If you are reading this, you have probably just heard the words “Feline Infectious Peritonitis” for the first time. You may have read that it is serious. You may have read older information that described it as a death sentence. This guide exists to give you accurate, current information — because what was true about FIP five years ago is not true today.
The simple version: what FIP is
FIP is a severe inflammatory disease caused by a mutation of the feline coronavirus (FCoV). The feline coronavirus itself is extremely common — the majority of cats in multi-cat households have been exposed to it, and in most cases it causes no serious illness.
The problem occurs when the virus mutates in a specific cat. That mutated version gains the ability to replicate inside macrophages, the very immune cells designed to destroy pathogens. When the virus hijacks the immune system’s own soldiers, it triggers a massive, uncontrolled inflammatory response that damages organs and tissues throughout the body. That is FIP.
Why it happened to your cat
This is the question every owner asks: why my cat? The honest answer is that science does not yet have a complete explanation.
What is known is that certain factors appear to increase the likelihood of the mutation occurring:
Stress. Major environmental changes — moving home, the arrival of new animals, frequent vet procedures — can temporarily compromise the immune system and create conditions that favor viral mutation.
Age. Young cats (6 months to 3 years) and senior cats (over 10 years) are overrepresented in FIP diagnoses.
Exposure to many cats. In shelters, colonies, and multi-cat homes, exposure to FCoV is higher and more sustained.
What you should not do is blame yourself or search for something you did wrong. FIP is not the result of neglectful care. It is the consequence of a viral mutation that science cannot yet reliably predict or prevent.
The four types of FIP
Wet FIP (effusive)
The most common form. The inflammatory response causes fluid to leak into body cavities: the abdomen (ascites), the chest (pleural effusion), or the sac around the heart. A distended, pear-shaped abdomen or breathing difficulty are the typical alarm signs. Wet FIP progresses faster — early action matters.
Dry FIP (non-effusive)
Instead of producing fluid accumulation, the virus forms granulomas — small inflammatory lesions — in internal organs. Symptoms are more subtle: progressive weight loss, intermittent fever, lethargy. Diagnosis typically takes longer.
Ocular FIP
Can appear alone or alongside dry FIP. It causes eye inflammation (uveitis), iris color changes, corneal opacity, or bleeding inside the eye.
Neurological FIP
When the virus affects the central nervous system, it can cause seizures, ataxia, limb weakness or paralysis, behavioral changes, or hydrocephalus in young cats. It is the most challenging form to treat, though antiviral treatment achieves positive results in the majority of neurological cases when the right doses are used.
2019: the year everything changed
Until 2019, FIP was considered incurable. Dr. Niels Pedersen and his team at the University of California Davis changed that reality. Their studies with GS-441524 demonstrated cure rates exceeding 90% in wet FIP and above 80% in neurological FIP.
Since then, thousands of cats worldwide have completed the treatment protocol and now live completely normal lives. What was a death sentence in 2018 is today a treatable disease with a favorable prognosis in the vast majority of cases.
What to do if your cat was just diagnosed
The first step is to take a breath. A FIP diagnosis today does not carry the same meaning it did a decade ago.
The second step is to get informed: about the types of FIP, treatment options, and what the 84-day protocol involves. That is what this site is here for.
The third step is to contact a vet with FIP experience as soon as possible. Time matters.