What is FIP?

Q: Mr. Mistoffelees, our 9-month-old cat, had little energy and a poor appetite, so we took him to the veterinarian. The vet noted that he had lost weight and his abdomen was full of fluid. She did some tests and said he has FIP, which is always fatal. We were so overwhelmed that we didn't hear what she said about FIP. What can you tell us about this horrible disease?

A: FIP, or feline infectious peritonitis, develops when a common, harmless virus, the feline coronavirus, mutates within a cat's body. Feline coronavirus is easily transmitted from cat-to-cat, infecting half the cats in single-cat homes and up to 90% of cats in multi-cat households and catteries. The virus does not infect dogs, humans or other species. Coronavirus may cause mild diarrhea that resolves on its own or produce no clinical signs at all.

However, in a small percentage of infected cats, the usually innocuous feline coronavirus mutates into the deadly FIP virus. While the coronavirus is contagious, the mutated FIP virus is not. Though FIP can occur at any age, half of affected cats are under 2 years old.

FIP occurs in two forms: wet and dry. Both are characterized by decreased energy, appetite and weight. Cats with the wet form, like Mr. Mistoffelees, also have fluid in the abdomen (known as the peritoneal cavity, which gives the disease its name) or sometimes the chest.

Cats with the dry form don't accumulate fluid in their body cavities, but they often have abnormalities of the eyes or nervous system. Other clinical signs depend on which organs are affected.

The wet form usually appears one to two months after a stressful event, such as rehoming, surgery or illness. The wet form is more rapidly progressive than the dry form, which develops over months to years.

Sadly, there is no cure for FIP. After diagnosis, the typical survival time is days to weeks for the wet form, and weeks to months for the dry form.

My heart is with you and Mr. Mistoffelees.

Lee Pickett, V.M.D. practices companion animal medicine in North Carolina. Contact her at vet@askthevet.pet.