Feline Infectious Peritonitis

By Kay Hisano | Dated December 20, 2025

Leo is a 25-week-old male kitten who came to us because he was unusually lethargic and had lost his appetite. When a young cat presents like this, one of the conditions we keep in mind is Feline Infectious Peritonitis (FIP), a serious disease that mainly affects kittens and young cats.

FIP is caused by a mutation of a very common virus called feline coronavirus (not related to COVID-19). Most cats that carry this virus never become unwell, but in a small number, the virus mutates and triggers an abnormal immune response. This can lead to widespread inflammation and, in some cases like Leo’s, a build-up of fluid in the chest or abdomen. For many years FIP was considered untreatable, but fortunately, effective treatment is now available!

To investigate his symptoms, we performed blood tests and also collected a small sample of fluid that had built up in his chest. These results confirmed that Leo did have FIP. He has since started oral treatment, and the improvement has been remarkable — he is already looking brighter, more comfortable, and much more like his normal playful self.

Look how happy Leo looks during his recent recheck with Dr. John!