What is feline infectious peritonitis?
Feline infectious peritonitis (FIP) is a complex disease caused by a virus that triggers an abnormal immune system response. It occurs worldwide in cats of all ages, but is most common in young cats, less than two years of age.
What causes FIP?
FIP is associated with a viral infection called feline enteric coronavirus. There are many strains of feline coronavirus, and they differ in their ability to cause disease. Once a cat ingests feline enteric coronavirus, it infects the intestine and may cause vomiting and diarrhea.
In addition, feline enteric coronavirus can mutate (change) into a more harmful type of virus called feline infectious peritonitis virus (FIPV), which can infect white blood cells and cause FIP. Even when cats are infected with FIP-causing strains, many do not develop FIP disease. It is unclear why one infected cat becomes diseased while others are unaffected.
How common is feline coronavirus compared to FIP disease?
Many cats are infected with one or more strains of feline coronavirus at some time (up to 50% of cats in single-cat households and as many as 80% to 90% in multi-cat environments). Most cats with feline enteric coronavirus remain healthy (about 90% or more). It is believed that less than 10% of cats with feline enteric coronavirus infections will develop FIP.
Are certain cats more susceptible to FIP?
FIP appears to be more common in certain cats.
- Cats that live in multi-cat households, shelters, or catteries are more likely to develop FIP.
- Cats that are stressed due to re-homing or have recently had surgery may be more susceptible to FIP.
- Cats that have concurrent infections (more than one infection at a time) may be more susceptible to FIP.
- Genetic factors are thought to contribute to the development of FIP.
- Male cats may be affected more often than female cats.
- Purebred cats may be predisposed to developing FIP, including Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and Devon Rex breeds.
How does a cat become infected with feline coronavirus?
Most cats become infected with feline coronavirus through 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.
The virus is quite fragile and does not survive for more than 24 to 36 hours in the environment, but it is believed that cold temperatures may preserve the virus for months. It can likely be transmitted on clothing or other objects, but only within a few hours of contact.
While most cats are infected with relatively harmless strains of feline coronavirus, 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. In some cats, feline coronavirus can remain dormant or inactive in the body for months to years before mutating and causing disease. Many cats that develop FIP have no history of contact with other cats that show clinical signs of FIP.
When are cats 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 cats can develop the disease at any age.
What are the clinical signs of FIP?
In cats that develop FIP, the first signs of illness may be vague. Common clinical signs include:
- Listlessness
- Lethargy
- Decreased or absent appetite
- Weight loss
- Fluctuating fever
After a period of several days to a few weeks, other symptoms typically begin to occur. At this stage, most cats 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, which involves little to no fluid accumulation. The dry form of FIP often involves severe inflammation in one or more organs, including the eyes, brain, liver, intestine, or other organs, leading to a variety of clinical signs. Many cats with non-effusive FIP have ocular (eye) symptoms as their only clinical sign.
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, once disease develops, most cats deteriorate rapidly, although some cats remain normal for several weeks. Until recently, cats that develop the disease almost always die without prompt treatment.
How is FIP diagnosed?
Many of FIP’s clinical signs are vague and occur with other diseases found in cats, making FIP particularly difficult to diagnose. Certain abnormalities in routine labwork may support a diagnosis of FIP (e.g., elevated protein levels), but there are no specific blood tests that can diagnose FIP.
- Blood tests: Certain abnormalities in routine labwork may support a diagnosis of FIP (e.g., elevated protein levels), but there are no specific blood tests that can diagnose FIP.
- Imaging: X-rays may help 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.
- Fluid analysis: Laboratory analysis of fluid removed from the chest or abdomen can be valuable, because few other diseases produce the same type of fluid as FIP. Fluid analysis does not always provide a definitive diagnosis.
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 a cat can be infected with FIP while also having other conditions, such as disease induced by feline leukemia virus.
Currently, the only way to positively diagnose FIP is by microscopic examination of a piece of affected tissue by a pathologist, or by post-mortem examination. 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.
How reliable are FIP blood tests?
Some tests can detect antibodies to feline coronavirus in the blood, but they cannot distinguish typical feline enteric coronavirus from a mutated form that causes FIP. Some laboratories provide tests such as polymerase-chain reaction (PCR) tests, which can detect tiny amounts of viral DNA, but there is no unique genetic sequence associated with FIP.
Some tests claim to be able to distinguish between the FIP strains, and to detect strains more likely to be associated with FIP, but many independent experts disagree with these claims. For that reason, a positive FIP test in a healthy cat is not a strong predictor of subsequent FIP disease. If a cat has clinical signs consistent with 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 FIP.
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. Now that there are some effective treatments for FIP, your veterinarian may use a positive response to treatment to support a diagnosis.
Is there any treatment for FIP?
Historically, FIP has been fatal in almost all cases. However, newer anti-viral medications have changed the prognosis for many cats. Antiviral medications have been used alone and in combination to neutralize the mutated coronavirus, including GS 441524, remdesivir, molnupiravir, EIDD-1931, nirmatrelvir/ritonavir (Paxlovid®), and recombinant feline interferon-omega.
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 your cat is too sick to take oral medication, your veterinarian may recommend an injectable product until your cat can take an oral formulation. Other antiviral treatments are under investigation.
"FIP treatment is rapidly evolving, so talk to your veterinarian about the latest options."
If the cat responds to treatment, improvement is often seen within a few days. Legally-approved treatments are important, as they ensure that the product has the correct amount of medication and doesn’t include any potentially dangerous compounds. FIP treatment is rapidly evolving, so talk to your veterinarian about the latest options.
Additional supportive treatments include glucocorticoids (e.g., prednisolone), which may be recommended in certain situations to reduce inflammation and negative effects of the immune response, and polyprenyl immunostimulant, to bolster the immune system. Supportive care to address clinical signs includes:
- Fluid support for dehydration
- Anti-nausea medications for vomiting or reduced appetite
- Pain medication
- Therapeutic drainage if fluid in the chest or abdomen is causing trouble breathing
- Eye drops for ocular inflammation
- Anti-seizure medication
Is there a vaccine for FIP?
Vaccines have been developed to help prevent FIP, but they have not been shown to be effective. Vaccine development is believed to be difficult because FIP is caused by a mutation of a common viral infection, the cause of the mutation is unknown, and the infection is not generally spread cat to cat. Also, in many cases, cats can be infected with feline enteric coronavirus before they can be vaccinated. Currently, FIP vaccines 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 of being infected with feline coronavirus. Fortunately, infection with feline coronavirus will lead to FIP in only a minority of cats. Ensure your cats’ environment is as low stress as possible to reduce the risk of FIP developing in other cats.
Infection risk can be reduced further by separating cats into groups of three or less and maintaining strict hygiene. Clean litter boxes frequently, with dilute bleach to kill the virus (1 part bleach to 32 parts water). Keep adequate numbers of litter boxes to help minimize exposure to other cats’ feces.
As a precaution, if an affected cat passes away, many veterinarians recommend that you wait about a month 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. Even if they do not develop disease, these previously exposed cats could be carriers and could potentially infect any new cats.
