many FIP cats go "back and forth to the vet, given antibiotics, briefly better, relapse again" before the diagnosis is finally established.
If your young cat shows this pattern, don't delay a deeper evaluation. For owners in Jakarta and Greater Jakarta who find it hard to bring a very weak sick cat to the clinic, WhatsApp Prabasa Vet for an initial discussion, we'll help assess the situation and schedule a visit for an initial examination at home.
Why is FIP difficult to diagnose?
This is one of the most frequent questions: "Why can't the vet just run a positive/negative FIP test like a Covid test?"
The answer: to this day, there isn't a single test that can confirm FIP 100% in a living cat via an easy, non-invasive method. What are commonly called "FIP tests" (corona antibody, PCR) don't actually distinguish ordinary FCoV from the FIP mutation. An FIP diagnosis is usually built from a combination of several examinations + clinical context assessed by an experienced vet.
1. History and physical examination
The cat's profile (age, breed, environment), the pattern of signs (persistent fever, weight loss, ascites/dyspnea), the response to previous treatment.
2. Blood tests
Several markers that are highly suggestive of FIP (not diagnostic, but supportive):
- Hyperglobulinemia, globulin levels in the blood are highly elevated (an increased chronic immune response).
- Low A:G ratio, the albumin-to-globulin ratio drops, often below 0.4–0.6 in many FIP cases. The lower it is, the more suggestive (not diagnostic).
- High AGP (alpha-1 acid glycoprotein), an acute phase protein, often sharply increased in FIP (often cited above 1500 mcg/ml as a suggestive cut-off, but not universal). Not all labs in Indonesia run AGP.
- Non-regenerative anemia, often appears in advanced FIP.
- Hyperbilirubinemia without hemolysis, suggestive of liver involvement.
- Leukocytosis with lymphopenia, a pattern that often appears.
3. Fluid analysis (if wet FIP)
If there's ascites or pleural effusion, thoracocentesis or abdominocentesis (a needle fluid tap) is one of the most informative steps. FIP fluid is usually:
- Straw-yellow in colour, viscous, sometimes foamy
- Very high total protein (often >3.5 g/dL)
- Low A:G ratio in the fluid (similar to the blood pattern)
- Positive Rivalta test, a simple test mixing a drop of fluid with a dilute acetic acid solution; if the drop holds its shape and sinks slowly (positive), it's highly suggestive of FIP or another exudative disease. The Rivalta test has high sensitivity and specificity for FIP in cats with effusion, especially when combined with blood tests and imaging.
- FCoV PCR from the fluid, if positive in effusion fluid, it's far more suggestive of FIP than PCR from blood/feces.
4. Imaging: ultrasound, MRI/CT
Abdominal ultrasound helps assess involvement of the kidneys, liver, and mesenteric lymph nodes, and the presence of small amounts of non-palpable fluid. For dry FIP with neurological signs, MRI or CT can be considered if available (limited in Indonesia), to look for granulomatous lesions in the brain/spinal cord.
5. Histopathology
Tissue examination (biopsy or post-mortem) with immunostaining remains the gold standard. For a living cat, biopsy is rarely performed because it's invasive, the diagnosis is usually established from a combination of points 1–4 above.
The bottom line: an FIP diagnosis is a composite diagnosis, a combination of the cat's profile + signs + lab + fluid + imaging, assessed together by an experienced vet. Don't conclude FIP from a single test result alone.
FIP treatment: from "always fatal" to hope of recovery
This is the part that has changed dramatically in the last 5–7 years.
Before 2018: prognosis almost always fatal
Up until around 2017–2018, FIP was considered a disease that almost always ended in death. All a vet could do was palliative: reducing fluid (periodic taps), corticosteroids for immune suppression, pain management, nutritional support. Most wet FIP cats died within weeks to months.
2018–2019 onwards: the GS-441524 breakthrough
Research by Pedersen et al. at UC Davis (a key publication in JFMS 2019) showed that GS-441524 , an antiviral nucleoside analogue, could achieve remission in the majority of FIP cats treated with a 12-week protocol. Early studies reported a very promising response rate, and many follow-up studies and global clinical experience since then have been consistent with those findings.
GS-441524 is structurally the active metabolite of remdesivir (the drug used for human Covid). It works by inhibiting FCoV replication inside cells.
