Feline Infectious Peritonitis (FIP) is one of the most serious and challenging diseases affecting cats. It develops when a common virus called feline coronavirus (FCoV), which usually causes no symptoms or only mild gastrointestinal upset, mutates inside a cat’s body. This mutation allows the virus to infect immune cells, triggering an abnormal and often fatal inflammatory response.
Although feline coronavirus is widespread—especially in multi-cat households—only a small percentage of infected cats go on to develop FIP. Until recently, FIP was considered almost universally fatal. However, advances in antiviral therapy have significantly changed the outlook for affected cats, making early recognition and accurate diagnosis more important than ever.
Feline Infectious Peritonitis (FIP) in cats is often misdiagnosed in its early stages because symptoms overlap with other chronic feline illnesses.
Feline coronavirus (FCoV) is commonly found in environments where multiple cats live together, such as homes with several cats, shelters, and catteries. Most cats exposed to FCoV either show no signs of illness or experience mild, self-limiting diarrhea.
In a small number of cats, the virus mutates within the body. This mutated form gains the ability to survive and multiply inside certain immune cells. The cat’s immune response to this mutation leads to widespread inflammation, which is what ultimately causes Feline Infectious Peritonitis.
Importantly, FIP itself is not directly contagious. Cats transmit feline coronavirus, not the mutated FIP-causing form.
Feline coronavirus spreads primarily through the fecal–oral route. Cats become infected by ingesting virus particles from contaminated environments.
Common routes of transmission include:
Shared litter trays
Contaminated food and water bowls
Grooming and close contact between cats
Surfaces contaminated with infected feces
Kittens are especially vulnerable and are often infected at a young age. Some cats may continue to shed the virus intermittently or chronically without appearing ill.
FIP generally presents in two classic forms, although some cats may show features of both.
Wet FIP is the more acute and rapidly progressive form of the disease. It is characterized by the accumulation of protein-rich fluid within body cavities.
Common features include:
Fluid buildup in the abdomen (ascites), causing visible abdominal enlargement
Fluid accumulation in the chest, leading to breathing difficulty
Lethargy and weakness
Persistent fever
Dry FIP progresses more slowly and is characterized by inflammatory lesions (granulomas) within organs.
Organs commonly affected include:
Eyes
Brain and spinal cord
Liver
Kidneys
Intestines
Cats with dry FIP may show neurological signs, eye inflammation, weight loss, chronic fever, and a gradual decline in overall health.
The signs of FIP vary depending on the form and organs involved, but commonly include:
Persistent fever that does not respond to antibiotics
Loss of appetite
Weight loss and muscle wasting
Lethargy and reduced activity
Abdominal distension due to fluid accumulation (wet FIP)
Labored or rapid breathing (wet FIP)
Eye changes such as cloudiness or inflammation
Neurological signs including seizures, poor coordination, or behavioral changes (dry FIP)
Because these symptoms can overlap with other diseases, FIP is often difficult to diagnose in its early stages.
There is no single test that can definitively diagnose FIP in every case. Diagnosis is typically based on a combination of clinical signs, laboratory findings, imaging studies, and exclusion of other diseases.
Common blood test abnormalities seen in cats with FIP include:
Elevated total protein levels, particularly globulins
Low albumin-to-globulin (A:G) ratio
An A:G ratio below 0.8 is suggestive
Ratios below 0.5 are highly suspicious for FIP
Non-regenerative anemia
Elevated liver enzymes in some cases
Diagnostic imaging can help identify changes associated with FIP:
Ultrasound or X-rays may reveal fluid in the abdomen or chest
Granulomatous lesions may be visible in organs in dry FIP cases
Polymerase Chain Reaction (PCR) testing detects feline coronavirus genetic material and can support the diagnosis of FIP when interpreted correctly.
Key points to understand:
Standard PCR tests detect feline coronavirus, not necessarily the mutated FIP-causing strain
Specialized PCR tests targeting mutations in the spike (S) gene can improve diagnostic specificity
PCR testing is most useful when performed on body cavity fluid or tissue samples rather than blood
A positive PCR result supports a diagnosis of FIP but must be correlated with clinical findings
A negative PCR result does not completely rule out FIP, particularly in dry cases
Recent advances have transformed FIP from a fatal disease into a treatable condition in many cases. The antiviral drug GS-441524, which is related to remdesivir, has shown high success rates in treating both wet and dry forms of FIP.
Key points about treatment:
Therapy typically lasts at least 12 weeks
Dosage and duration depend on disease severity and organ involvement
Close veterinary supervision and regular monitoring are essential
Many cats achieve remission and long-term survival with appropriate treatment
In addition to antiviral therapy, supportive treatment plays a vital role and may include:
Fluid therapy
Nutritional support
Anti-inflammatory medications
Management of secondary infections
Blood transfusions in cases of severe anemia
An intranasal vaccine for FIP exists, but it is not recommended for routine use.
Limitations of the vaccine include:
It is ineffective in cats already exposed to feline coronavirus
Most cats in multi-cat environments have already been exposed
Real-world effectiveness is low
It is not recommended by major veterinary organizations
Current prevention strategies focus on environmental management rather than vaccination.
While FIP cannot always be prevented, certain measures can reduce the risk:
Maintain excellent litter box hygiene
Keep food and water bowls separate from litter areas
Reduce overcrowding in multi-cat households
Minimize stress, especially in young cats
Isolate cats showing signs of illness when possible
Schedule regular veterinary check-ups for early detection of health issues
Feline Infectious Peritonitis remains one of the most feared diseases in cats, but advances in diagnostics and antiviral therapy have dramatically improved outcomes. Early recognition, thorough diagnostic evaluation, and timely treatment can give affected cats a real chance at recovery.
At nextdoorVets, our veterinary team stays up to date with the latest diagnostic tools and evidence-based treatments to provide the best possible care for cats affected by complex conditions like FIP.
👉 If your cat shows persistent fever, unexplained weight loss, or ongoing lethargy, seek immediate help at nextdoorVets.
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