Feline Infectious Peritonitis (FIP): A Comprehensive Guide for Cat Parents
Once a death sentence, FIP now has effective treatments. This guide provides cat parents with essential information about this complex viral disease, from understanding the virus to modern treatment options.
Understanding Feline Coronavirus (FCoV)
What is FCoV?
Feline coronavirus (FCoV) is an extremely common virus in cat populations worldwide. In most cats, it exists as feline enteric coronavirus (FeCV), primarily infecting intestinal cells. The vast majority of infected cats show no symptoms or experience only mild, temporary issues like diarrhea or minor respiratory problems.
How is it Transmitted?
FCoV is highly contagious among cats, primarily through the fecal-oral route. Infected cats shed the virus in feces, and others become infected by ingesting viral particles, often through shared litter boxes. Transmission can also occur through direct contact, mutual grooming, or contaminated surfaces.
Who Gets Infected?
Given its contagious nature, FCoV infection is extremely common, especially in multi-cat environments. Antibody prevalence can reach 80-100% in catteries, shelters, and rescues. Most cats encounter FCoV at some point, often during kittenhood.
How FCoV Transforms into FIPV
Initial Infection
Cat becomes infected with the relatively harmless feline enteric coronavirus (FeCV), which primarily targets intestinal cells.
Internal Mutation
During replication within the cat's intestines, the FeCV undergoes spontaneous genetic mutations, often involving genes like the spike protein.
Cell Targeting Change
The mutated virus (FIPV) gains the ability to infect and replicate within macrophages, a type of white blood cell that travels throughout the body.
Immune Response
The cat's immune system recognizes infected cells and mounts an intense inflammatory response, causing damage to blood vessels and organs.
Who is at Risk for Developing FIP?
Age
Young cats are most vulnerable, with highest incidence between 3 months and 2-3 years old. Peak risk is often between 6-18 months, likely due to immature immune systems.
Genetics
Purebred cats appear overrepresented compared to mixed breeds. Persians, Birmans, Abyssinians, Bengals, Himalayans, Ragdolls, and Rex breeds show higher incidence in studies.
Environment
High-density housing situations significantly increase risk. Stress events like rehoming, surgery, or introduction of new pets can trigger disease progression.
Immune Status
Cats with weakened immune systems face higher risk, including very young kittens and those with concurrent diseases like Feline Leukemia Virus (FeLV).
Recognizing FIP: Early Warning Signs
Lethargy
Decreased activity or playfulness, often one of the first noticeable signs
Appetite Loss
Reduced food intake or complete inappetence
Weight Loss
Progressive weight loss or failure to grow/thrive in kittens
Persistent Fever
Fluctuating fever that doesn't respond to antibiotics
The Two Main Forms of FIP
Wet (Effusive) FIP
Characterized by accumulation of inflammatory fluid in body cavities. Often more common in younger cats and progresses more rapidly.
  • Abdominal fluid causing swollen, "pot-bellied" appearance
  • Chest fluid making breathing difficult or labored
  • General illness signs (fever, lethargy, weight loss)
  • Possible jaundice due to liver involvement
Dry (Non-Effusive) FIP
Characterized by inflammatory lesions (granulomas) forming in various organs without significant fluid buildup. Often develops more gradually.
  • Organ-specific symptoms depending on affected areas
  • Eye inflammation (30% of dry cases)
  • Neurological signs like ataxia or seizures (30% of cases)
  • Enlarged organs or lymph nodes
Ocular and Neurological FIP
Ocular Signs
Eye involvement occurs in about 30% of dry FIP cases, sometimes as the only sign. Symptoms include inflammation inside the eye (uveitis), changes in iris color, cloudiness, debris in the eye, and potentially blindness.
Neurological Signs
Neurological FIP affects about 30% of dry cases. Signs depend on lesion location and can include uncoordinated movement, difficulty jumping, seizures, behavioral changes, increased sensitivity to touch, and even paralysis.
Treatment Implications
Cats with ocular or neurological FIP require higher doses of antiviral medication to ensure adequate drug penetration into these protected sites. These forms may also have a higher risk of relapse.
Diagnosing FIP: A Multi-pronged Approach

Definitive Diagnosis
Biopsy with immunohistochemistry or response to treatment
Specialized Tests
PCR, fluid analysis, imaging, antibody tests
Blood Work
CBC, chemistry panel, protein patterns
Clinical Assessment
Age, breed, history, physical exam findings
No single test can definitively diagnose FIP in a living cat. Veterinarians build a case by considering multiple factors together, including the cat's age and breed, clinical signs, physical examination findings, and results from various diagnostic tests.
Key Diagnostic Tests for FIP
1
Blood Work
Common findings include low lymphocyte count, high neutrophil count, anemia, elevated globulins with low albumin (low A:G ratio), and possibly elevated bilirubin and liver enzymes.
2
Fluid Analysis
If fluid is present (wet FIP), analysis is highly valuable. FIP effusion is typically thick, sticky, yellow to amber, with high protein concentration. The Rivalta test can be a useful screening tool.
3
Antibody Tests
These detect exposure to FCoV but cannot differentiate between harmless FeCV and FIP-causing FIPV. A high titer in a symptomatic cat increases suspicion, but many healthy cats also test positive.
4
PCR Tests
Detect viral RNA in various samples. Most significant when virus is found outside the intestinal tract, such as in effusions, CSF, or organ samples.
5
Imaging
X-rays detect fluid, ultrasound examines organs, and MRI is preferred for suspected neurological FIP.
Revolutionary Treatment: GS-441524 and Remdesivir

Mechanism of Action
These drugs interfere with the virus's ability to replicate by disrupting its RNA copying process
Administration Methods
Available as injectable (subcutaneous) or oral (tablets/suspension) formulations
Treatment Duration
Standard recommended treatment is a minimum of 12 weeks of continuous daily therapy
For decades, FIP was considered untreatable, but the development of specific antiviral drugs has revolutionized the outlook, offering a high chance of cure. The breakthrough came with two closely related nucleoside analogs: GS-441524 and its prodrug, Remdesivir, which were initially developed for human viral diseases.
Treatment Protocols: Dosage Guidelines
Treating FIP requires adherence to specific protocols developed through research and clinical experience. Dosing is critical and based on the cat's current body weight and the form of FIP. Dosages need to be recalculated and adjusted upwards frequently as the cat gains weight during recovery. Underdosing is a major risk factor for treatment failure or relapse.
It is absolutely essential to work with a veterinarian to determine the correct protocol for an individual cat. Oral bioavailability is lower than injectable, requiring higher doses, and twice daily oral dosing may be beneficial to maintain consistent blood levels.
Hope for FIP Cats: Treatment Success
80-90%
Success Rate
Overall cure rate with proper treatment protocol
24-72h
Initial Response
Time to see first signs of improvement
12+
Treatment Weeks
Minimum recommended duration
2
Post-Recovery Weeks
Continue treatment after clinical remission
The landscape of FIP has dramatically changed in recent years. What was once a fatal diagnosis now offers genuine hope for cure and long-term survival. Cats with FIP typically show rapid improvement in clinical signs within days of starting appropriate antiviral treatment, with fever reduction, increased appetite and activity being early positive indicators.