FIP in Cats: Signs, Diagnosis and the New Treatment Options

What FIP is and which cats get it

Feline coronavirus is very common, especially where many cats share litter boxes, and in most cats it causes nothing worse than mild, short-lived diarrhea. In a small minority, under 10 percent of infected cats according to both Cornell Feline Health Center and VCA Animal Hospitals, the virus mutates inside the cat into a form that infects white blood cells. Those cells carry it around the body, and the immune system's intense reaction to it is what causes FIP.

Young cats carry most of the risk. Cornell reports that about 70 percent of cases are in cats under 18 months old. Cats from shelters, catteries and crowded multi-cat homes are more likely to be affected, and stress, recent surgery, other infections, being male and some purebred lines may all play a part. The mutated virus is not thought to spread readily from cat to cat; it is the common coronavirus that spreads, mainly through shared litter.

Signs of FIP: the wet and dry forms

FIP usually starts with vague signs that are easy to miss: low energy, a poor or patchy appetite, weight loss or a kitten that stops growing, and a fever that comes and goes and does not respond to antibiotics. After that the disease tends to take one of two forms, although some cats show features of both.

In the wet form, fluid builds up in the abdomen or chest. The belly may swell like a potbelly, or the cat may breathe fast and with effort if fluid surrounds the lungs. In the dry form, inflammation affects organs such as the eyes, brain, liver, kidneys or intestines. VCA notes that some cats show only eye problems, and Cornell notes that brain involvement causes neurologic signs such as wobbliness or seizures. These signs overlap with many other illnesses, so they point toward testing rather than to a diagnosis.

When to call your vet today

Call your vet the same day if a young cat has a fever that keeps returning, is losing weight, is eating poorly for more than a day, or has a belly that is getting rounder while the rest of the body gets thinner. A change in the look of an eye, a new wobble when walking, or a kitten falling behind its littermates also deserves a prompt visit.

Because the newer treatments work best when started early, do not wait weeks to see whether vague signs settle. Tell your vet if your cat came from a shelter, breeder or multi-cat home, or if another cat in the household has had FIP, since that history helps them decide how hard to look for it.

When it's an emergency

Go to your vet or the nearest emergency vet immediately if your cat is breathing fast or with effort, breathing with its mouth open. Fluid around the lungs can make breathing very hard and may need to be drained urgently. A seizure, collapse, sudden inability to stand, or a cat that has stopped eating and drinking entirely also needs emergency care.

Keep the cat calm and quiet in a carrier on the way, and phone ahead so the team can be ready. If you have a record of temperatures or weights, or blood test results from a recent visit, bring them with you.

What you can safely do at home

Do not give any medicine, human or animal, unless your vet has prescribed it, and do not buy unregulated FIP antivirals online. Cornell warns that black-market products vary widely in strength and purity, which can mean an under-treated cat or a harmful one. Your vet can tell you what prescription options are available.

At home, keep your cat warm, quiet and with easy access to food, water and a litter box. Offer tempting, familiar food and note how much is eaten, along with weight, breathing rate at rest and any new signs. If you have other cats, ask your vet about keeping litter boxes very clean and separate while you wait for answers.

What the vet will likely check

There is no single blood test that proves FIP. Coronavirus antibody and PCR tests cannot reliably tell harmless infection from the disease, so vets build the diagnosis from several pieces: the cat's age and history, a physical exam, routine blood work, imaging such as X-rays or ultrasound, and, where there is fluid, an analysis of a sample drawn off with a needle. A tissue sample examined by a pathologist gives the most certain answer.

Treatment has changed dramatically. Cornell reports that the antiviral GS-441524 has been safe and effective in studies and is now available in the US as a compounded oral medicine with a veterinary prescription. VCA notes that antivirals are given over several months and that many cats improve within days of starting. Supportive care such as fluids, draining fluid, anti-nausea medicine or eye drops may be needed alongside. Treatment details are evolving, so ask your vet about current options.

Prevention

The only sure prevention is avoiding coronavirus infection, which is difficult in multi-cat settings. Both Cornell and VCA advise keeping groups to about three cats or fewer, providing plenty of litter boxes, scooping daily, keeping boxes well away from food and water, and keeping stress low. Vaccinating against other common viruses keeps cats healthier overall. The one FIP vaccine that exists has not been shown to work well and is not generally recommended.

If a cat in your home has had FIP, VCA suggests waiting about a month before bringing in a new cat, and longer in a multi-cat home to watch the others. Coronaviruses from cats are not known to pass to people.

This page is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your pet.