FIP in Cats: Understanding Feline Infectious Peritonitis Symptoms and New Treatments

A feline infectious peritonitis (FIP) diagnosis was once considered a death sentence for cats. Today, in 2026, the landscape has changed dramatically — effective antiviral treatments now exist that can cure many cats with FIP when diagnosed and treated promptly. But recognizing FIP early requires knowing what to look for, because its early signs are easy to miss or misattribute to other conditions.

What Is FIP?

FIP is caused by a mutation of feline coronavirus (FCoV). Many cats (particularly those in multi-cat environments) carry feline coronavirus asymptomatically in their intestines without ever developing FIP. In a small percentage of cases — estimated at 5–10% of infected cats — the coronavirus mutates and escapes the intestinal tract, infecting macrophages (immune cells) and causing systemic disease (AAFP-AAHA Feline Life Stage Guidelines, 2021).

FIP occurs most commonly in:

• Young cats under 2 years old (peak incidence)
• Cats over 10 years old (second peak, possibly related to immune decline)
• Cats from shelters or multi-cat households where coronavirus circulates freely
• Purebred cats (may have a genetic predisposition)

Two Forms of FIP

FIP presents in two distinct clinical forms:

Wet (Effusive) FIP

The wet form is characterized by accumulation of fluid in body cavities:

• Abdominal (belly) fluid — the cat develops a pot-bellied appearance from fluid accumulation in the abdomen
• Chest fluid — fluid in the pleural space makes breathing difficult
• Signs: abdominal distension, labored breathing, weight loss, fever, lethargy

The wet form tends to progress more rapidly than the dry form.

Dry (Non-Effusive) FIP

The dry form causes granulomatous inflammation in organs rather than fluid accumulation:

• Commonly affects the eyes (clouding, redness, bleeding in the eye), brain and spinal cord (seizures, behavioral changes, paralysis, tremors), liver (jaundice), kidneys, and intestines
• Signs depend on which organ is affected but include neurological signs, eye changes, jaundice, weight loss, persistent fever

Early Symptoms of FIP

The earliest signs of FIP are often nonspecific and can be present weeks before the condition becomes clearly identifiable:

• Recurring fever that rises and falls repeatedly
• Chronic, progressive weight loss
• Poor appetite or refusal to eat
• Lethargy and weakness
• Failure to thrive in kittens — not growing as expected compared to littermates

These early signs are shared with dozens of other conditions, which is why FIP is frequently misdiagnosed initially.

When to Worry: See a Vet Immediately If

• Your cat's abdomen appears visibly distended with fluid
• Your cat is breathing with effort and their belly is moving with every breath (fluid in chest)
• Sudden neurological signs — seizures, circling, sudden blindness, head tilt, loss of coordination
• Yellow-tinged skin, gums, or eye whites (jaundice)
• Your cat is losing weight rapidly — see cat losing weight rapidly for what this can signal
• Persistent fever over several days that doesn't resolve

A Note on Treatment — Important 2026 Update

FIP was previously nearly 100% fatal without specific treatment. The introduction of nucleoside analogue antivirals, particularly GS-441524 (the active metabolite of remdesivir), has transformed FIP outcomes. Studies and clinical experience show remission rates of 85–90% in cats who complete a full treatment protocol.

As of 2026, licensed FIP treatments are available in many countries. If your cat is diagnosed with FIP, ask your vet specifically about antiviral treatment options — this is no longer a hopeless diagnosis.

Still Not Sure if Your Cat Needs a Vet?

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Frequently Asked Questions

Q: Is FIP contagious to other cats? A: The underlying feline coronavirus (FCoV) that can lead to FIP is contagious between cats via fecal-oral transmission. However, FIP itself (the mutated form that causes disease) is not directly passed from cat to cat — it develops through individual mutation. Cats in a FIP-affected household should be monitored but are not automatically doomed to develop FIP.

Q: How is FIP diagnosed? A: FIP diagnosis is challenging. Vets use a combination of clinical signs, bloodwork (low albumin-to-globulin ratio, elevated proteins), fluid analysis (Rivalta test for effusions), and the FCoV antibody titer or a newer FIP PCR test on fluid or tissue. No single test is 100% definitive.

Q: What is the survival rate for FIP with treatment? A: With a full course of antiviral therapy (typically 84 days minimum), remission rates are approximately 85–90% for both wet and dry forms. Neurological FIP has a slightly lower but still significant remission rate.

Q: Can FIP come back after treatment? A: Relapse is possible, particularly if treatment is stopped too early. Cats should be monitored closely for at least 1 year after completing treatment.

This article is for informational purposes only and is not a substitute for professional veterinary advice. Please consult a licensed veterinarian for any health concerns about your cat.