FIP Symptoms in Cats: A UK Owner's Visual Guide

The earliest FIP symptoms are often quiet: a cat that eats less, hides more, runs a fever that will not settle, and slowly loses condition. Feline Infectious Peritonitis (FIP) takes four forms, wet, dry, ocular, and neurological, and each shows a different pattern of warning signs. Spotting these early and asking your veterinarian for the right tests gives you the best window to act.

This guide is written for UK caregivers who are mid-search and want clear, specific signs to look for. We walk through each of the four forms of FIP, what you can actually see at home, and when to escalate to your veterinarian.

A cat resting comfortably at home.

What is FIP and why do the symptoms vary so much?

Feline Infectious Peritonitis (FIP) is a serious disease caused by a mutation of a common feline coronavirus. The symptoms vary because FIP presents in four distinct forms, and each form targets different parts of the body, so two cats with the same diagnosis can look very different.

FIP often begins with vague, non-specific signs. Many cats show a fluctuating fever that does not respond to antibiotics, reduced appetite, lethargy, and gradual weight loss over days or weeks.

Young cats under two years old and cats from multi-cat homes or shelters are seen more often, though FIP can affect a cat of any age. If you notice a cluster of these early signs together, that pattern matters more than any single symptom on its own.

For a wider checklist of the earliest red flags, our guide to the 12 early warning symptoms of FIP breaks them down one by one.

What are the early FIP symptoms in cats?

Early FIP symptoms are usually subtle and easy to mistake for a minor illness: a persistent fever that comes and goes, a drop in appetite, low energy, and slow weight loss. The key pattern is that these signs continue or worsen rather than resolving.

Watch for these early feline infectious peritonitis warning signs:

• A fever above normal that keeps returning and does not respond to standard antibiotics

• Reduced or picky appetite over several days

• Lethargy, hiding, and reduced interest in play

• Gradual weight loss and a dull, unkempt coat

• Stunted growth in kittens

• Pale or yellow-tinged gums

None of these signs alone confirms FIP. They point you toward your veterinarian, who can run bloodwork and imaging to build a fuller picture.

What do the four forms of FIP look like?

FIP presents in four forms, wet, dry, ocular, and neurological, and recognising which pattern your cat shows helps you and your veterinarian move faster. The four subsections below mirror the way treatment is later dosed by form.

Wet (effusive) FIP symptoms

Wet FIP, also called effusive FIP, is defined by fluid building up inside body cavities. The most visible sign is a swollen, rounded belly that feels full or fluid-filled, and this can develop quite quickly.

When fluid collects in the chest instead of the abdomen, you may see laboured or rapid breathing, open-mouth breathing, or a cat that sits hunched and reluctant to move. Wet FIP often progresses faster than the dry form, so breathing changes are an urgent reason to contact your veterinarian.

Other signs alongside the fluid include ongoing fever, weight loss over the spine and hips even as the belly swells, and marked lethargy.

Dry (non-effusive) FIP symptoms

Dry FIP, also called non-effusive FIP, produces little or no fluid, which makes it harder to spot and often slower to develop. Instead of swelling, it causes inflammatory lesions in organs such as the liver, kidneys, and lymph nodes.

The signs are frequently vague: a persistent fever, steady weight loss, poor appetite, and a cat that simply seems unwell without one obvious problem. Some cats show jaundice, a yellow tint to the gums, ears, or eyes, or enlarged organs your veterinarian can feel on examination.

Because dry FIP is so non-specific, diagnosis usually leans heavily on bloodwork. Our guide to understanding cat bloodwork during FIP explains what the key results mean.

Ocular FIP symptoms

Ocular FIP shows up in the eyes and is often one of the more visible forms. Look for a change in eye colour, cloudiness, or a hazy, inflamed look to one or both eyes.

Common signs include colour changes in the iris, a visibly cloudy or foggy appearance, blood or debris inside the eye, and an altered or unequal pupil size. Some cats develop sudden vision problems, bumping into objects or becoming hesitant in familiar spaces.

Ocular signs can appear on their own or alongside dry or neurological FIP, so any unexplained eye change deserves a prompt veterinary examination.

Neurological FIP symptoms

Neurological FIP affects the brain and spinal cord and produces some of the most alarming signs. These can include wobbliness or loss of balance, seizures, tremors, unusual behaviour changes, and weakness in the limbs.

You may notice your cat stumbling, circling, appearing disoriented, becoming unusually sensitive to touch, or losing control of movement. Because these signs point to the nervous system, they are always a reason to seek veterinary care quickly.

Neurological and ocular forms are typically harder to treat than wet or dry FIP, which is one reason early recognition genuinely matters.

How is FIP diagnosed once I spot symptoms?

