Feline Infectious Peritonitis (FIP) – 2026 Update | Little Big Cat

Updated 2/9/26

FIP is a particularly nasty disease—one that causes great confusion and distress. The name itself is misleading; the only absolutely true point about the name is the “feline,” since it is not especially infectious, nor is it always peritonitis (inflammation in the abdominal cavity). Unfortunately, the disease is virtually 100% fatal in its active form.

As a rule, FIP develops primarily in young cats under 2 years of age, or in older cats age 10 and up. It is fairly rare in the middle years. A study of more than 800 cats in Great Britain found that in homes where a cat had died of FIP, the transmission rate to other cats in the home was less than 5%. Since these other cats continued to go outside where they could also have been re-exposed to another source, it is clear that FIP does not easily pass from one cat to another.

FIP is considered to be caused by a virulent form of an otherwise harmless bug called Coronavirus. This virus causes mild diarrhea in very young puppies and kittens, but is generally self-limiting and doesn’t cause much of a problem. However, the virus is also found in cats who do ultimately contract FIP, in which it is thought to have mutated to a virulent form (though this has never been proven).

Saying that Coronavirus causes FIP might be a little like saying “flies cause garbage” just because the two are usually found together. Whether or not coronavirus is the real trigger is unknown. Many perfectly normal cats who will never develop FIP will test positive for Coronavirus; in my experience, about 40% of normal cats are positive. A positive test is only a reflection of the cat’s having been exposed to the virus at some time in its life, and doesn’t mean very much otherwise.

FIP is very difficult to diagnose correctly, at least while the cat is still alive. Most confirmed cases are recognized at necropsy (the technical term for an autopsy on a non-human animal). The early symptoms are vague, and commonly found with many other conditions, not just FIP. Symptoms include poor appetite, failure to thrive (in kittens), weight loss, ratty-looking fur, eye problems such as uveitis (inflammation), fever, anemia, lethargy, jaundice, neurological symptoms, and what vets refer to simply as “ADR”—which stands for “ain’t doin’ right.” FIP is typically diagnosed when a symptomatic cat has a positive test for Coronavirus along with other typical laboratory abnormalities that support the diagnosis.

The actual FIP disease symptoms are actually caused by the cat’s own immune system. For some reason, the immune system over-reacts and creates many patches of white blood cells that produce tons of antibodies. In some cats, this results in chronic inflammation, usually without clear symptoms; this is called “dry” FIP and is very hard to diagnose correctly. In the “wet” or “effusive” form of FIP, the body also produces large amounts of fluid—usually in the abdomen but sometimes in the chest instead. The fluid is characteristically yellow and sticky; its high protein content is diagnostic for FIP. Both forms of FIP typically cause rapid deterioration and death.

Antiviral Treatment: A Major Shift in FIP Outcomes

For many years, feline infectious peritonitis (FIP) was considered almost uniformly fatal once clinical signs appeared. This belief was based on decades of experience in which supportive care and immune-modulating therapies were unable to stop disease progression. From a conventional standpoint, treatment was largely palliative, and euthanasia was often recommended once a definitive diagnosis was made.

That grim prognosis, however, is no longer universally accurate.

In recent years, a growing body of research has shown that antiviral medications targeting the feline coronavirus itself can dramatically change outcomes for many cats with FIP. These drugs do not “cure” coronavirus infection in the population at large, but they can interrupt viral replication in affected cats long enough for the immune system to regain control.

A study conducted at Colorado State University found that an antiviral drug (molnupiravir) was effective in approximately three-quarters of cats treated, a striking improvement compared with historical survival rates. Similar success has been reported with related antiviral compounds, including GS-441524 and remdesivir, which act directly against viral replication.

While not every cat responds, and relapses can occur, FIP is no longer an automatic death sentence in all cases.

Integrating Antiviral Therapy with a Holistic Approach

Antiviral drugs represent a major shift in how FIP can be managed, but they do not negate the importance of supporting the cat as a whole. FIP remains a disease of immune dysregulation, not a viral infection. The virus is common; the disease is not.

For cats receiving antiviral treatment, supportive and holistic care still matters:

• Nutritional support to prevent muscle wasting and support immune resilience

• Anti-inflammatory strategies, used judiciously, to reduce immune-mediated damage

• Stress reduction, since stress is a known contributor to immune dysfunction

• Support of liver and kidney function, especially during prolonged treatment courses

From a holistic perspective, antiviral therapy can be viewed as buying the immune system time — reducing viral load so the body can re-establish balance. Cats with robust baseline health, good nutrition, and early intervention appear more likely to respond favorably.

It is also important to note that access to these medications varies by location and regulatory environment, and treatment protocols are still evolving. Any antiviral therapy should be undertaken with veterinary guidance and careful monitoring.

The Take-Home Message

• FIP is still a serious and complex disease, but it is no longer uniformly fatal.

• Antiviral therapies have shifted the conversation from “nothing can be done” to “many cats can recover.”

• A combined approach — antiviral treatment plus thoughtful supportive care — offers the best chance for a good outcome.

As research continues, the outlook for cats with FIP continues to improve. Staying informed and working with a veterinarian who is up-to-date on current options is essential.

Since so little is really known about the true nature of FIP, it’s hard to recommend specific preventive measures. However, a nutritious diet and appropriate immune-supporting supplements are always the best bet to create optimal health for your cat. We recommend a primarily wet food diet (either canned, raw, or homemade) along with four basic supplements: digestive enzymes, probiotics, antioxidants, and omega-3 fatty acids.