Receiving a FIP diagnosis for your cat is the kind of news that changes everything in an instant. A disease that until very recently meant an inevitable ending now has, thanks to advances in veterinary medicine, a 93% cure rate. This guide is written to give you the groundwork you need: what FIP is, why it happens, how it shows up, and what that diagnosis means today.
The simple version: what FIP is
FIP, Feline Infectious Peritonitis, is a severe systemic inflammatory disease caused by a mutation of the feline coronavirus (FCoV). It is not the same coronavirus that causes COVID-19 in humans: they belong to the same family, but they are completely different viruses, with no risk of cross-species transmission.
FCoV is a very common virus in the cat population. In multi-cat households, an estimated 80% to 90% of animals have been exposed to FCoV at some point. In the vast majority of cases the infection passes without serious consequences: the virus circulates through the intestinal tract and the immune system keeps it under control.
The problem occurs when FCoV mutates in a specific cat. That mutated version of the virus gains the ability to replicate inside macrophages, the immune cells responsible for clearing pathogens. By infecting the very cells that should be fighting it, the virus triggers a massive, uncontrolled inflammatory response that damages organs and tissues throughout the body.
Why it happened to your cat
This is the question owners ask most: why my cat? The honest answer is that science does not yet have a complete explanation.
What is known is that certain factors make an FCoV mutation more likely:
Stress. Major changes in a cat’s environment, moving home, the arrival of new animals, frequent veterinary procedures, can temporarily compromise the immune system and give the virus an opening to mutate.
Age. Young cats (between 6 months and 3 years) and cats over 10 years old are overrepresented in FIP diagnoses. In young cats, the immune system is not yet fully mature. In older cats, it may be weakened by other causes.
Contact with many cats. In shelters, colonies and multi-cat homes, exposure to FCoV is higher and more sustained, which statistically increases the chance that one of the animals develops the mutation.
What you should not do is look for blame in the past. FIP is not the result of neglectful care. It is the consequence of a viral mutation that science still cannot predict or prevent with certainty.
The four types of FIP
Wet FIP (effusive)
This is the most frequent form, and usually the one diagnosed earliest because its symptoms are more visible. The virus triggers an inflammatory response in the blood vessels that causes fluid to leak into body cavities: the abdomen (ascites), the chest (pleural effusion), or the pericardial sac around the heart.
The most recognizable sign is a distended abdomen, pear-shaped as the cat walks. If fluid builds up in the chest, the cat has difficulty breathing. Both situations are urgent and require immediate veterinary assessment.
Dry FIP (non-effusive)
Instead of causing fluid accumulation, the virus forms granulomas, small inflammatory lesions, in internal organs such as the kidneys, the liver, the spleen or the lymph nodes. Symptoms are more subtle: progressive weight loss, intermittent fever, lethargy. Diagnosis usually takes longer because the signs are unspecific.
Ocular FIP
It can appear on its own or alongside the dry form. It causes eye inflammation (uveitis), color changes in the iris, opacity, or bleeding inside the eye. In some cases it is the first visible sign of FIP.
Neurological FIP
When the virus affects the central nervous system, it can cause seizures, loss of coordination (ataxia), paralysis or behavioral changes. It is the most challenging form to treat, although it also responds to antiviral treatment in most cases when it is diagnosed and treated at the right doses.
2019: the year everything changed
Until 2019, FIP was considered incurable. The treatments available, palliative care, interferon, corticosteroids, could briefly improve quality of life but did not stop the disease.
Dr. Niels Pedersen and his team at the University of California Davis changed that reality. Their studies with GS-441524, a nucleoside analog that blocks coronavirus replication, showed that FIP had become a treatable disease, including its neurological form. The studies published in 2019 in the Journal of Feline Medicine and Surgery established GS-441524 as the new reference treatment.
Since then, thousands of cats worldwide have completed the treatment 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 mean what it meant ten years ago.
The second step is to get properly informed: about the types of FIP, about the treatment options, about how the 84-day protocol works. That is what this site is here for.
The third step is to talk to a vet with FIP experience as soon as possible. Time matters: in general terms, the sooner treatment starts, the better the prognosis.
You do not need to have all the answers tonight. But you can take the first step.