Feline infectious peritonitis is a worldwide, always fatal infectious disease that only affects cats. It is still one of the leading causes of death in cats today.
FIP disease is caused by a normally harmless virus that is widespread. It is the so-called “feline coronavirus”.
In most cats, infection with it causes diarrhea, if any, and occasionally fever. In cats suffering from FIP, the coronavirus has mutated, resulting in the FIP virus. This happens in about 5% of cats infected with Corona. Whether or not the mutation occurs in a corona-infected cat depends on the age, immune status, genetic susceptibility of the infected animal, the aggressiveness of the infection strain and the amount of infection.
Cats that are kept in groups in a confined space and where there is frequent change, such as in animal shelters, cat kennels or cat boarding houses, are particularly frequently infected with the feline coronavirus if the animals are kept together there.
Young animals are particularly affected. 50% of all FIP cats are under 1 year old.
Course of infection
At the beginning of the infection, the feline coronavirus, which is mainly excreted with the faeces of infected animals, is taken up through the nose and throat. The mutated FIP virus itself is not transmissible.
The ingested corona viruses settle in the small intestine on the surface of intestinal cells, into which they then penetrate. They multiply in these cells and destroy them in the process. The viruses released in the process then infect new cells and are excreted with the faeces.
In the environment, the corona virus usually only survives for a short time, but can remain infectious on surfaces for up to 7 weeks and can thus also be transmitted via clothing and objects.
In 5-10% of cats infected with the coronavirus, the virus mutates into the FIP virus days or even years after infection. Now the macrophages, a certain cell group of the immune system, are also attacked and the virus spreads with them in the body.
The more often or the more viruses are ingested and the higher the virus multiplication, the greater the risk of mutation. For this reason, cats in multi-cat households are more at risk because they pick up viruses each time they use the shared litter box if there is a virus-shedding cat among them.
About 14 days after the mutation, the virus has spread throughout the body. The immune system tries to attack the FIP virus with antibodies. This results in so-called “antigen-antibody complexes”. In turn, these complexes and the altered responses of the immune system caused by the FIP virus cause the majority of symptoms. So it’s not really the virus itself that causes the symptoms, but the immune response of your own body.
The first symptoms appear a few weeks up to a maximum of 18 months after the mutation.
Once established, FIP is always fatal, usually within a few days to weeks.
The clinical symptoms of FIP disease vary greatly, depending on which organs are primarily affected. In particular, the intestines, kidneys, liver, pancreas, eyes and central nervous system are often the targets of the infection. In them, inflammation of vessels and organ membranes is triggered, which leads to considerable organ damage and fluid secretions. The organs fail and fever develops, which can hardly be influenced with medication, and changes in the blood values of the affected organs. Sick cats stop eating, become lethargic, look unkempt and lose weight. In many cases, effusions occur mainly in the abdomen or chest, which can lead to an enlarged abdominal circumference.
Other symptoms may include jaundice, diarrhea, vomiting, eye inflammation, shortness of breath, and neurological symptoms.
The diagnosis of FIP disease is often very difficult because there is no simple test that provides a safe, unambiguous result.
Basically, a distinction must be made between tests that detect the mostly harmless feline coronavirus (most standard tests) and detection of the deadly FIP pathogen. Most tests do not allow a distinction between the harmless and the dangerous, mutated variant of the corona virus.
The so-called “FIP test” is usually a detection of antibodies against the feline coronavirus. However, a maximum of 10% of cats that are positive in this test will develop FIP at some point in their lives. Therefore, the positive detection of antibodies against corona viruses is never a reason for euthanasia.
The level of the “corona titer” also says nothing about whether a cat is more likely to develop FIP or not, but something about how many viruses are excreted with the faeces. Usually, the higher the titer, the more viruses the cat excretes. This can also be proven with a PCR test in the faeces. But beware. A single negative test does not mean that the cat is not a shedder of the virus, as the virus is not always shed. To be on the safe side, you should do 3 tests in 14 days.