General protocol:
- Daily subcutaneous injection or daily oral administration for 84 days (12 weeks), the base protocol.
- The dose is adjusted to the FIP form (effusive, dry, neurological, ocular), neurological cases usually need a higher dose because of drug penetration into the brain.
- Routine monitoring during treatment: weight, fever, A:G ratio, AGP (if available), appetite, general condition. Many cats show dramatic improvement in the first 1–2 weeks.
- Post-treatment observation period of at least 12 weeks after the last injection to assess cure vs relapse.
Regulatory status in Indonesia, a grey area
What's important to understand: GS-441524 to date does not have official registration as a veterinary medicine in many countries, including Indonesia. What's available are products from unofficial sources (often called "compounded" or "research-grade"). The implications:
- The quality, concentration, and safety of the product vary greatly between sources
- There's no guarantee of authenticity/purity like a registered pharmaceutical product
- Prices can vary greatly
- A vet handling FIP treatment must be experienced with this protocol and have a reputable product source
Because of this still-grey regulatory situation, we do not list specific product recommendations or exact costs in this article. Discussion of sources, quality, protocol, and cost must be done directly with a vet experienced in handling FIP.
Important supportive treatment during the protocol
- Good nutrition, if the cat won't eat, it may need a nasogastric tube, esophagostomy tube, or an appetite stimulant (mirtazapine is often used)
- Fluid management, periodic taps if a massive effusion impairs breathing/comfort
- Pain management as needed
- A quiet, low-stress environment with easy access to food and water
- Monitoring comorbidities (parasites, dehydration)
Prognosis and hope of recovery
The prognosis of FIP depends greatly on several factors:
- Access to quality GS-441524 + an experienced vet, this is the biggest factor
- Clinical form, classic wet FIP without neurological involvement usually responds best. Neurological FIP needs a higher dose and its prognosis is more guarded.
- Stage at diagnosis, the earlier treatment starts, the better. A cat that's already very weak (very low PCV, very dehydrated, multi-organ failure) has a more reserved prognosis.
- Adherence to the daily protocol over the full 84 days without a missed dose, very important to prevent viral resistance.
- Comorbid conditions, a cat with another concurrent disease (FeLV, FIV, etc.) has a more complex prognosis.
Without antiviral treatment, classic FIP is still considered fatal. With access to GS-441524 + an experienced vet + a sufficiently early diagnosis, many cats successfully achieve long-term remission.
FIP prevention
Because FIP arises from a mutation of FCoV already present in the cat's body, 100% prevention is difficult. What can help lower the risk:
- Reduce cat density in one environment, the fewer cats per space, the lower the FCoV circulation
- Litter box hygiene, at least 1 box per cat + 1 extra, scoop at least once a day, separate from the feeding area
- Quarantine a new cat for several weeks before contact with the resident cat, at minimum to observe for clinical signs
- Minimise stress, especially for kittens and young cats; consider the timing of spay/neuter, adoption, and house moves
- Good nutrition and general health, a healthy immune system is better able to control FCoV without mutation
- Avoid buying a cat from a breeder/cattery with a history of recurring FIP
An intranasal FIP vaccine was once developed but its efficacy and use recommendation remain controversial and it is not standard care in many international guidelines.
FAQ about FIP
If one cat at home is FIP positive, will my other cats certainly be infected?
No. What's contagious is FCoV (very common), not "FIP" itself. If there are other cats, they likely also already have FCoV antibodies (if they've shared an environment), but their chance of developing FIP is still small. Maintain hygiene + minimise stress for the other cats.
My cat's corona antibody test is positive, does this mean it has FIP?
No. An FCoV antibody test only shows that the cat has been exposed to FCoV, which is very common, especially in a multi-cat household. A positive antibody is NOT an FIP diagnosis. The majority of cats with a positive antibody never develop FIP.
How long is FIP treatment with GS-441524 and does it require hospitalisation?
The standard protocol is 84 days (12 weeks) of daily injection/oral, plus a 12-week post-treatment observation. It doesn't have to be inpatient, most cats undergo treatment at home with periodic vet checks (weekly/biweekly for monitoring). But if the cat is very weak early on (severe dehydration, complete anorexia, laboured breathing), initial hospitalisation may be needed for stabilisation.
