Feline infectious peritonitis (FIP) is a viral disease of cats. FIP most commonly affects young cats (less than 2 years or age), particularly purebreds, but cats of any age or breed could potentially develop this disease.
FIP is caused by a mutated (changed) form of Feline Coronavirus. Many cats within the general population are infected with Feline Coronavirus, with the highest infection rates occurring in cats kept in multi-cat environments such as breeding colonies, rescue facilities or multi-cat households.
Feline coronavirus itself rarely causes significant disease. It infects cells of the gut wall and may cause mild digestive upset or no signs of illness at all. Occasionally, however the virus mutates (changes) and is able to infect cells of the immune system. This ability to enter cells of the immune system, allows the mutated virus to spread in the blood, causing immune-mediated inflammation and disease of other organs such as the kidneys, liver, lungs, brain, eyes or blood vessels. It is this mutated virus that produces the disease syndrome known as FIP.
Unfortunately, the exact mechanism that causes the relatively harmless feline coronavirus to mutate into a fatal FIP virus is not known. It is important to note that although feline coronavirus can be spread between cats, the mutated FIP virus can not be transmitted between cats.
The clinical signs associated with FIP are diverse and vary considerably. In the early phases of the disease, affected cats may have waxing and waning episodes of lethargy and go off their food. They may also experience weight loss, or fail to grow well as kittens. As the disease progresses, affected cats may develop a build up of fluid within their abdomen and/or chest, often presenting with a distended abdomen or breathing difficulties. Depending on which organs are affected, cats may also have: vision problems; signs of neurological disease (wobbling, altered behaviour, tremors); kidney disease or liver disease.
As the clinical signs associated with FIP are often vague, it is important to rule out other potential causes of the clinical signs. Blood and urine tests are often the first step in investigating disease. Diagnostic imaging such as radiographs and ultrasound may also allow identification of fluid within the chest or abdomen and collection of fluid and tissue samples.
Only recently has testing become available to allow us to test fluid (via direct immunofluorescence) and tissue samples (via histopathology and immunohistochemistry) for the presence of the virus within the infected immune cells. There are still limitations to these tests. If a positive result comes back, a diagnosis of FIP can be made. If a negative result comes back, FIP still cannot be ruled out.
Until recently, a diagnosis of FIP was a death sentence. Although some previous treatments (omega-interferon and polyprenyl immunostimulant) had reasonable efficacy in some cases, most cats and kittens diagnosed with FIP died or required euthanasia. That all changed a few years ago with the discovery of an anti-viral drug called GS-441524 that was found to be effective for treating FIP.
For a while, this drug was only available on the black market. Ironically the COVID 19 pandemic led to the development of a very similar drug called remdesivir (GS-5734) to treat human coronavirus. Following registration of remdesivir by the TGA for human use, it can now be readily used off-label in our feline patients.
Remdesivir can be administered as an intravenous injection in severe cases, or as an injection under the skin. It is often recommended to commence cats on injectable remdesivir for 1-2 weeks, prior to the initiation of oral GS-441524 treatment. Both remdesivir and GS-441524 are administered as a once daily dose over 12 weeks. The dose is dependent on the cats weight and their clinical signs. In cats with involvement of their eyes or nervous system, a higher dose is recommended.
A third antiviral drug molnupiravir appears to be a highly effective treatment for FIP, comparable to remdesivir/GS-441524, and may be of use in cats that don’t respond well to GS-441524 or remdesivir. It can be compounded to an appropriate strength for kittens and cats.
Remdesivir and GS-441524 appear to have good efficacy in the management of FIP, resulting in a high chance of clinical control and cure.