Very few admit they have experienced it, but everyone has heard of someone who has. Experienced the unspeakable: Having had a cat that died of FIP. Or have sold a cat that died of FIP. It’s like saying you’ve got leprosy in your cattery.

By Connie Garfalk
Not many breeders like to speak about disease in their catteries, especially so if it concerns severe diseases. There is one disease though, that hardly anyone admits they have had close contact with: FIP. Three simple letters. Loaded with fear. With prejudice. And with ignorance.

FIP is the abbreviation of feline infectious peritonitis. A disease that first and foremost hits young cats. One usually estimates the danger zone to be from six months up to one or two years of age. After that the cat is relatively safe until it reaches old age. Then the risk increases again. For FIP as with so many other diseases it’s the very young and the old that are vulnerable. The individuals with the weakest immune system. However, it isn’t quite that simple after all.

FIP is caused by feline coronavirus. The virus is ubiquitous among cats and is extremely common where a number of cats are kept together. Feline coronavirus (FCoV) is "the familyname" for all coronaviruses that affect cats. Within this family you find the feline enteric coronavirus (FECV) and its mutant, the feared feline infectious peritonitis virus (FIPV).

According to English estimates the prevalence of coronavirus lies between 80–100 % where several cats live together. American estimates are a trifle more moderate, they say 80–90 % of cats in a multicat environment at one time or another have been infected with the virus. Probably they have been infected more than once. Usually this doesn’t lead to anything more serious than maybe a mild diarrhoea that passes by itself after a couple of days.

Cats get infected mainly via the litter boxes. The virus is present in the fæces of infected cats, and they can keep shedding the virus for long periods. Cats that are newly infected can also shed the virus in their saliva and thus infect other cats they groom, or share food and water bowls with. In short, the virus is ingested. The medical term is a fecal-oral route of transmission.

In spite of the fact that most cats in multicat homes at one time or another, probably many times, get infected with the virus, an exceptional few develop the deadly disease FIP. A disease one so far has no cure for. Some say that less than five percent of infected cats develop FIP, some say less than 10 percent.

As to why these few cats develop this horrible disease, while the majority don’t, noone knows for sure. One does know, however, that it has got something to do with the immune system. The belief today is that FIP develops in close co-operation with the individual cat’s immune system.

According to what research says today, the FIP-virus does not infect as such. The coronavirus has to mutate (change) inside the individual cat’s own body to become the deadly FIP-virus. That said, most coronaviruses mutate easily, but mostly these mutations are of a harmless character. Recently researchers have found that there are different strains of coronavirus, where some are more virulent than others.

Anyway, a coronavirus can not mutate into a FIP-virus without help from the cat’s own immune system. In that manner, the FIP-virus is a highly personalized virus, developed inside the individual cat, and this virus means nothing to other cats. Other cats can be infected with the original coronavirus, even by the more virulent viruses, but the FIP-virus in itself the specific cat has to develop itself.

– How can one explain that only a few cats develop FIP, while the vast number of corona-infected cats don’t?

– It’s a complex situation. The kitten’s immune system must kill enough virus to keep the viral load at low levels. This keeps a check on the virus’ ability to reproduce. If the kitten’s immune system becomes less effective for some reason, the virus will not be inhibited, and will reproduce at will and build up large numbers, says Dr Susan Little, a Canadian veterinarian that also is well known in Europe.

– What can cause the cat’s immune system to react in an inadequate way?

– There are many factors that could impair the kitten’s immune response, such as being infected at a young age, being infected with a high viral load, being infected with a particularly virulent strain, being stressed in some way, says Susan Little.

– Why does the virus mutate into the FIP-virus?

– Once the virus starts to reproduce at will, the risk that it will mutate into the disease-causing form within the kitten increases. If such a mutation occurs, the virus then has increased ability to invade the immune system and spread throughout the body. The kitten’s immune system tries to counteract this, but the usual defences simply add to the pathology of the disease, mainly through inflammation and formation of granulomas in tissues. FIP is essentially a vasculitis, in other words an inflammation around the blood vessels. The antibodies the kitten produces can bind with the virus and become deposited in tissues as antigen-antibody complexes, says Susan Little.

Antigen is a medical term for something the immune system reacts to as a foreign substance or material, something that has no business inside the body this particular immune system is protecting. As soon as an antigen is spotted by the immune system, it starts to produce antibodies to kill it. This can lead to a close embrace where the two enemies cling to each other in a deadly fight, and when this happens one talks about an immune-complex. The result being that neither of the enemies kill each other, but they form a unity that is even more deadly than each of them per se.

