Effusive (wet) FIP is a rapidly progressing, immune-complex disease caused by a mutated feline coronavirus that triggers fluid accumulation in the chest or abdomen. Without treatment, most cats deteriorate within weeks — but antiviral therapies have transformed outcomes for cats diagnosed early.
Last reviewed: June 2026
What Is Effusive FIP and Why Does Fluid Build Up?
Effusive FIP develops when a mutated form of feline enteric coronavirus (FECV) spreads through the bloodstream, activates a misdirected immune response, and causes blood vessels to leak protein-rich fluid into body cavities. The two main compartments affected are the abdomen (peritoneal effusion) and the chest (pleural effusion); rarely, fluid collects around the heart. The wet form progresses faster than the dry (non-effusive) form because the vasculitis driving fluid leakage is more severe.
Feline infectious peritonitis virus (FIPV) is not contagious the way common cat respiratory viruses are — cats acquire the ancestral feline enteric coronavirus through fecal-oral exposure in multi-cat environments, and the mutation to FIPV occurs sporadically inside an individual cat. As described in Greene's Infectious Diseases of the Dog and Cat, the mutation rate correlates with high intestinal viral load, which is why cats under two years and cats in shelters or catteries are disproportionately affected.
Recognizing the Signs of Wet FIP
Signs of effusive FIP emerge over days to a few weeks and reflect both fluid accumulation and the underlying systemic illness.
Abdominal effusion signs:
- Visibly distended, pear-shaped belly that feels fluid-filled but non-painful on palpation
- Rapid weight loss despite previously normal appetite
- Lethargy and reluctance to jump or play
- Waxing and waning fever
Pleural effusion signs:
- Rapid, shallow breathing or open-mouth breathing (cats are obligate nasal breathers — open-mouth breathing is always urgent)
- Extended neck and elbows held out when sitting (orthopneic posture)
- Exercise intolerance progressing to collapse
Systemic signs common to both forms:
- Persistent fever (39.5–40.5 °C / 103–105 °F)
- Jaundice (yellow tinge to gums, eyes, or skin)
- Neurological signs — ataxia, seizures, or nystagmus if CNS is involved
- Uveitis — cloudy or color-changed iris in one or both eyes
A large proportion of cats with confirmed FIP present with concurrent ocular or neurological signs, which help distinguish it from other causes of effusion such as heart failure, pyothorax, or lymphoma. As detailed in Ettinger's Textbook of Veterinary Internal Medicine, the Rivalta test — placing a drop of effusion in dilute acetic acid — is a practical bedside screening tool with high sensitivity for FIP effusion due to its elevated protein and inflammatory mediator content.
How Vets Diagnose Effusive FIP
No single test confirms FIP with absolute certainty, but a combination of findings creates high diagnostic confidence.
Diagnostic steps:
- Imaging — ultrasound confirms fluid and assesses lymph nodes, liver, and kidneys for concurrent lesions
- Effusion analysis — straw-colored to amber, viscous fluid with total protein typically >35 g/L; positive Rivalta test; cytology showing non-septic mixed cellularity strongly supports FIP
- Serum biochemistry and CBC — hypergammaglobulinemia, low albumin:globulin ratio (<0.8), elevated bilirubin, anemia, and lymphopenia together support the diagnosis
- RT-PCR on effusion — detects viral RNA directly in the fluid with high specificity when combined with clinical presentation; Felten et al., 2019, JFMS found RT-PCR on effusion samples highly reliable for confirming FIPV in clinical cases
- Immunohistochemistry — staining for FIPV antigen in macrophages provides the closest to a definitive diagnosis but requires tissue biopsy
Hartmann et al., 2003, JFMS established that a low albumin:globulin ratio combined with a positive Rivalta test correctly identifies effusive FIP in approximately 90% of compatible cases, enabling treatment before biopsy results are available.
Treatment: Antiviral Options for FIP
The landscape for FIP treatment changed dramatically with the development of nucleoside analogue antivirals. GS-441524 (the active metabolite of remdesivir) and the protease inhibitor GC376 both demonstrate efficacy against FIPV replication in clinical trials.
Pedersen et al., 2019, JFMS published the pivotal trial showing GS-441524 induced sustained remission in 25 of 31 cats with FIP across wet, dry, and neurological forms — a transformative result for a disease previously considered universally fatal.
Current treatment approach:
- Antiviral therapy (GS-441524 or licensed equivalent) for a minimum of 12 weeks; neurological and ocular forms typically require higher doses and longer courses
- Supportive care: fluid drainage for respiratory distress, appetite stimulation, vitamin B supplementation
- Serial monitoring: recheck bloodwork every 2–4 weeks during treatment to confirm improving protein ratios, lymphocyte counts, and inflammatory markers
- Response is typically visible within 1–3 days: fever breaks, appetite returns, fluid volume decreases
As described in Ettinger's Textbook of Veterinary Internal Medicine, early initiation of antiviral therapy before severe organ damage occurs correlates strongly with remission rate — underscoring why rapid diagnosis is critical.
