Feline Infectious Peritonitis

By Krista Williams, BSc, DVM, CCRP; Tammy Hunter, DVM; Ernest Ward, DVM

What is feline infectious peritonitis? 

Feline infectious peritonitis (FIP) is an important disease of domestic cats. It occurs worldwide in cats of all ages, but the disease is most common in young cats less than two years of age.

What is the cause of FIP?

FIP is associated with a viral infection called feline coronavirus. There are many different strains of feline coronavirus, which differ in their ability to cause disease.

Previously, there had been an attempt to classify these strains as either feline infectious peritonitis virus strains (capable of causing the FIP disease) or feline enteric coronavirus strains (essentially harmless strains mainly found in the intestinal tract). It is now recognized that feline enteric coronavirus strains can mutate (change) to the more harmful type of virus and cause FIP.

"The factors determining why one cat becomes diseased while others remain unaffected are unclear."

Diagnosing FIP is very challenging for many reasons. Unfortunately, there are no laboratory tests available that can distinguish between the enteric coronavirus and the FIP-causing strains. Even when infected with known FIP-causing strains, many cats do not develop FIP disease. The factors determining why one cat becomes diseased while others remain unaffected are unclear.

How common is infection with feline coronavirus in comparison with FIP disease?

Many cats (up to 50% in single cat households and as high as 80%-90% in multi-cat environments) become infected with one or more strains of feline coronavirus at some time in their lives. Most cats with feline enteric coronavirus (about 90% or more) remain healthy. The incidence of feline infectious peritonitis disease is low (only 5% to 10% of infected cats and less than 1% of cats admitted to veterinary hospitals).

Are certain cat breeds more susceptible to FIP?

FIP appears to be more common in cats that live in multi-cat households, shelters, or catteries. Cats that are stressed due to re-homing, have recently had surgery, or have concurrent infections (more than one infection at a time) may also be more susceptible to developing FIP. Genetic factors are also thought to contribute to the development of FIP. Male cats are affected more often than females. Purebred cats, including Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and Devon Rex may be predisposed to developing FIP.

How does a cat become infected with feline coronavirus?

Most cats become infected with feline coronavirus through the fecal-oral route (oral contact with infected feces). It is estimated that about one-third of these cats shed the virus in their feces. Most cats only shed the virus for a few months, but a small percentage will shed the virus continuously for life. Although the virus is quite fragile and does not survive for more than 24–36 hours in the normal environment, it is believed that cold temperatures may preserve the virus for months. It likely can be transmitted on clothing or other objects only within a few hours of contact.

"The virus can remain dormant or inactive in the body for months to years"

As explained above, most cats are infected with relatively harmless strains of feline coronavirus. Unfortunately, this initial benign infection may later mutate to cause FIP in some cats. Even with the more harmful strains, apparently healthy cats may be carriers of the virus, and may shed the virus without ever showing signs of disease. Many cats that develop FIP have no history of contact with other cats showing clinical signs of FIP. The virus can remain dormant or inactive in the body for months to years before the cat eventually develops disease.

What age is a cat most at risk of developing FIP?

It is believed that most cats are exposed to feline coronavirus at a very young age, perhaps during the first few weeks of life. Most cats that develop FIP are between three months and two years of age, although a cat of any age can develop the disease.

What clinical signs does a cat infected with FIP develop?

In cats that develop FIP, the first signs of illness may be very vague. Listlessness, lethargy, decreased or absent appetite, weight loss, and a fluctuating fever are commonly reported clinical signs. After a period of several days to a few weeks, other symptoms typically begin to occur.

At this stage, most cats will develop the "wet" or effusive form of FIP, which refers to the accumulation of fluid in body cavities; fluid may accumulate in the abdomen, leading to a swollen abdomen, or in the chest cavity, resulting in difficulty breathing.

Some cats develop "dry" or non-effusive FIP where little to no fluid accumulates. The dry form often involves severe inflammation in one or more organs including the eyes, brain, liver, intestine, or other organs of the body, leading to a variety of clinical signs. Many cats with non-effusive FIP will have ocular (eye) symptoms as their only clinical sign.

Once disease develops, without treatment, most individuals deteriorate rapidly, although some cats remain normal for several weeks. Until recently, the disease was fatal in almost every case.

Most cats exposed to feline coronavirus, even to the potentially FIP-inducing strains, can develop an immune response that protects them, so only a small proportion of infected cats develop clinical disease. However, as stated above, those that do develop the disease almost invariably die without treatment.

How can FIP be diagnosed?

