Feline infectious peritonitis, commonly known as FIP, is a severe viral disease affecting cats. It arises from specific strains of the feline coronavirus. Most feline coronavirus strains reside in the gastrointestinal tract and are benign, known as feline enteric coronavirus (FeCV). Typically, cats infected with FeCV show no initial symptoms, though transient diarrhea or mild upper respiratory issues might occur, from which they recover without intervention. Within 7 to 10 days of FeCV infection, cats usually develop an immune response, producing antibodies against the virus. However, in about 10% of FeCV-infected cats, viral mutations can lead to a more aggressive form. This mutated virus infects white blood cells and spreads throughout the cat’s body. At this stage, the virus is termed FIPV. The body’s immune response to FIPV causes intense inflammation around blood vessels in affected tissues, often in the abdomen, kidneys, or brain. The development of FIP is a result of this complex interaction between the cat’s immune system and the mutated virus.
FIP manifests primarily in two forms: the “wet form,” characterized by fluid accumulation in body cavities like the abdomen and chest, and the “dry form,” which predominantly affects the central nervous system, leading to neurological symptoms. Clinical FIP is typically progressive and, without treatment, almost invariably fatal. However, effective therapies have emerged. Initially available in countries like Australia and the UK, an oral compounded formulation has recently become accessible in the US, requiring a veterinary prescription. The advent of safe and effective FIP treatments marks a significant advancement in veterinary medicine. If you suspect your cat has FIP, consulting with your veterinarian about these new therapies is crucial. Importantly, feline coronaviruses are not known to be transmissible from cats to humans.
Is Your Cat at Risk of Developing FIP?
Any cat carrying FeCV, the common and usually harmless intestinal feline coronavirus that can mutate into FIP, is potentially at risk. Younger cats are significantly more susceptible, with approximately 70% of FIP cases diagnosed in cats under 18 months old, and half of these cases occurring in cats younger than seven months. FeCV transmission commonly occurs from infected queens to their kittens, usually between five and eight weeks of age. It can also spread between adult cats through feces. Cats in high-density environments such as shelters and catteries, as well as purebred, male, and geriatric cats, appear to have a higher risk of developing FIP, although the reasons for this remain unclear.
Recognizing the Symptoms of FIP
Initial FeCV exposure often presents no obvious symptoms in cats. Some may exhibit mild upper respiratory signs like sneezing, watery eyes, and nasal discharge, or mild gastrointestinal signs such as diarrhea. These symptoms are usually short-lived and resolve on their own. However, 5 to 10% of FeCV-exposed cats develop FIP, which can occur weeks, months, or even years after the initial FeCV infection.
FIP presents in two main forms: effusive (wet) and non-effusive (dry). Regardless of the form, cats with FIPV typically initially show non-specific signs of illness, including loss of appetite, weight loss, lethargy, and fever. It’s important to note that the effusive form can evolve into the non-effusive form and vice versa.
Generally, the non-effusive form progresses more slowly. Besides the non-specific signs, it may include neurological symptoms such as seizures and ataxia (uncoordinated movements).
The effusive form of FIP tends to develop and progress more rapidly. Cats exhibit the non-specific signs along with fluid accumulation in body cavities, such as the abdomen and chest. Abdominal fluid buildup can cause a pot-bellied appearance, and excessive fluid can lead to breathing difficulties.
Diagnosing FIP in Cats
Unfortunately, there is no single definitive test for diagnosing FIP. While coronavirus antibody levels (titers) can be measured, they cannot distinguish between FeCV and FIPV exposure. A positive result only indicates prior coronavirus exposure, not necessarily FIPV. Despite this limitation, young cats with fever unresponsive to antibiotics and high coronavirus titers are often presumptively diagnosed with FIP, which is frequently accurate. This is especially true if characteristic fluid (yellowish, high in protein and white blood cells) accumulates in body cavities. However, a healthy cat with a high coronavirus titer is not inherently more likely to develop FIP or carry FIPV than a cat with a low titer. In cats with weakened immune systems, FIPV infections may not even cause elevated coronavirus titers due to impaired antibody production.
Other tests aim to detect the virus directly. The immunoperoxidase test can identify viral proteins in infected white blood cells in tissue samples obtained via biopsy. The immunofluorescence test can detect viral proteins in infected cells in tissue or body fluids. Polymerase chain reaction (PCR) technology can detect viral genetic material in tissue or body fluid. While these tests can be helpful, none are completely accurate and may yield false negative or false positive results due to various limitations.
In most cases, a presumptive FIP diagnosis is reached by considering a combination of the cat’s history, physical examination findings, and laboratory tests.
Treatment Options for FIP
Until recently, FIP was considered untreatable and almost always fatal. While some uncertainties remain regarding new antiviral drugs for FIP, including long-term effectiveness, optimal dosage, potential for drug resistance, and ideal mechanism of action, research in laboratories and in cats with naturally occurring FIP indicates that GS-441524, a well-studied drug, is a safe and effective treatment option. This medication has been available in Australia and the UK for several years and has recently become available in the US as an orally compounded formulation. Before this recent US availability of reliably produced GS-441524, “black market” sources offered the drug, but reports suggest variable drug concentration and purity in these sources. Therefore, the availability of a reliably produced oral formulation in the US is significant. If considering GS-441524 therapy for a cat with FIP, discussing the risks and benefits with your veterinarian is crucial, as a veterinary prescription is required to obtain it. Supportive care, such as fluid therapy, drainage of accumulated fluids, and blood transfusions, may also be necessary in certain cases.
Protecting Your Cat from FIP
The only definitive way to prevent FIP is to prevent FeCV infection, which is challenging due to its widespread nature. This is particularly relevant for cats in high-density environments like shelters and catteries. Housing cats at densities of three or fewer per room is recommended to reduce stress associated with overcrowding. While FeCV is highly contagious, spreading through feces and saliva and primarily infecting cats orally, FIPV is not believed to be contagious, pending further research. Instead, FIP is thought to develop in individual cats after FeCV infection and subsequent viral mutation into FIPV. (It is currently believed that FIPV itself is not transmitted through feces or other secretions, although ongoing studies may change this understanding.) Maintaining optimal cat health, including preventing infections from other viruses like feline leukemia virus and calicivirus through appropriate vaccination, where advised, may reduce FIP risk. Litter boxes should be kept clean and away from food and water. Some advise separating new cats or cats suspected of FeCV infection, but the effectiveness of this is debatable.
Only one FIP vaccine is licensed, but its effectiveness is questionable, and the American Association of Feline Practitioners Feline Vaccine Advisory Panel does not routinely recommend it. The vaccine appears safe, but the risks and benefits should be carefully considered in consultation with a veterinarian to decide if vaccination is appropriate for your cat.
Updated 2024