– What I have described is a simplification of the process. We are still learning about the complex interplay of the virus and the immune system in this disease, Susan Little says.

Again. To make a long and complicated story short: The FIP-virus is a personally developed virus, and this personalized virus has no impact on other cats.

Recent research (the 90ies) point to a factor of inheritance when it comes to the way the specific cat’s immune system deals with the coronavirus.

Two researchers, Dr Janet E. Foley and Dr Niels C. Pedersen in California, have identified three risk factors: Genetical susceptibility, the presence of a chronical coronavirus-shedder and a dense cat population. In 1996 these two researchers studied 10 generations of pedigreed cats and found that inheritance could be alarmlingly high, as high as about 50 percent, actually.

In a guidebook for veterinarians, The Feline Patient – Essentials of Diagnosis and Treatment, by Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee and Larry P. Tilley (1998 Lippincott Williams & Wilkins), it says: "Recently, it was reported that there is a genetic influence on susceptibility to FIP. It appears that certain breeding toms may pass this trait to offspring."
In an article about FIP Dr Susan Little claims, a claim supported by others, that this seems to be a polygenetic trait. If that is so one cannot pinpoint a specific gene that passes this susceptibility on, it’s more like a "gang assault".

– If this is a polygenetic trait, wouldn’t it then be plausible to assume that both parents contribute at an equal basis?

– The issue of inheritance of susceptibility is often misunderstood. You are correct that it is likely a trait that can be passed by either or both parents. It gets confusing because in the study the textbook refers to, as in most catteries, a few males sire all the kittens. Breeders tend to keep more females than males. So a given male spreads his genes to more kittens than any given female. Therefore he can have a greater genetic impact. Males that have sired kittens that died of FIP have the ability to pass this susceptibility on to many kittens, so I think we need a stricter standard when evaluating them, says Susan Little.

She continues:
– We might choose to keep a queen in the breeding programme that has had a kitten die of FIP because she only produces so many kittens in her lifetime (and we can breed her to a different male the next time). But a male that produces FIP-kittens could potentially produce a lot of FIP-kittens. It is suspected that about 50% of the incidence of FIP is due to inherited susceptibility, and this susceptibility likely has to do with immune system function. So not every kitten that dies of FIP has inherited susceptibility, Susan Little says.

Translated to layman language, it might be a wise decision to take out cats from the breeding programme if they have produced two or more kittens who died of FIP.

FIP manifest itself in two forms: the effusive one (also called wet FIP) and the non-effusive FIP (also called dry FIP). The wet form of FIP is by far the most common. It is also easier to diagnose than the non-effusive FIP. For both forms, however, there are some other diseases that give similar symptoms.

In both cases the usual symptoms are fever without any obvious cause and typically this fever doesn’t respond to any treatment by antibiotics. Weight loss is a symptom for both cases, as are lethargy, loss of appetite and maybe a slight tendency to dehydration. Cats with the wet form develop later on a yellowish fluid with a characteristically high protein content in their abdomen or chest. The dry form leads to severe problems in several organs, such as kidneys, mesenteric lymph nodes and in the brain and eyes.

However severe the symptoms are, they could in most cases also relate to other diseases. Today, however, there are different tests that together make a diagnosis fairly conclusive. A biopsy from the live cat is put through a so called immunochemical staining-test, where one can identify the coronavirus in the affected tissues. But in many cases the cat is not diagnosed until after it’s dead.

Some scientific milieues say that catteries or homes where there are less than six cats living together, are relatively safe. The cats get rid of the virus by themselves reasonably quick. In cases where there is a massive amount of virus involved, or some of the cats have an immune system that does not cope that easily with the virus, or the virus load is heavy or contains a particularly virulent strain, you may want to consider some extra measure.

Keeping everything clean is obvious. The virus is easily killed by mild detergents. Another good advice is to have at least one litterbox for every two cats. Keep food and water bowls far away from the litterboxes. Fæces should be removed at least once a day, and litter completely changed at least weekly (ideally daily), and the trays disinfected.

The most important advice, however, is to reduce the number of cats that have to live together. See to it that the cats you have are not subjected to undue stress in any way, give them appropriate food and try to make all of your cats enjoy life. Joy and happiness are great immune system boosters!

But when all is said and done, the fact remains: Noone can guarantee that no cat from them is going to develop FIP. The coronavirus is ubiquitous. The only comfort is, that only an exceptionally few will develop the deadly disease FIP.