A healthy cat that has no detectable corona antibodies is highly unlikely to be infected with it, nor will it excrete viruses. Therefore, she cannot get FIP either. However, if cats show the typical symptoms of FIP, they can also appear negative in the corona test towards the end of the disease, since all antibodies against the virus are bound in so-called “antigen-antibody complexes” and can no longer be detected.
In general, the diagnosis of FIP consists of the overall picture of symptoms and laboratory results. If the symptoms are correct and if specific laboratory values have changed accordingly, it is almost certainly a FIP infection, with the consequence that the affected animal will not live much longer.
Frequent laboratory changes in the blood serum of sick cats are high total protein (especially if it is over 100g/l) with high gamma globulins (especially > 40g/l) and low albumin. From this you can calculate the so-called “albumin-globulin quotient” which is <0.5 for most FIP cats.
However, this does not have to be the case for every cat with FIP, nor does every cat with these changes have to have FIP.
More reliable than detection in the blood are tests of mostly existing fluid from the chest or abdomen.
A so-called “Rivalta test” can be carried out, which is positive in most FIP cats, and the liquid, which is usually yellowish and stringy, can be examined for its contents and the cells it contains. FIP often contains a lot of protein but few cells. In a group of these cells, the “macrophages”, the FIP virus itself can be detected with the help of a PCR or a special stain. If the result is positive, this represents the only reliable proof of the dangerous mutated virus form, since only mutated viruses can be detected in macrophages in sufficient quantities for detection. However, if the result is negative, it does not rule out FIP disease.
No single test method, laboratory change and no single symptom allows a reliable diagnosis of FIP disease, since all changes can, but do not have to, occur in diseased animals and also occur in other diseases. However, if enough typical symptoms and laboratory changes occur together, the diagnosis “FIP” can be made with great certainty.
A diagnostic laparotomy (opening of the abdominal cavity) with sampling and subsequent histopathological examination can provide a clear diagnosis.
Many different medications have been used, but once FIP disease has broken out, the diseased animal currently always dies. Symptomatic treatment (especially antibiotics, corticosteroids, and antipyretics) can temporarily stop the disease in some cats for a few months, but on average, affected animals die within 9 days of diagnosis.
As protection, there is a vaccination that triggers immunity locally at the point of entry (drops that are instilled into the nose) and is intended to prevent the virus from being absorbed. It can be administered from the 16th week. Before the first vaccination, cats should be tested, since the vaccination makes no sense in animals that are already corona-positive. It does not make sense to vaccinate the remaining cats if one cat in the stock has just died of FIP, since the remaining animals have very likely already had contact with the virus.
Overall, however, the effectiveness of the vaccination is very controversial. According to studies, protection ranges from 0% to 80%. Vaccinated animals appear positive in the corona antibody test.
Since the main transmission takes place with faeces and faecal dust and the risk of mutation of the virus increases with the increasing quantity and frequency with which cats ingest corona viruses, attempts should be made to limit or prevent intake.
In a household that does not yet have any corona-positive animals, all new arrivals should be tested before the first contact and only animals that have tested negative and show no symptoms of the disease should be admitted.
In a household with cats that have tested positive for Corona, the litter boxes should be kept as clean as possible and ideally one litter box should be available for each cat to reduce constant virus ingestion from using the same litter box. Free running is an advantage here, as the cat is less likely to defecate in the same spot frequently.
After the death of a FIP cat everything should be thoroughly cleaned and disinfected and wait 3 months before new cats are allowed to move in.
If you still have a cat that is corona-positive, you should ideally wait until it has overcome the infection and is corona-negative again in the test before purchasing a new animal. This can take months or years.
When should you take your cat to the vet?
Any sick cat should be presented to a veterinarian.
Since the main symptoms of FIP (fever, loss of appetite, apathy) are relatively non-specific, there may be other diseases that nevertheless require veterinary treatment.
Kittens that have high fevers that are difficult to control with medication and/or fluid in the abdominal or chest cavity are highly suspect for FIP.
Every cat that is to be brought into a previously corona-free stock should be tested for FIP before the first contact.