Many of the clinical signs of FIP are vague and occur with other diseases found in cats, making FIP particularly difficult to diagnose. There may be abnormalities in a routine blood analysis, but none are specific for FIP. X-rays may be helpful to determine the presence of fluid in the abdomen or chest. If fluid is present, some of it can be removed by tapping the chest or the abdomen. Analysis of this fluid at a veterinary laboratory can be particularly valuable, as few other diseases produce the same type of fluid that FIP creates.

"Sometimes FIP is a diagnosis of exclusion, meaning that a variety of similar conditions have been ruled out."

Nevertheless, fluid analysis does not always provide a definitive diagnosis of the disease. Sometimes FIP is a diagnosis of exclusion, meaning that a variety of similar conditions have been ruled out. The diagnosis may be further complicated because FIP may exist at the same time as some other conditions such as feline leukemia virus diseases. See the handout “Feline Infectious Peritonitis Testing” for further information on diagnosing FIP.

Currently, the only way to make a positive diagnosis of FIP is by histological examination of affected tissue (or by post-mortem examination) by a pathologist at a laboratory. If there is any doubt about the diagnosis, your veterinarian may advise that a biopsy be taken from your cat, so that FIP can be distinguished from another disease.

I understand there are specific blood tests. How reliable are these?

Veterinary laboratories provide tests that detect antibodies to feline coronavirus in the blood, but these tests are non-specific and cannot be used alone to diagnose FIP. Some laboratories provide tests such as polymerase-chain reaction (PCR) tests, which can detect very small amounts of the virus but no unique genetic sequence associated with FIP has been identified. Although some of these tests claim to be able to distinguish between the strains, and to detect strains more likely to be associated with FIP, many independent experts disagree with these claims. Therefore, a positive test in a healthy cat is not a strong predictor of subsequent FIP disease.

"FIP is one of the most challenging diagnoses for your veterinarian to make"

If a cat has clinical signs consistent with a diagnosis of FIP, then a positive test is supportive of the diagnosis, but not conclusive. Likewise, a negative test in the presence of advanced signs does not rule out the diagnosis of FIP.

As you can see, FIP is one of the most challenging diagnoses for your veterinarian to make because of the complexity of the disease and the limitations of current tests.

Is there any treatment for FIP?

Historically, FIP has been fatal in almost all cases. Supportive treatments may extend longevity and improve quality of life, but there is no proven cure. Anti-inflammatory drugs such as corticosteroids (e.g., prednisolone), in combination with certain drugs that suppress the immune system (e.g., cyclophosphamide) may temporarily reduce inflammation and improve the cat’s quality of life. While experimental treatments are being investigated, access to these treatments has been limited and has prevented a specific treatment protocol from being proven effective; however, they may offer a possible cure for a disease that was considered inevitably fatal.

"Supportive treatments may extend longevity and improve quality of life, but there is no proven cure."

Newer anti-viral medications, including Remdesivir and GS 441524, were legalized in 2021 for treatment of FIP in the United Kingdom and Australia and are available now for legal import to certain countries including Canada (by Emergency Drug Release). These medications are given as a series of injections or oral treatments over several months, and have shown more success than traditional treatment, so far.

If the cat responds to treatment, improvement is often seen within a few days. Remdesivir is legally available in the US, although difficult to obtain currently; GS 441524 is not currently legally available. Legally-approved treatments are important, as they ensure that the product has the correct amount of medication and there aren’t any other potentially dangerous compounds. Other medications are in development. Talk to your veterinarian about the latest options for FIP treatment.

Is there a vaccine for FIP?

In recent years, some manufacturers have developed vaccines to help in the prevention of FIP. The success of vaccination is not certain because of the method of transmission, because the sequence of events leading to clinical FIP disease are poorly understood, and because infection may have occurred before vaccination. Currently, the vaccines for FIP are not recommended for general use. You and your veterinarian can discuss whether vaccination is appropriate for your cat.

Are other cats in the household at risk?

If your cat has FIP, other cats in your household may be at a greater risk for becoming infected with feline coronavirus. Fortunately, infection will lead to this fatal disease in only a minority of cats. As a precaution, many veterinarians recommend that you wait about a month after an infected cat dies before introducing a new cat into the house, to minimize the chance of exposure to the virus. In a multi-cat household in which an infected cat has died, it is recommended to wait at least three months to see if any other cats develop clinical disease. However, these previously exposed cats could be carriers of the disease and could potentially infect any new cats.

Cleaning with dilute bleach (1:32) is adequate to kill the virus. Keeping adequate numbers of litter boxes can also help minimize exposure to other cats’ feces.

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