Feline Infectious Peritonitis (FIP) is a potentially fatal viral disease of cats. This article explains what FIP is, which cats are most at risk, common clinical signs, how veterinarians approach diagnosis, and the recent advances in treatment. Learn how a typically mild intestinal virus can, in some cats, change and spread through the body, and how modern antiviral therapies are offering new hope for affected cats and their families.

WHAT IS FELINE INFECTIOUS PERITONITIS?

Feline infectious peritonitis (FIP) is a viral disease of cats. FIP most commonly affects young cats (less than 2 years of age), particularly purebreds, but cats of any age or breed can develop this disease.

FIP is caused by a mutated form of feline coronavirus. Many cats in the general population are exposed to feline coronavirus, especially in multi-cat environments such as breeding colonies, rescue facilities or multi-cat households. The non-mutated virus usually infects cells of the gut wall and causes mild digestive upset or no illness at all.

Occasionally the virus changes and is able to enter immune cells. This allows the mutated virus to travel in the blood and trigger immune-mediated inflammation that can affect other organs such as the kidneys, liver, lungs, brain, eyes or blood vessels. The exact reason this change happens is not fully understood. While feline coronavirus can spread between cats, the form that causes FIP typically arises by internal mutation and is only rarely transmitted between cats.

WHO IS MOST AT RISK?

• Kittens and young cats (less than 2 years of age)
• Purebred cats
• Cats living in multi-cat environments such as breeding colonies, rescue facilities, or multi-cat households
• Any cat can be affected

WHAT ARE THE CLINICAL SIGNS OF FIP?

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 behavior, tremors); kidney disease or liver disease.

WHEN SHOULD I SEE MY VET?

• Persistent lethargy or reduced appetite
• Failure to gain weight or weight loss in kittens
• Distended abdomen or breathing difficulties
• Vision changes or neurological signs such as wobbling, tremors, or behavior changes
• Concerns about kidney or liver disease

HOW IS FIP DIAGNOSED?

Because signs can mimic other conditions, veterinarians usually start with blood and urine tests to narrow the possibilities. Imaging such as X‑rays or ultrasound may reveal fluid in the abdomen or chest and help guide sampling. In some cases, specialized laboratory tests can examine fluid or tissue for evidence of viral proteins inside immune cells. A positive result can confirm the diagnosis, while a negative result does not always rule it out.

CAN FIP BE TREATED?

Historically, FIP was considered untreatable. Supportive care could help, but most cats did not survive. In recent years, modern antiviral medications have changed this outlook. These medicines target the virus directly and have shown high rates of success in controlling or even clearing the disease.

Treatment plans are tailored to the individual cat. Some cases begin with a short course of injections, followed by oral medication for several weeks to a few months (often about 6 to 12 weeks), as directed by your veterinarian. Duration and dose depend on the cat’s weight, age, and whether the eyes or nervous system are involved. Many cats treated promptly now go on to live healthy, normal lives.