How much does FIP treatment cost in Indonesia?
It varies greatly because GS-441524 is still a regulatory grey area, the cost depends on the product source, dose (cat's weight), clinical form (neurological needs a higher dose), duration, and the vet's fee. Because the variation is so large, we do not list specific figures in this article. Cost discussion must be directly with a vet experienced in handling FIP, including the treatment + lab monitoring + supportive care components.
My kitten was adopted 3 months ago and is perfectly healthy, should I worry about FIP?
No need to be paranoid, but stay vigilant. Cats aged 4 months – 1.5 years are in the highest-risk window. What you can do: reduce stress (don't make many environmental changes at once), maintain nutrition and hydration, avoid intense contact with many other cats from unclear sources, and observe for clinical signs, persistent fever, gradual weight loss, a bloated belly, or persistent lethargy. If there's a suspicious sign, consult a vet early.
After finishing the 12-week treatment, is my cat free of FIP forever?
Not automatically. After the 12-week protocol, there's a 12-week (sometimes longer) observation period to assess for relapse. If after the observation period the cat remains stable with no clinical signs and good lab parameters, many vets consider it long-term remission. Some cats still need periodic monitoring. Consult your vet for an individual follow-up schedule.
Closing
FIP used to be one of the most devastating diagnoses for a cat owner, a verdict that almost always meant parting within weeks to months. Now the situation has changed. With the GS-441524 breakthrough, many FIP cats that previously had no hope can now achieve remission and live for years with good quality of life.
But the main key remains the same: early diagnosis + an experienced vet + a consistent protocol. If your young cat shows a combination of suspicious signs, persistent fever unresponsive to antibiotics, weight loss, a bloated belly, unusual lethargy, don't delay a deeper evaluation. Every week lost before the diagnosis is established can affect the prognosis.
For cat owners in Jakarta and Greater Jakarta who suspect FIP or have just received a tentative diagnosis from a vet and want to discuss the next steps, WhatsApp Prabasa Vet for an initial discussion. We'll help assess the situation and, if possible, schedule a visit for an initial examination at home so that an already-weak cat doesn't have to endure the stress of travelling to the clinic.
Read also: Signs of a feline emergency you must not delay, Cat won't eat: causes and when to see a vet, Senior cats at 10 years: changes and geriatric care, Pet emergency guide (pillar).
Medical references used in this article
This article was prepared with reference to the following sources, verified per clinical statement:
- Addie DD, Belak S, Boucraut-Baralon C, et al; ABCD (European Advisory Board on Cat Diseases). Feline Infectious Peritonitis, ABCD Guidelines (update Dec 2021). The European consensus for FIP diagnosis + management, including the composite diagnosis approach and discussion of GS-441524 status
- Pedersen NC. A review of feline infectious peritonitis virus infection: 1963–2008. Journal of Feline Medicine and Surgery (JFMS) 2009 , a classic review of pathophysiology + epidemiological patterns (age, breed, multi-cat environment)
- 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. Journal of Feline Medicine and Surgery (JFMS) 2019 , the key study that validated GS-441524 as an effective FIP treatment
- Tasker S, Addie DD, Egberink H, et al; ISFM/ABCD. 2022 ISFM Guidelines on the Diagnosis and Management of FIP. Journal of Feline Medicine and Surgery (JFMS) 2022 , an update of the diagnostic protocol (A:G ratio, AGP, Rivalta, fluid analysis) + the GS-441524 treatment framework
- Hartmann K. Coronavirus infections (canine and feline), FIP section. In: Ettinger SJ et al, Textbook of Veterinary Internal Medicine, a standard internal medicine reference
- Plumb's Veterinary Drug Handbook 7e, mirtazapine (appetite stimulant), maropitant (anti-emetic), prednisolone (supportive palliative) dosing references
This article is a general guide based on the ABCD/ISFM consensus + textbooks + the key GS-441524 study. For the diagnosis and treatment of your cat's specific FIP, including the antiviral protocol, product source, dose, and cost, consulting a vet experienced in handling FIP is the right step. The regulatory status of GS-441524 in Indonesia is still evolving; a discussion with a vet is mandatory before starting any protocol.
