Understanding Feline Infectious Peritonitis

Apr 22
4 min read

Updated: Apr 22

What is FIP?

Feline Infectious Peritonitis (FIP), also known as short-form FIP, is one of the most devastating diagnoses a cat owner may receive. Knowing the different forms of FIP, recognizing symptoms early, and understanding current treatment options can make a critical difference for a cat with FIP.

How It Develops

FIP is caused by a mutation of the feline coronavirus (FCoV), a common virus in felines. There are many strains of feline coronavirus, most of which are found in the gastrointestinal tract, and they do not cause significant issues. Feline enteric coronavirus (FeCV) strains can mutate into more virulent strains that cause FIP.

Chi, X., Deng, X. (2025). Feline Coronaviruses. In: Wang, L. (eds) Veterinary Virology of Domestic and Pet Animals. Springer, Cham. https://doi.org/10.1007/978-3-031-54690-7_123-1

How do Cats get FIP?

FeCV mainly passes between cats through their environment and close contact. Common ways it spreads include:

From mother cat to kittens: it is possible for the mother cat to spread FIP through her placenta to her unborn kittens.
Fecal-oral route (most common): the virus is mainly spread through cat feces, especially when cats share the same litter box.
Saliva: FIP can spread through food bowls and when cats groom each other.
Sneezing (less common)

Types of FIP

FIP can manifest in three different forms: Effusive (Wet), Non-Effusive (Dry), and Mixed.

Effusive (Wet) Form

Churchill 5 yo M, diagnosed with Wet FIP early 2025.

Pleural Effusion meant that it was a struggle for him to breathe

The rapid deterioration of his condition in a matter of days required him to undergo intensive care at Toronto Animal Health Partners

Wet FIP is characterized by fluid buildup in the abdomen or chest.

This fluid accumulation can cause:

Visible abdominal swelling (often mistaken for weight gain or pregnancy)
Breathing difficulties if fluid accumulates around the lungs
Rapid decline, as wet FIP often progresses quickly

Wet FIP is often easier to recognize than other forms as the symptoms can become dramatic in a short period of time.

Non-Effusive (Dry)
Nova - 9 mo, F

Dry FIP involves little to no fluid buildup, however, causes inflammation throughout the body instead.

Causes inflammation in organs such as:

Eyes
Brain and nervous system
Liver and kidneys

Symptoms may develop more slowly, but can be complex and harder to diagnose.

Mixed

Nova has dry FIP and weighs approximately 4 pounds

Mixed FIP is a combination of both wet and dry forms

Cats with mixed FIP may show symptoms of:

Fluid accumulation
Organ inflammation
Neurological/eye-related symptoms

These forms are influenced by the immune response stronger cellular immunity may lead to dry FIP, while weaker responses often result in the wet form

Symptoms to Watch For

FIP symptoms can be difficult to notice at first, and it often induces symptoms similar to other illnesses. Common signs include:

Persistent fever that does not respond to antibiotics
Lethargy and decreased appetite
Weight loss
Abdominal swelling (wet form)
Difficulty breathing (if chest fluid is present)
Jaundice (yellowing of skin/eyes)
Eye changes (cloudiness, colour changes)
Neurological signs (wobbling, seizures, behaviour changes)

Because symptoms vary widely, diagnosis can be challenging and often requires multiple tests.

Is It Contagious?

FIP itself is not considered directly contagious. However:

The underlying coronavirus (FCoV) is contagious between cats and typically spreads through feces (litter boxes, shared spaces).
Many cats carry FCoV without ever developing FIP
Only a small percentage (estimated around 1–5% in high-density environments) go on to develop FIP

This is why good hygiene and reducing stress in multi-cat environments are critical!

Is Your Cat Vulnerable to FIP?

Any cat that carries FeCV is at a potential risk of developing FIP. Other factors include:

Young age (commonly during first few weeks after birth)
Cats that live in multi-cat households, shelters, or catteries
Stress
Genetic factors
Male cats are affected more often than females

Treatment & Prognosis

Historically, FIP was considered fatal. Once the disease develops, without treatment, most felines deteriorate rapidly. However, recent advances have dramatically changed the outlook.

Without Treatment

Untreated FIP is generally considered progressive and often fatal. The speed of progression varies depending on the form of FIP:

Wet (effusive) FIP often progresses rapidly, sometimes over days to weeks
Dry (non-effusive) FIP progresses slower than wet FIP, but can still become severe

With Treatment

Prognosis for cats receiving antiviral treatment has improved dramatically. Two treatment options that have shown high success rates in treating FIP well are: Remdesivir and GS-44152; however, they are not yet widely available.

Supportive care can extend longevity and improve quality of life for cats with FIP:

Thoracentesis to drain fluid from the chest
Abdominocentesis to drain fluid from the abdomen
Anti-inflammatory drugs such as corticosteroids
Immunosuppressants
Fluid therapy
Nutritional support (such as feeding tube)

While access and regulation of treatments may vary by region, there is now real hope for recovery, which is a major shift from the past.

GS-441524 Dosage Guidelines for Cats with FIP

Dosage Guideline for Injection

FIP Type	Starting Dose
Wet (effusive) FIP with no ocular/neurological disease signs	4-5mg/kg daily for 12 weeks
Dry (non-effusive) FIP with no ocular/neurological disease signs	5-6mg/kg daily for 12 weeks
Ocular lesions and no neurological signs	8mg/kg daily for 12 weeks
With neurological signs	10mg/kg daily for 12 weeks
Wet/Dry FIP developing ocular/neurological signs	Appropriate ocular/neurological dosage

Do not inject dosage above 10mg/kg daily. Dosage is to be adjusted with weekly weight checks as weight can fluctuate.

Impact on Treatment Costs:

The cost of GS-441524 treatment will vary through several key factors such as:

Limited suppliers
Dosage based on weight and severity
Different vial concentrations
Duration of treatment

The typical cost for a 20mg/ml vial is ~$110 - $160 CAD.

Why This Matters for Rescues

FIP thrives in stressful environments— understanding FIP is critical for cat rescues like Finally Home Cat Rescue because it allows for early prevention and knowing the treatment options beforehand.

Edited by: Esther Kim and Jason Tsang