NAVLE Multisystemic

Feline Infectious Peritonitis Study Guide

📅 March 28, 2026 ⏱ 25 min read

Feline infectious peritonitis (FIP) is one of the most important infectious diseases and causes of death in cats, particularly in young cats under 2 years of age.

Overview and Clinical Importance

Feline infectious peritonitis (FIP) is one of the most important infectious diseases and causes of death in cats, particularly in young cats under 2 years of age. FIP develops when a ubiquitous feline enteric coronavirus (FECV) undergoes mutation within an infected cat, transforming into the virulent feline infectious peritonitis virus (FIPV). This mutation enables the virus to infect macrophages and disseminate systemically, triggering a severe immune-mediated pyogranulomatous vasculitis.

Approximately 5-12% of cats infected with FCoV will develop clinical FIP. The disease was historically considered universally fatal; however, recent advances in antiviral therapy with nucleoside analogues (GS-441524, remdesivir) have revolutionized treatment, with survival rates now exceeding 80-90% in treated cats.

High-YieldFIP is the most common cause of abdominal effusion in cats less than 2 years of age. When you see a young cat with fever unresponsive to antibiotics, weight loss, and ascites or pleural effusion, FIP should be at the top of your differential list.

Property	Description
Classification	Alphacoronavirus, closely related to canine coronavirus
Serotypes	Type I (80-95% of field cases) and Type II (less common, arises from recombination with CCoV)
Transmission	Fecal-oral route; virus shed in feces for weeks to months
Environmental Survival	Survives up to 7 weeks in dry environment; susceptible to most disinfectants

Etiology and Pathogenesis

The Feline Coronavirus

Feline coronavirus (FCoV) is an enveloped, positive-sense, single-stranded RNA virus belonging to the order Nidovirales, family Coronaviridae, genus Alphacoronavirus. Two biotypes exist: the avirulent feline enteric coronavirus (FECV) and the virulent feline infectious peritonitis virus (FIPV). These biotypes cannot be distinguished morphologically or serologically.

Key Viral Properties

FECV to FIPV Mutation

The transformation from benign FECV to virulent FIPV occurs through spontaneous mutations within the individual cat (internal mutation theory). Key mutations have been identified in the spike (S) protein gene, particularly at positions M1058L and S1060A in the fusion peptide, which are found in greater than 95% of FIPV isolates. Additional mutations in the 3c accessory gene are also associated with virulence.

The critical change is the acquisition of ability to efficiently replicate within monocytes and macrophages. This enables systemic dissemination via infected macrophages (Trojan horse mechanism) and triggers the immune-mediated pathology characteristic of FIP.

NAVLE TipRemember FIPV is NOT directly transmissible cat-to-cat. Each FIP case develops independently through mutation of FECV within that individual cat. FECV is highly contagious, but FIPV is not.

Immune-Mediated Pathophysiology

FIP is fundamentally an immune-mediated disease. The host immune response, rather than direct viral cytopathology, causes the tissue damage. The balance between cell-mediated immunity (CMI) and humoral immunity determines disease outcome and clinical presentation.

Memory Aid - FIP Immune Response: "Th1 = Thrives (cat survives), Th2 = Trouble (wet FIP), T-mixed = Troubled but slow (dry FIP)"

Immune Response	Disease Outcome	Clinical Form
Strong CMI (Th1)	Virus eliminated; cat recovers	No clinical disease
Strong Humoral (Th2)	Antibody-dependent enhancement; immune complex formation; vasculitis	Effusive (wet) FIP
Partial CMI/Mixed	Granuloma formation; slower progression	Non-effusive (dry) FIP

Epidemiology and Risk Factors

Prevalence and Demographics

FCoV infection is highly prevalent in multi-cat environments, with seropositivity rates of 80-90% in catteries and shelters compared to 20-60% in single-cat households. However, clinical FIP develops in only 5-12% of infected cats.

High-YieldThe classic FIP patient on the NAVLE is a young (less than 2 years), purebred cat from a multi-cat environment (shelter, cattery) with recent stress history. This signalment combined with typical clinical signs should trigger immediate consideration of FIP.

Risk Factor	Details
Age	70% of cases in cats less than 1.5 years; 50% in cats less than 7 months
Breed	Purebred cats overrepresented; Bengals, Birmans, Ragdolls, Abyssinians at higher risk
Housing	Multi-cat households, catteries, shelters (high FCoV exposure)
Stress	Recent adoption, surgery, vaccination, boarding; immunosuppression
Sex	Males slightly overrepresented, particularly intact males

Clinical Presentation

General Clinical Signs

Regardless of the form, most cats with FIP present with nonspecific signs including persistent fluctuating fever (unresponsive to antibiotics), anorexia, lethargy, and weight loss or failure to gain weight in kittens. Icterus occurs in many cases, particularly with effusive disease.

