FIP in Cats: Diagnosis, Treatment & Recovery Guide
What is FIP?
Feline Infectious Peritonitis (FIP) is a serious disease caused when feline coronavirus (FCoV) infection develops into FIP within an individual cat. FIP can affect different parts of the body and may present with fluid accumulation (often called wet FIP), without obvious fluid (dry FIP), or with ocular (eye) and neurological signs.
Common signs include persistent fever, loss of appetite, weight loss and lethargy. Cats with wet FIP may develop a swollen abdomen or breathing difficulty due to fluid accumulation, while ocular or neurological FIP can cause eye changes, poor coordination, tremors or seizures.
How Is FIP Diagnosed?
There is no single simple test that can diagnose every case of FIP. Veterinarians usually combine the cat's clinical signs, blood test results, imaging and FCoV-specific testing to determine how likely FIP is.
When abdominal or chest fluid is present, testing this effusion is particularly useful and should be prioritised. For cats without fluid, further investigation such as ultrasound and sampling of affected organs may be required. Diagnosis should always consider the overall clinical picture rather than relying on a coronavirus-positive result alone.
Wet Form
Wet FIP causes fluid to build up in the abdomen or chest. Cats may develop a swollen belly, difficulty breathing, persistent fever, poor appetite, weight loss and lethargy. Testing the accumulated fluid can be particularly helpful in diagnosing FIP.
Dry Form
Dry FIP develops without obvious fluid accumulation and may affect organs such as the kidneys, liver, intestines or lymph nodes. Common signs include persistent fever, poor appetite, weight loss and lethargy, although symptoms vary depending on the organs affected.
Neurological FIP
Neurological FIP affects the brain or nervous system. Cats may show unsteady walking, poor coordination, weakness, tremors, behavioural changes or seizures. Neurological involvement can occur alone or together with other forms of FIP.
Ocular FIP
Ocular FIP affects the eyes, causing inflammation and visible changes such as cloudiness, abnormal pupils, colour changes or reduced vision. Eye involvement may occur on its own or alongside other signs of FIP.
How Is FIP Treated?
FIP can now be treated with antiviral medication. GS-441524 is currently the most extensively used antiviral for FIP, while remdesivir may also be used, particularly in severely affected cats that initially cannot tolerate oral medication.
Treatment needs to be matched to the individual cat and the type and severity of FIP. Cats should also be weighed regularly during recovery because the antiviral dose may need adjustment as they regain weight.
Historically, FIP treatment was commonly given for 84 days (12 weeks). Newer clinical evidence has shown that a 42-day (6-week) GS-441524 course can also be effective in appropriately monitored cats, so treatment duration should be determined by response and veterinary assessment rather than stopping solely because a fixed number of days has been reached.
FIP Antiviral Treatment Comparison
Details | GS-441524 | Remdesivir | Molnupiravir |
|---|---|---|---|
Route | Oral / Injection | IV / Injection | Oral |
Dosage | Based on weight & FIP form | Based on weight & condition | Based on weight |
Course | 42–84 days* | Commonly up to 84 days | Commonly up to 84 days |
Evidence | Extensively studied for FIP | Established clinical evidence | Growing clinical evidence |
Key point | Commonly used antiviral for FIP | Useful when oral medication cannot initially be given | Alternative in selected cases |
Side Effects & Safety
Treatment | Side Effects & Safety |
|---|---|
GS-441524 | Generally well tolerated. Vomiting, diarrhoea or temporary liver-enzyme changes may occur. Injectable treatment can cause pain or skin irritation at the injection site. |
Remdesivir | Generally well tolerated, but injections may be painful. Temporary liver-enzyme changes can occur. |
Molnupiravir | Has more significant safety concerns than GS-441524. Reported effects include vomiting, diarrhoea, reduced white blood cells, increased liver enzymes and changes such as broken whiskers. Mutagenicity is also a concern. |
Why Is GS-441524 Commonly Used for FIP?
GS-441524 has become an important antiviral option for FIP because high recovery rates have been reported across multiple clinical studies and it is generally well tolerated by cats. Oral formulations also make treatment easier for many cats, while injectable formulations can be useful when oral medication cannot initially be given.
When Can FIP Treatment Stop?
The decision to stop FIP treatment should be based on recovery, clinical examination and laboratory results.
Before stopping antiviral treatment, the cat should be clinically well and previous abnormalities should have substantially improved or normalised. Blood tests can help confirm recovery, and inflammatory markers such as AG Ratio may provide additional guidance when available.
A cat that still has significant clinical or laboratory abnormalities may require further investigation or continued treatment rather than automatically stopping at 42 or 84 days.
What Happens After FIP Treatment?
After completing treatment, continue monitoring your cat for changes in appetite, weight, activity, fever, abdominal swelling, eye changes or neurological signs.
Some abnormalities can remain after successful treatment and do not automatically mean that FIP has returned. However, new or worsening symptoms should be assessed by a veterinarian promptly to determine whether they represent relapse or another health problem.
With successful antiviral treatment and appropriate follow-up, many cats can return to a normal quality of life after FIP.
Can FIP Be Cured?
FIP is now a treatable disease. Antiviral therapy such as GS-441524 has allowed many cats diagnosed with FIP to recover successfully. Early treatment, an appropriate dose and regular monitoring throughout the treatment course are important for achieving a good response.