Feline infectious peritonitis (FIP) is a fatal incurable disease that affects cats. It is caused by feline coronavirus. This is different to the human coronavirus that causes COVID-19 in people. Experts do not always agree on the specifics of FIP, making it a very complicated disease to understand and often diagnose.
However, the most common theory is that the normally benign (not harmful) coronavirus mutates causing feline infectious peritonitis. This means that a cat may be infected with coronavirus but not have FIP. Feline coronavirus is very common and usually doesn’t cause any serious issues, aside from mild diarrhoea. But when the feline coronavirus changes to a specific strain of the coronavirus, FIP can develop. The mutated virus is able to invade and grow in certain white blood cells. The immune system’s response causes an intense inflammatory reaction in various tissues. It can cause an extreme inflammatory reaction in the tissues surrounding the abdomen, kidney, or brain. This disease is generally fatal. Although FIP is not believed to be contagious, it is a very serious disease.
Feline coronavirus is very common, especially in places where large groups of cats are kept together. Cats become infected by inhaling or ingesting the virus. The most common source is faeces, although contaminated surfaces such as food dishes and clothing can also transmit the virus.
Most infected cats, however, do not develop FIP. A cat without clinical signs may still be a carrier and may pass the virus to another cat. The virus in an infected cat might mutate into the FIP causing kind. This chance is increased for cats that are immune compromised, including very young and very old cats. There is also thought to be a genetic component to susceptibility to viral mutation.
There are two main forms of FIP: wet and dry. While both types are fatal, the wet form is more common and progresses more rapidly than the dry form.
The most common sign in the wet form is the accumulation of fluid within the abdomen or chest, which can cause breathing difficulties. Other symptoms include lack of appetite, fever, weight loss, icterus (yellow colouring), and diarrhoea.
Dry FIP will also present with lack of appetite, fever, icterus, diarrhoea, and weight loss, but there will not be an accumulation of fluid. Typically, a cat with dry FIP will show ocular (eye) or neurological signs. Cats may develop difficulty in standing up or walking, becoming paralysed over time or experience loss of vision.
The signs associated with FIP are often non-specific, which can cause diagnosis to be very difficult. There is as yet no definitive diagnostic test for FIP. Diagnosis may include a combination strong clinical suspicion, physical examination findings, presence of abdominal fluid with characteristic and blood tests to screen for coronavirus and levels of certain proteins.
Quarantine is not necessary as FIP is not truly an infectious disease. Coronavirus is shed in the faeces and can be passed on to other cats; however, it is the mutated form of the virus that leads to FIP. This form is only found in certain white blood cells called macrophages and is therefore not shed and not contagious. The feline coronavirus is specific for cats and cannot infect other species, including humans.
There is no cure for FIP, although research is continuous. Treatment is symptomatic in order to make the cat as comfortable as possible. Newer approaches using immune modulators are being developed by several companies overseas. Wet FIP usually progresses too rapidly for any meaningful therapy to be attempted. With FIP prognosis remains extremely poor.