FIP

This article was published in March 2022. We have added an update note to some of the statements.

If you're anything like us, you'd probably be happy never – and we really mean NEVER – to hear the word corona again. For two years it has felt as if hardly anything else mattered but lockdowns, infection cases or tragic figures from hospitals and care homes. So why an article on it here, of all places? Well, simply because corona unfortunately also occurs in our beloved house cats. And not just since the end of 2019, when SARS-CoV2 was officially described for the first time, but for decades.

Coronaviruses in cats

Feline coronavirus (FCoV), which infects cats and can be detected in almost every cat population, is in itself fairly harmless. It was first described back in the 1960s and, especially in young cats, mainly causes mild diarrhoea or other so-called non-specific symptoms such as fever. Often an infection also runs its course without any outward signs. However, it is also highly contagious, which makes it almost impossible to get it out of a cat population again. What does that mean for us as owners? For now, only this: a great many of our house tigers carry antibodies against the coronavirus, meaning they have been in contact with the virus at some point, successfully fought it off and recovered.

From harmless corona infection to deadly FIP

What unfortunately makes the feline coronavirus frightening is the fact that it can mutate – and it does so busily. In fact, FCoV mutates relatively easily in our beloved pets, in as many as 5–10% of all cats infected with the “harmless” coronavirus. This mutation then always leads to a very well-known and highly dangerous disease. You and all of us have heard of it before: it is FIP. FIP stands for feline infectious peritonitis (in case your head is already spinning from all the technical terms and abbreviations: we have listed all the important terms for you once more at the end of the article).

What happens during an infection?

At the start of an infection, the harmless feline coronavirus, which is shed mainly in the faeces of infected animals, is taken up through the nose and throat. The ingested coronaviruses attach to the surface of cells in the small intestine and enter them. They multiply inside these cells and destroy them. The viruses released in the process then infect new cells and are subsequently shed in the faeces or also via saliva. So we can see: simply sharing a litter box can lead to healthy cats becoming infected. Very important: FCoV in itself is hardly dangerous, and many cats already carry it at birth, as it can also be transmitted by the mother cat in the womb.

Mutation – and what it means for our cat

Unfortunately, in 5–10% of cats infected with the coronavirus, the virus mutates into the FIP virus days to years after the infection. It then infects the macrophages, a particular group of cells of the immune system, and spreads through the body with them. About 2 weeks after the spontaneous mutation, the virus has spread throughout the body. The immune system tries to fight the FIP viruses with antibodies. In the process, so-called “antigen-antibody complexes” are formed.

These complexes, in turn – together with the immune system reactions altered by the FIP virus – cause most of the symptoms. So it is not really the virus itself that leads to the symptoms but, paradoxically, the body's own immune response. The first symptoms appear a few weeks to a maximum of 18 months after the mutation. What's important to know here: the feline coronavirus is contagious, but FIP is not!

FIP, the spectre

Worldwide, FIP is one of the most common – if not THE most common – infectious causes of death in cats. Several million house cats live in Germany; the cat is our most popular pet. Few things worry a pet owner as much as the wellbeing and health of their animal. This makes the feline coronavirus, or rather its mutation, all the more frightening for all of us. Because unlike many other infectious diseases, there is currently still no cure for the disease. FIP always leads to the death of our cat, usually within just a few weeks. In some cases, the cat was able to live with FIP for several more months – unfortunately, though, this is not the norm. (Update 2026: With the active substance GS-441524, FIP is today considered curable in many cases if treated early. In Germany, legal treatment is possible via a compounded prescription (Rezeptur), see below.) Cats under one year of age are most frequently affected by FIP.

Symptoms of FIP

The so-called clinical symptoms of FIP, i.e. what we can see, vary greatly. Depending on which organs are mainly affected, the cats show different signs. The intestines, kidneys, liver, pancreas, eyes and central nervous system are frequent targets of the infection. Inflammation is triggered in them, which leads to considerable organ damage and fluid effusions. The organs fail, and the cat develops a fever that does not respond to medication. The blood count then also shows the altered values of the affected organ in each case. Most commonly, inflammation of the abdominal lining – the so-called peritoneum – is observed. This symptom also gives the disease its name: peritonitis means inflammation of the peritoneum. When fluid accumulates, this is known as wet FIP. In contrast, there is dry FIP, which occurs without effusions. This form is harder to diagnose and is therefore sometimes not recognised as the first possibility. Sick cats stop eating, are apathetic, often look unkempt and lose weight. The vet can only ever provide relief for the symptoms.

Is there any hope at all?

A disease, quickly and easily transmitted, with a high mutation rate, that will certainly kill my cat? Is there any hope at all in all these horror scenarios? Thank goodness – yes! First and foremost, the most important point for owners with several cats at home: FCoV is contagious; FIP is NOT! In other words, should one of our cats tragically suffer from FIP, the other cats in the family can stay perfectly healthy. Furthermore, in 2021 there was a first, promising study with a new type of medication. In this study, all participating cats were actually cured. Since it was only a very small study (18 animals took part) and the medication has not yet been approved in the EU, we can only hope that research in this field of science will continue eagerly. (Update 2026: While the active substance GS-441524 still has no official EU or national approval as a finished veterinary medicinal product against FIP in Germany, legal treatment is nevertheless possible via a compounded prescription (Rezeptur).) There is also a vaccine against FIP. This vaccine is quite controversial, as different studies have come and still come to different results: an efficacy of between 0% and 80% has been shown. Quite a wide range? We think so too! One drawback of the vaccine is that only animals that have never had contact with coronaviruses are eligible for vaccination – because the vaccine can only prevent infection with corona. There is no vaccine against a mutation inside the cat's body.

Is there anything we can do?

Here, too: yes. As far as can be assessed scientifically, there are preventive options to reduce the likelihood of a mutation. It is mainly young cats in larger cat populations (animal shelters, breeders, multi-cat households) that develop FIP. This suggests that stress could act as a driver of mutation. In addition, increased hygiene can help ensure that the viral load does not become too high. In other words, if infected cats shed the virus via faeces or saliva, rigorous hygiene can noticeably reduce the viral load for the other animals. Because the fewer viruses are taken up, the lower the viral load in the body. Studies indicate that an increased viral load, for example due to repeated infections, can also contribute to mutation.

What's left if my cat has FIP?

Until now, the situation was this: FIP led to death, and all we could do, with the help of a vet, was make sure our sick cat did not suffer for too long. In the meantime, however, there is hope that the medication mentioned above can genuinely cure cats. We sincerely hope that the medication will become freely available in Germany in the future and soon be at our vets' disposal – because then there will be a real chance of a cure for those house tigers treated with it early enough in the infection. (Update 2026: Treatment with GS-441524 is now legally possible in Germany via a compounded prescription (Rezeptur), see above.) We hope that nobody ever loses their house tiger to this devilish disease; but we also know that, unfortunately, at present it will keep happening.

Glossary

• FCoV: feline coronavirus
• Feline (derived from Felidae, cats) means that all members of the cat family, so theoretically lions or tigers too, can be affected.
• Mutation: a spontaneously occurring, permanent change in the genetic material of a cell or, in our case, of a virus
• FIP: feline infectious peritonitis
• Peritoneum: the lining of the abdominal cavity
• Peritonitis: inflammation of the peritoneum
• Antibodies: produced by the body after recovering from an infectious disease or after a vaccination