Feline Infectious Peritonitis

FIP is caused by a coronavirus (RNA virus). This virus integrates itself into the white blood cells in the body. Once infected, the body will begin to generate a cell-mediated immune response. Such a response involves macrophages, cytotoxic T-cells and other cells which digest and kill virus-containing cells. If this response does not successfully remove the virus from the system, limit the amount of virus significantly, or allow the cat to become immune such that it can survive with the virus in the body, the cat will mount an antibody-mediated response. In this response antibodies will be created to seek out and kill virus and cells containing virus. It is the antibody-mediated response which actually causes the disease symptoms and eventually can kill the cat. Thus cats who can mount an effective cell-mediated response and avoid the antibody-mediated response usually can live with or fight off the disease, while animals who get to the point at which they mount an antibody-mediated response generally show signs of disease. On average cats will develop symptoms of the disease within two weeks after infection if they are to develop the disease. And for-the-most-part only the very young, very old, stressed or immuno-compromised will become ill from exposure to the virus.

Currently much deliberation occurs as to how FIP is transmitted and spread. It is known that cats shed coronavirus in their saliva and feces. For a long time it was thus thought that the virus was transmitted through casual contact with saliva or feces, or from a mother to kitten. Thus catteries or multiple cat homes with an FIP+ cat were thought to be in danger of infecting all animals. However, new studies show that this may not be the case at all. These studies propose that FIP is acquired due to mutations of other coronavirus’ to the FIP virus. These hypotheses are based on numerous studies which show that the FIP virus is not very stable in the environment and that it is rapidly inactivated by household cleaners, that indoor-only cats in single-cat homes who originally test FIP- can suddenly become FIP+, and that infection of other cats in multiple cat homes with one FIP+ cat is sporadic and limited at best (the number of cats the do become FIP+ correspond with the number of cats which would become infected due to a mutation anyway). Of course until definitive studies are completed it is still best to keep a FIP+ cat separated from other cats to avoid any chance of possible infection.

There are two types of FIP, wet (effusive) and dry. Both forms will affect multiple organs of the body (kidney, liver, eyes, brain, lungs, stomach, nervous system) and such symptoms may be non-specific and vary from cat to cat. Wet FIP is rapidly progressive and characterized by fluid accumulation in different parts of the body (lungs, chest, stomach). An infected cat may appear pot-bellied and have trouble breathing. The dry form is characterized by non-specific symptoms like fever, loss of appetite, lethargy, and neurological problems. Such cats can have trouble walking or moving and can have anisocoria in which one pupil fully dilates and stays that way.

Symptoms to look for:

  • Fever (most vets agree that the largest cause of chronic fever of unknown origin is FIP)
  • Lethargy
  • Depression
  • Weight loss
  • Rough hair coat
  • Vomiting
  • Diarrhea
  • Bloated belly (with wet FIP)
  • Anisocoria (one pupil fully dilated)
  • Trouble walking or moving
  • Incontinence
  • Trouble breathing
  • Anemia
  • Neurological problems (seizure, paralysis, behavioral changes, balance problems)
  • Kidney problems/failure
  • Liver problems/failure


Unfortunately tests for FIP are shaky at best. This is why it is very important to treat the symptoms, not the test. Thus if your cat has all the symptoms of FIP and there is no other explanation for his/her illness, even if the test is negative, treat for FIP. Cats who test negative can do so for multiple reasons: 1) the cat has not been exposed to the virus 2) the cat has the disease but all antibodies are tied up trying to kill virus 3) the immune system is so weak from the disease that it no longer can produce antibodies 4) antibodies are currently in a growth/death cycle and are currently not present in high amount. Similarly if your cat is tested and is shown as FIP+, yet has no symptoms, do not assume he or she has the disease and do not treat for the disease. Your cat could have been exposed to FIP and have antibodies but does not have the disease, or your cat may have antibodies to FECV.

There are over a dozen different coronavirus species and in cats coronavirus’ fall into two types, FIP or FECV (Feline Enteric Corona Virus, a mild virus which causes diarrhea). The standard FIP test (a serum antibody test like an ELISA or IFA) simply tests for the presence of antibody to coronavirus, not specifically to FIP. Results to such a test will be in the form of a titer (example 1:4000). What that means is the number of times you can dilute the serum by half and still see presence of virus. So the larger the second number, the more antibodies present. Other ways to test for FIP that can sometimes be more accurate include but are not limited to:

  • Immunooperoxidase test – detects virus-infected cells in tissue rather than just free virus or antibodies to virus.
  • PCR (Polymerase Chain Reaction) – tests for the presence of virus not antibody. However it tests for the presence of coronavirus in general.
  • Serum Protein Levels – high levels of total serum protein AND an A:G ratio that is small (ratio of two different proteins) can be a sign that a cat has FIP as only a few diseases cause this.
  • Abdominal or Chest Fluid analysis – if your cat has wet FIP or dry and has some fluid accumulation, testing the fluid for certain characteristics and indicate a strong candidacy for FIP.
  • X-rays of the Chest or Abdomen – this can identify fluid presence in wet FIP.
  • Needle aspiration to the liver or kidneys – FIP produces a distinct inflammatory pattern in these organs. This test can show strong possibility of FIP.
  • White blood cell count – as white blood cells are created in the immune response to combat FIP a high level of WBC’s can indicate that FIP is a possibility (many cats however have normal WBC levels while having FIP).


Most vets will tell you that FIP is always fatal and that there is nothing you can do but euthanasia. Although FIP is often fatal there are treatments that you can try to help your pet force the virus into remission, to form a symbiosis with the virus so both can live quite happily, or to simply live a good quality of life for months to years after diagnosis. The goal of treatment should be to boost the cell-mediated immune response and suppress the antibody-mediated immune response. And as suppressing any part of the immune system can allow the cat to be susceptible to other diseases you need to keep the cat as safe and healthy as possible during this time so as to not catch another illness. Also minimize stress in your cat’s life, keep the cat happy and on a high-quality diet. Your vet may also prescribe supportive care for other symptoms or problems caused by the FIP such as anemia or kidney problems. Medications that can be used to accomplish the above goals include interferon, prednisone or other corticosteroids, antibiotics, cytotoxic drugs, and anti-virals.

For specific treatments/drug therapies (with full explanation) or if you have a cat with FIP please read Taz’s story. At 6 months of age Taz was diagnosed with a very bad case of wet FIP (2 vets said to put him to sleep before we found a vet willing to work with us and work on a treatment) in 1996. However through careful drug therapy and treatment he is now 5 years old and healthy as a horse although his viral titer is still as large as it was when he was first diagnosed.

Precautionary measures:

There is a vaccine for FIP; however, this vaccine is very controversial and it hasn’t been determined if it can actually cause more severe cases of FIP in cats. At this time, until further research can be made, we can not advocate the use of such a vaccine. For more information ask your veterinarian about the vaccine and any new finding on its efficacy.