Effusive (Wet) FIP

The effusive form accounts for 60-70% of FIP cases and is characterized by accumulation of protein-rich, straw-colored, viscous fluid in body cavities. Abdominal effusion (ascites) is approximately four times more common than pleural effusion.

Non-Effusive (Dry) FIP

The non-effusive form is characterized by pyogranulomatous lesions in various organs without significant fluid accumulation. This form progresses more slowly but is more challenging to diagnose. Ocular and neurological involvement occurs in approximately 70% of dry FIP cases but in only 9% of wet FIP cases.

Ocular Manifestations

Ocular involvement is more common in non-effusive FIP and results from breakdown of the blood-ocular barrier:

Anterior uveitis: iris color change, aqueous flare, fibrin in anterior chamber
Keratic precipitates: "mutton fat" precipitates on corneal endothelium
Chorioretinitis: perivascular cuffing, retinal hemorrhage, detachment
Hypopyon/hyphema: inflammatory cells or blood in anterior chamber
Secondary complications: glaucoma, lens luxation, blindness

Neurological Manifestations

Neurological FIP results from pyogranulomatous meningitis, ependymitis, and/or encephalitis:

Ataxia: often progressive, may be vestibular or cerebellar
Seizures: focal or generalized
Nystagmus: horizontal, vertical, or positional
Behavioral changes: dementia, aggression, compulsive behaviors
Paresis/paralysis: posterior paresis common with spinal involvement
Hydrocephalus: secondary to ependymitis and CSF obstruction

NAVLE TipFIP is the most common cause of inflammatory CNS disease in cats. In any young cat presenting with progressive neurological signs and fever, FIP should be high on your differential list, especially if ocular signs are also present.

Abdominal Effusion Signs	Pleural Effusion Signs
Progressive abdominal distension Fluid wave on palpation Mesenteric lymphadenopathy Palpable organomegaly (kidneys, liver)	Dyspnea, tachypnea Rapid shallow breathing pattern Muffled heart and lung sounds Cyanosis in severe cases

Diagnosis

Definitive antemortem diagnosis of FIP remains challenging because no single test is pathognomonic. Diagnosis requires a multimodal approach, building an "index of suspicion" through signalment, history, clinical signs, and multiple diagnostic tests.

Hematology and Serum Biochemistry

Memory Aid - A:G Ratio: "0.8 = Okay (FIP unlikely), 0.4 = Awful (FIP very likely)"

Effusion Analysis

Analysis of cavitary effusion is critical for diagnosis of wet FIP. FIP effusions have characteristic features that distinguish them from other causes.

Characteristic FIP Effusion Features

The Rivalta Test

The Rivalta test is a simple, inexpensive point-of-care test useful for ruling out FIP:

Procedure:

Mix 7-8 mL distilled water with one drop of 98% acetic acid (or white vinegar)
Carefully place one drop of effusion on the surface
Observe the drop behavior

Interpretation:

Positive: Drop retains shape, stays at surface or slowly sinks ("jellyfish" appearance) - FIP possible
Negative: Drop dissipates, solution remains clear - FIP very unlikely (high negative predictive value 91-100%)

High-YieldThe Rivalta test has excellent sensitivity (91-100%) for RULING OUT FIP - a negative result makes FIP very unlikely. However, specificity is only 66-81%, as positive results also occur with bacterial peritonitis and lymphoma. Always combine with cytology!

Additional Diagnostic Tests

Imaging Findings

Radiography: Abdominal effusion (loss of serosal detail), pleural effusion, hepatosplenomegaly

Ultrasonography: Anechoic to echogenic effusion; mesenteric lymphadenopathy; renomegaly with hyperechoic cortex; loss of corticomedullary distinction; irregular peritoneal/omental thickening

MRI (neurological FIP): Hydrocephalus; periventricular contrast enhancement; meningeal enhancement

Parameter	Typical Finding	Clinical Significance
Lymphocytes	Lymphopenia (greater than 50% of cases)	Reflects immune-mediated lymphocyte destruction
Neutrophils	Neutrophilia with left shift	Chronic inflammatory response
RBC	Non-regenerative anemia (normocytic, normochromic)	Anemia of chronic disease
Total Protein	Hyperproteinemia (greater than 7.8 g/dL)	Due to hyperglobulinemia
Globulins	Hyperglobulinemia (polyclonal gammopathy)	Present in approximately 89% of FIP cases; most common abnormality
Albumin	Hypoalbuminemia	Negative acute phase protein; more common in wet FIP
A:G Ratio	Less than 0.8 suspicious; less than 0.4 highly suggestive	KEY diagnostic parameter; ratio greater than 0.8 makes FIP unlikely
Bilirubin	Hyperbilirubinemia (without hemolysis)	More common in wet FIP; not from liver failure

Treatment

The treatment landscape for FIP has been revolutionized by the development of antiviral nucleoside analogues. What was once a universally fatal diagnosis now has survival rates exceeding 80-90% with appropriate treatment.

