Feline Infectious Peritonitis

By Ernest Ward, DVM

Infectious Diseases

November 21, 2008

What is feline infectious peritonitis? 

Feline infectious peritonitis (FIP) is an important disease of domestic cats and most members of the cat family (Felidae). It occurs worldwide in cats of all ages, but the disease is most common in young cats less than two years of age. Although FIP is not a particularly common disease, it is important because once a cat develops the disease, the outcome is almost invariably fatal.

What is the cause of FIP?

"Feline Enteric Coronavirus strains can mutate to the more harmful type of virus and cause FIP disease."

FIP is associated with a viral infection called Feline Coronavirus (FCoV). There are many different strains of Feline Coronavirus, which differ in their ability to cause disease. Previously there has 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 confined to the intestinal tract). It is now recognized that Feline Enteric Coronavirus strains can mutate to the more harmful type of virus and cause FIP disease.

"FIP remains one of the least understood of all cat diseases."

Because the Enteric Coronavirus and the FIP-causing strains of Feline Coronavirus are indistinguishable in laboratory tests (see below) and because many cats, even when infected with known FIP-causing strains do not develop FIP disease, the diagnosis of FIP is very challenging. The factors determining why one cat becomes diseased while others remain unaffected are unclear. Repeated re-infection and possibly genetic factors are thought to contribute to the development of FIP. FIP remains one of the least understood of all cat diseases.

How common is infection with Feline Coronavirus (FCoV) in comparison with FIP disease?

Based on blood test surveys, many cats (up to 50% in single cat households and as high as 80 to 90% in multi-cat environments) become infected with one or more strains of Feline Coronavirus at some time in their lives. Nevertheless, 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). As explained above, many infections are with relatively harmless strains, and even when infected with the strains that could cause disease, many cats remain healthy for long periods, even for life.

Are certain breeds more susceptible to FIP?

Certain breeds of cats may be more likely to develop FIP. Those breeds include the Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and Devon Rex. FIP may be more common in cats that live in multi-cat households or catteries. One study concluded that males were more likely to develop FIP than females.

How does a cat become infected with Feline Coronavirus (FCoV)?

"Cold temperatures may preserve the virus for months."

Most cats become infected with FCoV through the fecal-oral route (oral contact with infected feces). It is estimated that about one-third of FCoV –infected cats shed the virus in their feces. Most cats only shed the virus for a few months but approximately 13% will shed the virus continuously for life. Although the virus is quite fragile and does not survive for more than twenty-four to thirty-six hours in the normal environment, it is believed that cold temperatures may preserve the virus for months. Transmission on clothing or other objects is only likely within a few hours of contact.

As explained above, most infections are with relatively harmless strains of FCoV. 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 'latent' in the body for months to years before the cat eventually develops disease.

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

It is believed that most cats are exposed to FCoV at a very young age, perhaps during the first few weeks of life. Approximately half of all cats with FIP are less than two years of age although any age cat can develop the disease.

What clinical signs does a cat infected with FIP develop?

Most cats exposed to FCoV, even to the potentially FIP-inducing strains, are able to develop an immune response that protects them. Thus, only a small proportion of infected cats actually develop clinical disease. However, those that do develop the disease almost invariably die.

In cats that develop FIP disease, the first signs of illness may be very vague. Listlessness, lethargy, decreased or absent appetite, and a variable 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 that occurs in body cavities; fluid may accumulate in the abdomen, leading to a swollen abdomen, or in the chest cavity, resulting in difficulty with breathing. In some cats, little or no fluid accumulates, known as "dry" or non-effusive FIP as opposed to "wet" FIP. 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 noneffusive FIP will have ocular symptoms as their only clinical sign. Once disease develops, most individuals deteriorate rapidly, although some cats remain normal for several weeks. Unfortunately, the disease will eventually result in death in almost every case.

How can FIP be diagnosed?

feline_infectious_peritonitis-2Because FIP presents with a variety of clinical signs, and many of these signs occur with other diseases, FIP is particularly difficult to diagnose. There may be abnormalities in a routine blood analysis, but none is 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 this 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. 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.

"Your veterinarian may advise that a biopsy be taken from your cat, so that FIP can be distinguished from a treatable disease."

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 a treatable disease.

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

Several companies produce kits that detect antibodies to Feline Coronavirus in blood. Some veterinary laboratories provide tests such as polymerase-chain reaction (PCR) tests, which can detect minute amounts of the virus (antigen test). 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.

"If a cat has clinical signs consistent with a diagnosis of FIP then a "positive" test is supportive of the diagnosis, but not conclusive."

If a cat has clinical signs consistent with a diagnosis of FIP then a "positive" test is supportive of the diagnosis, but not conclusive. In advanced clinical cases, the virus antibodies may be bound into complexes that are not detected by the laboratory test, producing a "false negative" result. Therefore, 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?

FIP is fatal in almost all cases. There is no specific treatment. Anti-inflammatory drugs such as corticosteroids (sometimes used in combination with certain types of immunosuppressive drugs used in cancer therapy, such as cyclophosphamide), may temporarily suppress symptoms and improve the cat's quality of life. An experimental drug, polyprenyl immunostimulant (PI), is currently being investigated for use in the “dry” form of FIP. For the clinically ill cat, once FIP has been diagnosed, euthanasia may be the most humane and appropriate course of action.

Is there a vaccine for FIP?

In recent years, some manufacturers have developed vaccines to help in the prevention of FIP. Because the method of transmission and the sequence of events leading to clinical FIP disease are poorly understood, and because infection may have occurred before vaccination, the success of vaccination is not certain. 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?

"Infection will lead to this fatal disease in only a few cats."

If your cat has FIP, other cats in your household may be at a greater risk for becoming infected with the virus that can cause FIP. Fortunately, infection will lead to this fatal disease in 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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