FIP is diagnosed by your veterinarian using a combination of clinical signs, bloodwork, fluid analysis if effusion is present, and sometimes imaging, rather than by a single stand-alone test. No home observation replaces this, so your role is to notice the pattern and report it clearly.

Here is a practical sequence once you suspect FIP:

1. Write down every sign you have noticed, including when it started and whether it is getting worse.

2. Note your cat's appetite, weight changes, breathing, and any eye or balance changes.

3. Book a veterinary appointment and share your full timeline.

4. Ask your veterinarian about bloodwork, and about analysing any abdominal or chest fluid.

5. Discuss which form of FIP is most likely, because the form shapes the treatment plan.

Bringing a clear, dated symptom list helps your veterinarian move faster toward a diagnosis.

How does the form of FIP change the treatment plan?

The form of FIP directly shapes the antiviral dose, because ocular and neurological FIP generally require higher dosing than wet or dry FIP. GS-441524 is the antiviral at the core of modern FIP treatment, with a 92% success rate reported by UC Davis (Pedersen, 2019).

A standard injectable course of GS-441524 runs 84 days under veterinary supervision, dosed by the form your cat has. The table below shows the per-form dosing exactly as listed for the CureFIP injectable range, so you can see how symptoms and dosing stay aligned.

FIP form	Common visible signs	GS-441524 injectable dose
Wet	Swollen belly, fluid, breathing changes	6 mg/kg
Dry	Fever, weight loss, jaundice	8 mg/kg
Ocular	Cloudy eye, colour change, vision loss	10 mg/kg
Neurological	Wobbliness, seizures, tremors	10 mg/kg

The CureFIP injectable range includes the CureFIP™ GS-441524 Injectable 20mg/ml at €79.00, the CureFIP™ GS-441524 Injectable 30mg/ml at €89.00, and the Cure FIP Antiviral 40mg/ml at €119.00. All three follow the same schedule of 1 subcutaneous injection per day, 7 days a week, for 12 weeks (84 days), referencing Pedersen et al., UC Davis (PMC6435921).

There is also an oral route. The CURE FIP™ Dual Antiviral Oral Capsules at €179.00 combine GS-441524 with EIDD-1931 and are dosed by weight, with a schedule of 1 capsule per day for a recommended 12 weeks. This oral dual route is positioned for wet and dry FIP, and some regions note it is not recommended once ocular or neurological signs are present or the cat cannot eat or defecate.

Dual antiviral therapy is an evolving area. A CureFIP dual antiviral study reported 78.3% remission in cats that had already relapsed (Li and Cheah 2025), and you can read more about how dual antiviral therapy is changing FIP treatment. Your veterinarian is best placed to decide which route and dose fit your cat.

How quickly should I act if I see FIP symptoms?

You should contact your veterinarian promptly, and treat any breathing difficulty, seizure, collapse, or sudden vision loss as urgent. FIP can progress over days rather than weeks, especially the wet form, so early action widens your options.

We are clear about what the data shows and what it does not. Evidence shows that GS-441524 based treatment under veterinary supervision has helped a large number of cats, with 100,000+ cats treated since 2019 across the CureFIP network, but outcomes depend on the form, timing, and individual case.

If you want to understand what the weeks ahead look like, our GS-441524 treatment timeline walks through each stage of an 84-day course.

FAQ

What are the first signs of FIP in cats in the UK?

The first signs of FIP in cats are usually a fever that comes and goes and does not respond to antibiotics, reduced appetite, low energy, and gradual weight loss. These early symptoms are non-specific, so a persistent cluster of them is the cue to see your veterinarian.

Can you see FIP symptoms with the naked eye?

Some FIP symptoms are visible, such as a swollen fluid-filled belly in wet FIP, cloudiness or colour change in the eye with ocular FIP, or wobbliness and tremors in neurological FIP. Dry FIP is harder to see and often needs bloodwork, so visible signs guide you toward diagnosis but do not replace veterinary testing.

How fast does FIP progress once symptoms appear?

FIP can progress over days to weeks, and the wet (effusive) form often moves faster than the dry form. Breathing difficulty, seizures, collapse, or sudden vision loss should be treated as urgent and warrant immediate veterinary care.

Does the type of FIP change the treatment dose?

Yes. With the CureFIP GS-441524 injectable range, wet FIP is dosed at 6 mg/kg, dry at 8 mg/kg, and ocular and neurological at 10 mg/kg, over an 84-day course under veterinary supervision. Your veterinarian confirms the form and the correct dose for your cat.

Is FIP the same as feline calicivirus?

No, FIP and feline calicivirus are different diseases with different causes and signs. If you are trying to tell them apart, our article on whether feline calicivirus is dangerous explains that condition separately.

If your cat is showing any of these signs, you do not have to work through it alone. Learn more about the available treatment options and next steps with the CureFIP team here in the UK, and speak with your own veterinarian so that every decision about your cat's care is guided by a professional who knows the case.