What Is Feline Infectious Peritonitis (FIP)?

Feline infectious peritonitis (FIP) is a reaction to an infection with the feline enteric coronavirusa positive-stranded RNA virus that is infectious to cats worldwide. While most cats that become infected with this virus spontaneously clear it and never show signs of any serious disease, some cats have a more serious reaction that develops into the syndrome known as FIP. FIP causes a widespread infiltration of the body’s organs.

How Is it Transmitted?

Feline enteric coronavirus is common in groups of cats and is readily transmitted among cats. Transmission typically occurs through fecal-oral contact with infected feces; the litter box is the most common place that transmission occurs. The virus is less likely to be found in cats who live outdoors with no litter box or in one-cat households. The enteric virus attacks the lining of the intestines and causes gastrointestinal (GI) upset, resulting in vomiting and diarrhea. An active infection lasts for several weeks to months, during which time the cat may seem ill and can actively shed the virus in his or her stool. Inflammatory cells called macrophagesa clear the virus from the body.

As long as the infection remains confined to the GI tract and the coronavirus does not mutate, the cat will not develop FIP. Sometimes, however, the virus mutates. Mutation is most likely to occur in young cats or cats with compromised immune systems. Factors that cause immune-compromise in cats include stress, overcrowding, youth/immaturity and concurrent disease. Purebred cats, especially Bengals, are more likely to develop FIP. Age is a huge risk factor; the majority of cats that develop FIP are less than one year of age, but FIP also has been reported in geriatric cats, so it can develop at any age. It is newly understood that genetics are a risk factor for development of FIP. Kittens usually develop FIP at five to six weeks of age when the maternal antibodies have worn off. Stress or co-infection of a seropositive cat can make them develop FIP. More than 10 percent of coronavirus-infected cats develop FIP at some point in their life.

Instead of dying from the macrophage, the mutated virus uses the cell to “hitch a ride” to other parts of the body. The body senses the presence of the mutated virus and mounts a strong immune response to fight off the infection. The resulting large numbers of macrophages and immunologic proteins make up the inflammatory tissue called a pyogranuloma, which infiltrates the body’s organs and causes clinical signs of FIP. The mutated virus is not shed into the feces of an infected cat, though he or she will still shed the non-mutated virus.

What Does it Look Like?

FIP is classified as either effusive (wet) or non-effusive (dry). Because there are two forms of the disease, clinical signs can be widely variable and non-specific. The wet form is characterized by an accumulation of fluid in the abdomen and chest. Cats with effusive FIP often have trouble breathing. Their abdomens appear bloated. The dry form is characterized by pyogranulomatousan inflammatory process in which there is infiltration of polymorphonuclear cells into a more chronic area of inflammation growths that develop in the kidneys, lymph nodes, lungs, liver, spleen, abdomen and brain. This results in weight loss, a poor appetite and fever that is not responsive to antibiotics. Cats with the dry form can also show signs of neurologic or ocular (eye) disease.

How Does a Veterinarian Diagnose and Treat FIP?

Diagnosis of FIP can be challenging because these is not one single test. Bloodwork and fluid analysis are the two most common tests used, but neither is very telling. Most of the time, veterinarians come to a diagnosis by excluding other diseases.

How is FIP Treated?

There is no effective treatment for FIP. Treatment usually involves supportive care such as abdominal/chest fluid removal, appetite stimulants and fluid therapy. Because FIP is a disease created by the cat’s immune system, immunosuppressive drugs such as steroids have been used to slow the progression of FIP.

Can it Be Prevented? If So, How?

Most cats have already been exposed to the coronavirus in their lifetime. Decreasing the number of cats in your house will help decrease the probability of your cat developing FIP. There is an available vaccine, but its efficacy has been questionable and its use remains controversial.