Feline Infectious Peritonitis (FIP) in Cats

Lorie Huston, DVM
Updated: March 25, 2016
Published: April 01, 2013
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I recently attended the American Animal Hospital Association’s 2013 conference in Phoenix, AZ. While there, I had the pleasure of listening to feline gurus Dr. Neils Pedersen and Dr. Alfred Legendre. One of the topics these two experts in feline health care covered was feline infectious peritonitis, more commonly known as FIP.

I thought I’d take the opportunity today to bring you up-to-date about what we know about FIP and to present to you a drug that may potentially offer some hope for cats with this deadly disease.

When I say deadly disease, I do mean that literally. It is believed that FIP is 100% fatal for cats that develop the disease. However, the development of the disease is far from simple. There is a complex mechanism that causes FIP in cats. It involves infection with a common and usually non-harmful virus known as the feline enteric coronavirus, a mutation within the virus itself, and a deficiency within the immune system of the affected cat.

We know that all cats infected with FIP are also infected with the feline enteric coronavirus. However, we also know that not all cats infected with the coronavirus develop clinical FIP. Under normal circumstances, the enteric coronavirus causes very few symptoms other a mild transient diarrhea for some kittens. Many display no symptoms at all when infected. There is a mutation that occurs within the virus that makes the virus virulent. Actually, there are two genes that need to mutate in order for the virus to morph into the FIP virus. The FIP virus looks just like the enteric coronavirus but acts much differently because of these mutations.

But a mutation within the virus alone is not enough to cause the clinical disease known as FIP. The immunity of the infected cat also comes into play. Most cats, when exposed, develop antibodies to the virus. Antibodies are proteins within the blood stream and they are a natural part of the body’s immune system. However, there are a number of elements that work together to make the immune system function effectively in ridding the body of a pathogen, or disease-causing organism. Antibodies are only part. Cell-mediated immunity is another part of the equation.

In cats that develop FIP, cell-mediated immunity does not occur as it should. Cats that mount a normal effective cell-mediated immune response do not get the disease. They recover fully and do not become ill. However, cats that do not mount any cell-mediated immune response develop the wet (or effusive) form of FIP. Cats that manage a partial cell-mediated immune response develop the dry (or non-effusive) form of the disease.

Cats with the wet form of the disease develop effusions (a form of fluid accumulation) in the abdominal cavity and sometimes the chest cavity. Cats that develop the dry form of the disease do not typically accumulate fluid but they develop characteristic lesions in various organ systems, including the pleural cavity, abdominal cavity, central nervous system, and the eyes. These lesions and where they occur determine the clinical signs seen in these cats. Both forms of the disease are considered to be fatal though.

Several drugs have been looked at as potential treatments for FIP. Both Dr. Pedersen and Dr. Legendre agreed that pentoxifylline and feline omega interferon are not effective against FIP. However, both also agree that a drug known as polyprenyl immunostimulant (or PI) is showing promise as being helpful at least for some cats with FIP. Dr. Legendre has found that cats with dry FIP treated with PI seem to have an improved quality of life and may even have a longer survival time. The verdict is still out and research on this medication is ongoing but the results obtained so far provide more hope than we’ve had previously.

Dr. Lorie Huston

Image: Seiji / via Shutterstock