Feline Infectious Peritonitis: A Clear Guide for Cat Owners
Feline infectious peritonitis, usually called FIP, is a serious disease caused by a feline coronavirus that can mutate inside a cat’s body. Most feline coronaviruses live in the intestinal tract and cause little or no illness, but in a smaller number of cats the virus changes in a way that allows it to spread through the body and trigger intense inflammation.
For many years, FIP was considered almost always fatal once a cat became clinically ill. That has changed in an important way: oral compounded GS-441524 became legally available in the United States in 2024 with a veterinary prescription, giving many cats a real treatment option that did not exist before.
What FIP really is
A point that often confuses owners is that FIP is not the same thing as ordinary exposure to feline coronavirus. Many cats are exposed to feline enteric coronavirus, especially in multi-cat environments, and most of them never go on to develop FIP.
Cornell explains that roughly 5 to 10 percent of cats exposed to the enteric coronavirus develop FIP, and this can happen weeks, months, or even years after the original exposure. Because the harmful form usually develops after mutation within an individual cat, FIP itself is not generally believed to spread from a sick cat in the same direct way the common enteric coronavirus spreads through feces and saliva.
Which cats are most at risk
FIP can occur in any cat, but young cats are affected most often. Cornell reports that about 70 percent of cases are diagnosed in cats younger than 18 months, and about half occur in kittens younger than 7 months.
Cats living in crowded or stressful environments also face higher risk. Shelters, catteries, foster settings, and large multi-cat homes tend to have more feline coronavirus circulation, which increases the chance that susceptible cats will be exposed. Purebred cats, male cats, and older cats may also be overrepresented in some reports, although not every cat in those groups will become sick.
Signs owners may notice
Early signs of FIP are often vague, which is one reason the disease can be so frustrating. Owners may first notice reduced appetite, weight loss, lethargy, depression, or a fever that does not improve as expected.
FIP is commonly described in two forms: wet and dry. In the wet form, fluid can build up in the belly or chest, leading to a swollen abdomen, labored breathing, or both; in the dry form, inflammation is more likely to affect organs, the eyes, or the nervous system, which can cause wobbliness, seizures, or other neurologic changes. Some cats show a mixture of features, and cases can shift over time rather than fitting neatly into one category.
Why diagnosis can be difficult
There is no single perfect test that proves every case of FIP. A coronavirus antibody test can show past exposure, but it cannot reliably tell the difference between the common enteric coronavirus and the mutated form associated with FIP.
Veterinarians usually diagnose FIP by putting several clues together: the cat’s age and history, physical exam findings, bloodwork, imaging, fluid analysis when fluid is present, and selected tests such as PCR or tests that detect viral proteins in tissues or body fluids. The 2022 AAFP/EveryCat diagnostic guidelines also emphasize that diagnosis depends on combining signalment, history, examination findings, and test results rather than relying on one simple screening test.
Treatment today
The biggest change in the FIP story is treatment. Cornell states that GS-441524 has been the antiviral drug studied most extensively and has been shown in laboratory work and in client-owned cats with naturally occurring FIP to be a safe and effective treatment option.
In the United States, a compounded oral form of GS-441524 became available beginning June 1, 2024, through legal veterinary prescribing channels. Cornell also notes that a veterinary prescription is required and warns owners against relying on unregulated products of uncertain concentration or purity.
Supportive care may still matter a great deal, especially in sicker cats. Depending on the case, this may include fluid therapy, drainage of accumulated chest or abdominal fluid, nutritional support, and other treatments recommended by the attending veterinarian.
What owners can do at home
If a cat shows signs that could fit FIP, the best next step is prompt veterinary care rather than waiting to see whether things improve on their own. Earlier evaluation helps a veterinarian decide whether the problem is FIP or another illness that may look similar.
In homes with multiple cats, sensible hygiene and stress reduction can help lower coronavirus spread and overall disease pressure. Cornell recommends keeping litter boxes clean, placing them away from food and water, and avoiding overcrowding when possible.
Owners should also know that the currently licensed FIP vaccine is not routinely recommended by the American Association of Feline Practitioners because its effectiveness is questionable. Vaccine decisions should be discussed with a veterinarian on a case-by-case basis.
A practical note of hope
An FIP diagnosis is still serious, but it is no longer automatically hopeless in the way it once was. With modern antiviral treatment, veterinary supervision, and careful follow-up, many owners now have a path forward that simply did not exist a few years ago.
That said, FIP remains a disease that requires professional guidance. Cat owners should work closely with their veterinarian to confirm the diagnosis as carefully as possible, discuss treatment options honestly, and make a plan that fits the cat’s condition and the household’s abilities.
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