Antiviral Therapy

Treatment Protocol

Duration: Minimum 84 days (12 weeks); some cats require extended treatment
Monitoring: Weekly weights (adjust dose accordingly); CBC/chemistry at weeks 2, 4, 8, 12
Response: Fever resolution within 24-72 hours; appetite improves in days; effusion resolves in 2-4 weeks
Relapse: Occurs in approximately 10% during or after treatment; increase dose by 5-10 mg/kg/day

High-YieldOcular and neurological FIP require HIGHER doses because the blood-brain and blood-ocular barriers exclude approximately 80% and 70% of drug respectively. Always use increased dosing (8-15 mg/kg) for these forms!

Supportive Care

Fluid therapy: Correct dehydration; avoid over-hydration with effusions
Nutritional support: High-quality diet; appetite stimulants if needed; feeding tubes for anorexic cats
Therapeutic drainage: Large volume effusions causing respiratory compromise may require therapeutic thoracocentesis/abdominocentesis
Anti-inflammatory therapy: Short-term prednisolone may provide symptomatic relief; NOT as sole therapy
Ocular treatment: Topical steroids (prednisolone acetate), mydriatics (atropine) for uveitis

Prognosis

With antiviral treatment:

Overall survival rate: 80-90%
Effusive FIP: Generally responds faster; effusion resolves within 2-4 weeks
Non-effusive FIP: May require longer treatment; monitor for relapse
Neurological FIP: More challenging; requires higher doses; some cats develop resistance
Relapse rate: Approximately 10%, most within 60 days post-treatment

NAVLE TipRemember that GS-441524 is now legally available via veterinary compounding pharmacies in many countries including the US, UK, and Australia. The FDA has indicated it will not enforce approval requirements for GS-441524 compounded for FIP treatment. This represents a major shift from the historical "universally fatal" prognosis!

Parameter	Typical Finding in FIP
Gross Appearance	Straw-colored to yellow, viscous, may form fibrin strands/clots
Total Protein	Greater than 3.5 g/dL (typically greater than 5 g/dL)
A:G Ratio (effusion)	Less than 0.4 highly suggestive; greater than 0.8 rules out FIP
Cell Count	Low to moderate (less than 5,000 cells/uL)
Cytology	Pyogranulomatous: macrophages, non-degenerate neutrophils, few lymphocytes; highly proteinaceous background
Classification	Modified transudate (by cell count) or exudate (by protein); often called "modified exudate"

Prevention

Reduce FCoV exposure: Maintain less than 3 cats per household; adequate litter boxes (one per cat plus one); regular cleaning
Minimize stress: Avoid overcrowding; gradual introductions; minimize rehoming/boarding
Early weaning: In catteries, isolate queen and kittens; wean at 5-6 weeks before FCoV infection typically occurs
Vaccination: Intranasal Primucell vaccine available (outside UK); limited efficacy; NOT routinely recommended by AAFP

Test	Utility	Limitations
FCoV Antibody Titer	Cats with FIP usually have high titers (greater than 1:1600); seronegative cats unlikely to have FIP	Cannot distinguish FECV from FIPV; many healthy cats seropositive
RT-PCR (effusion)	Positive RT-PCR in effusion is diagnostic for FIP	Detects FCoV RNA, not specifically FIPV
Alpha-1 Acid Glycoprotein	Values greater than 1.5 g/L highly suggestive of FIP	Elevated in other inflammatory conditions; not specific
Immunocytochemistry	Detection of FCoV antigen in macrophages within effusion or tissue - DEFINITIVE if positive	Requires adequate sample; false negatives possible
Histopathology + IHC	Gold standard: pyogranulomatous vasculitis with intralesional FCoV antigen = 100% specific	Requires biopsy; often only obtained postmortem

Drug	Dosing	Notes
GS-441524 (oral)	Wet/Dry FIP: 4-6 mg/kg PO q24h Ocular FIP: 8 mg/kg PO q24h Neurological FIP: 10-15 mg/kg PO q24h	First-line therapy; give fasted; 84-day minimum course; adjust for weight gain
Remdesivir (injectable)	Induction: 10-15 mg/kg IV/SC q24h x 4 days, then maintenance 6-15 mg/kg SC q24h	Prodrug of GS-441524; injection site discomfort common; often transition to oral GS-441524
Molnupiravir (EIDD-1931)	Reserved for refractory cases	Second-line therapy; used when GS-441524 fails or for cost concerns

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