Feline Infectious Peritonitis: What is FIP in Cats?

The letters ‘FIP’ stand for feline infectious peritonitis.

As the name suggests this is an infection that affects cats. The most common symptom is fluid in the belly, which distends the tummy. Sadly, there is no cure for this condition.

However, FIP as a disease is far from straightforward. Not only does it come in two forms (Wet form or dry form) but it can affect the chest, eye, or nervous system.

Most confusing of all, FIP is caused by a virus, unlike other viruses. This is because coronavirus doesn’t always cause severe illness but sometimes just simple diarrhea. But if the virus mutates, this means things get serious and signs of FIP can arise.

Feline infectious peritonitis is caused by a virus belonging to the coronavirus group. The feline coronavirus only causes illness in cats, and the seriousness of that illness can vary.

Exposure to coronavirus is actually very common. It’s estimated that around 80% of show cats test positive on a blood test, to exposure to this virus. However, exposure is very different from becoming ill. In healthy cats, the immune system mounts a defense against feline coronavirus and protects the cat from illness.

But things are even more complicated than this because feline coronavirus can mutate and become more deadly. The ‘innocent’ form of coronavirus is a common cause of diarrhea in cats. Whilst it is a more aggressive, mutated strain, which becomes the FIP virus and causes such severe illness.

Coronavirus thrives in feces. Indeed, the raw virus will dry out and die in just a few hours. However, coronavirus in feces can live on for about seven weeks.  This makes a dirty litter tray or several cats all sharing litter boxes, a great way to spread infection.

Worse than this, cats become infected when they come into contact with coronavirus. When a cat uses a litter tray being shared with a cat with diarrhea, they may get the virus on their paws. When that cat grooms, they then take the virus internally, where it will pass along the gut and out in the feces.

This is an example of the virus ‘passaging’ through a cat. Unfortunately, the more times the virus passages, the more chance it gets to mutate. Hence, multi-cat households are at far greater risk of a harmless coronavirus diarrhea mutating into the more serious FIP virus form.

A member of the coronavirus family, the canine coronavirus, infects dogs. However, canine coronavirus does not cause serious disease. Instead, it only causes mild diarrhea.

A different strain of canine coronavirus can also cause illness affecting the respiratory tract. Indeed, this virus can be on of the bugs that cause kennel cough in dogs. 

Those dogs most likely to be sick with canine coronavirus, are those with weak immune systems. Typically, puppies under 12 weeks of age are at greatest risk.

However, the dog form of coronavirus can infect other species; this includes cats and pigs. Happily, these other animals rarely show clinical signs and it does not cause feline infectious peritonitis.

FIP disease is most likely to occur in young cats and especially kittens. The initial signs are very vague and easily confused with other milder infections.

Typically a kitten may have a poor appetite, be feverish, and lose weight. The course of the illness of waxes and wanes, and becomes established as the kitten being sickly. One of the signs that will first alert the vet that FIP may be a possibility, is a persistent fever which doesn’t respond to antibiotics.

Once FIP is more established, the symptoms vary depending on which form the unlucky cat develops.

Wet or effusive FIP is so-called because it is characterized by fluid building up in the body cavities. This is most obvious as a kitten with a big swollen belly that is full of fluid.

Wet FIP progresses more rapidly than the dry form. Few cats survive more than a handful of weeks after diagnosis before euthanasia becomes the most humane option.

Dry or non-effusive FIP is less common than the wet form but is still very serious.

In this version of FIP, the cat doesn’t get the typical fluid in the belly. Instead, they get develop tissue inflammation, which often affects the brain or nervous system.

FIP is considered a disease of younger cats, especially those under two years of age. However, there is another group of cats, aged 11 years or over that also seem at risk.

The wet form of FIP is caused by the virus making the walls of blood vessels leaky, which allows fluids to leak out. Other symptoms include breathing difficulties, due to fluid in the chest which compresses the lungs, along with anemia and sometimes even jaundice.

The symptoms of dry FIP, once again are vague, but include:

Whilst the FIP virus prefers to multiply in the gut, sometimes it also replicates in the respiratory tract. This can cause enlarged tonsils, and signs of a chest infection.

This is an added concern because a sneezing kitten can transmit the virus in their saliva or nasal secretions. Unfortunately, these cats may carry on shedding virus for up to 10 months. This risks constant exposure to the virus for other cats in the house, with all that this implies.

The peritoneum is the membrane that lines the abdominal cavity. As part of FIP, the peritoneum becomes inflamed and leaky, which allows fluid to build up in the tummy. This gives the cat a balloon-like belly, which is filled with fluid.

Your vet may have a strong suspicion of feline infectious peritonitis from the cat’s age, clinical signs, and their history (belonging to a multi-cat household.) However, there is no blood test which can definitively diagnose this condition. All the blood tests can too strongly hint that the problem is FIP.

This difficulty is down to the overlap between the common but harmless diarrhea-causing coronavirus, and the rarer but more deadly FIP virus. Lab tests struggle to tell the two apart. This means a cat with a simple upset tummy from which they will recover may test positive for coronavirus, in the same way that a sick cat with FIP does.

What the vet does when diagnosing FIP is to try and fit as many clues together as possible. This is where the belly fluid proves very helpful. This fluid has certain distinctive properties such as having a high protein content and containing few cells. This gives it a straw color and the fluid becomes frothy when shaken.

One of the distressing things about FIP is that there is no cure. However, the body’s own immune system becomes overstimulated as a result of FIP, which causes some of the symptoms.

Treatment is therefore aimed at switching off this cell-mediated immunity so that the virus does less damage. Drugs such as the corticosteroid are commonly used to good effect. Steroids need to be given at high, immunosuppressive doses in order to be effective.

Another option for treatment is Virbagen Omega. This is a form of interferon, which has immune-modulating effects. Like , this does not cure the condition but it can extend life.

FIP is caused by a virus that mutates, the more contact it has with cats. To prevent this serious infection, you can short-circuit the viral mutation by keeping scrupulously clean litter trays. Scoop the poop as soon as it lands, and make sure each cat has their own tray.

Indeed, coronavirus is happiest in multi-cat households where it has lots of possibility for passaging through cats. With this in mind give careful consideration to how many cats you own, as this could increase the risk of acquiring this distressing condition.

In addition, if a cat develops diarrhea, keep them separate from other cats in the house. Whilst this doesn’t eliminate the risk of virus passing between felines, it does reduce it.

Sadly, FIP in cats has a gloomy long-term outlook.

Wet FIP (effusive FIP carries the worst prognosis. After diagnosis, the average survival time is as little as five weeks. Of course, some cats survive for longer than this. But this is balanced against FIP cats that deteriorate more quickly.

The dry form of FIP is still likely to prove fatal, but the cat may live a little longer.

What’s most important is to understand the serious nature of FIP. The wise owner concentrates on giving the cat the best quality of life possible. Tragically, sometimes the most humane thing for the cat is to recognize when they are suffering and let the cat go.