Prognosis and What to Expect
With antiviral treatment started promptly, reported remission rates exceed 80% in cats without advanced neurological involvement. Cats that relapse after a 12-week course are typically retreated at a higher dose or for a longer duration; many achieve sustained remission on a second course. Without treatment, median survival after wet FIP diagnosis is measured in days to weeks.
Post-treatment monitoring is essential — relapse most often occurs within the first three months after stopping antivirals. Cats that remain in remission for 12 months post-treatment are generally considered cured.
When to See a Vet
Call your vet today if:
- Your cat's belly looks visibly swollen or has a new, rounded shape that appeared over days
- Your cat has lost noticeable weight over the past 2–4 weeks
- Your cat has a persistent fever: feels warm, is lethargic, and is eating less
- You notice cloudiness, color change, or visible inflammation in one or both eyes
- Your cat is less active than usual and the change has persisted more than 48 hours
Go to the ER immediately if:
- Your cat is breathing rapidly with shallow breaths or is breathing with mouth open
- Your cat sits with elbows out, neck extended, and gums appear pale, blue, or grey
- Your cat collapses, loses balance, has a seizure, or cannot walk properly
- Your cat's gums or eyes look yellow (jaundice is a medical emergency in cats)
- Breathing is labored in any way — do not wait until morning
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Frequently Asked Questions
What does effusive FIP look like in a cat? The most visible sign is a distended, pear-shaped belly that develops over one to three weeks as fluid accumulates in the abdomen. If fluid is in the chest instead, you may see rapid, shallow breathing and reluctance to lie flat. Cats also typically appear underweight, have a dull coat, and are less active than usual. Some develop inflamed or cloudy eyes. These signs together warrant an urgent vet visit — not watchful waiting.
Can a cat survive wet FIP? Yes — with antiviral treatment, remission rates exceed 80% in many clinical series, compared to near-certain death without treatment. Cats that respond typically show improvement within one to three days: the fever breaks, appetite returns, and fluid volume decreases on ultrasound. Completing the full treatment course — a minimum of 12 weeks — is essential to prevent relapse.
Is FIP contagious to other cats? The disease itself is not directly contagious between cats — the mutation from harmless feline enteric coronavirus to disease-causing FIPV happens inside the individual cat. However, the underlying feline enteric coronavirus is shed in feces and can spread to housemates. The vast majority of exposed cats never develop FIP. Strict litter-box hygiene reduces fecal-oral FECV transmission.
How much does FIP diagnosis and treatment cost? Diagnostic workup typically runs $300–600, including exam, bloodwork, ultrasound, and effusion analysis. A 12-week antiviral treatment course commonly ranges from $1,500–4,000 in the US, depending on formulation and dose. Emergency visits for respiratory distress add a minimum of $500–800. Starting treatment before neurological involvement occurs keeps total costs significantly lower.
How is wet FIP different from dry FIP? Effusive (wet) FIP involves fluid accumulation in body cavities and progresses more rapidly, typically over days to weeks. Dry (non-effusive) FIP causes granulomatous lesions in organs without significant effusion and progresses more slowly but is harder to diagnose. Both forms can coexist and overlap over time. Both respond to the same antiviral therapies.
What is the Rivalta test? The Rivalta test is a quick bedside screening step where a drop of effusion fluid is added to dilute acetic acid: FIP effusion, which is high in protein and inflammatory mediators, forms a visible white precipitate (positive result). It is highly sensitive for FIP in cats with compatible clinical signs and is useful as a rapid triage tool. It cannot differentiate FIP from other protein-rich effusions such as lymphoma, so it is always interpreted alongside full clinical and laboratory findings.
Which cats are at highest risk for FIP? Cats under two years of age are at greatest risk, as high intestinal FECV viral loads are most common in young cats in multi-cat environments. A second smaller peak occurs in cats over 10 years, possibly related to age-related immune changes. Breed predispositions have been reported in Birmans, Ragdolls, Bengals, and Rex breeds, though any cat can develop FIP regardless of breed or history.
What photos help the vet assess my cat before the visit? Short video clips of your cat's breathing pattern, body posture while resting, and views of the abdomen from both sides are most useful. Note the date the belly first looked swollen, any fever-like behavior, and changes in appetite or litter-box habits. This information helps your vet assess urgency and determine whether referral to an internal medicine specialist is needed.
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