Feline infectious peritonitis (FIP) is a devastating viral disease in cats, triggered by specific strains of the feline coronavirus. Most feline coronaviruses, known as feline enteric coronavirus (FeCV), reside in the gastrointestinal tract and typically cause no significant harm. Cats infected with FeCV might not show any initial symptoms, or they may experience short periods of diarrhea or mild upper respiratory issues, from which they usually recover without intervention. Within 7 to 10 days of FeCV infection, cats generally develop an immune response, producing antibodies against the virus. However, in about 10% of FeCV-infected cats, the virus undergoes mutations, altering its behavior and enabling it to infect and spread through white blood cells throughout the body. At this stage, the virus is termed feline infectious peritonitis virus (FIPV). The body’s immune response to FIPV causes intense inflammation around blood vessels in affected tissues, commonly in the abdomen, kidneys, or brain. It’s this interplay between the cat’s immune system and the virus that leads to the development of FIP.
FIP manifests in two primary 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 generally progressive and almost always fatal if left untreated. Fortunately, effective therapies, previously available in countries like Australia and the UK, have recently become accessible in the US as an oral compounded formulation, requiring a veterinary prescription. The advent of these safe and effective FIP treatments marks a significant leap forward in veterinary medicine. If your cat is suffering from this serious disease, consulting with your veterinarian about FIP therapy options is highly recommended. It’s important to note that, to the best of our current understanding, feline coronaviruses cannot be transmitted from cats to humans.
Is Your Cat at Risk for FIP?
Any cat that carries FeCV, the common and usually harmless intestinal form of feline coronavirus that can mutate into FIPV, is potentially susceptible to developing FIP. However, younger cats are significantly more vulnerable, with approximately 70% of FIP cases diagnosed in cats younger than 18 months, and half of these cases occurring in cats under 7 months old. The primary mode of FeCV transmission is from infected queens to their kittens, typically when kittens are between 5 and 8 weeks old. FeCV can also spread through feces among more mature cats. Cats in high-density environments such as shelters and catteries, as well as purebred cats, male cats, and older cats, appear to have an increased risk of developing FIP, although the reasons behind these predispositions are not fully understood. Stressful conditions, such as those found in overcrowded environments, can weaken a cat’s immune system, potentially increasing the likelihood of the FeCV mutation that leads to FIP.
Recognizing FIP Symptoms
Initial FeCV exposure in cats usually presents with no noticeable symptoms. Some cats might exhibit mild upper respiratory signs like sneezing, watery eyes, and nasal discharge, while others may have mild gastrointestinal issues such as diarrhea. These symptoms are generally mild and resolve on their own. However, in 5% to 10% of FeCV-exposed cats, FIP develops, potentially weeks, months, or even years after the initial FeCV infection.
FIP has two main forms: effusive (wet) and non-effusive (dry). Regardless of the form, cats with FIPV typically initially show nonspecific signs of illness, including appetite loss, weight loss, lethargy, depression, and fever. It’s also important to note that a cat can transition between the wet and dry forms of FIP during the course of the disease.
The non-effusive form of FIP tends to progress more slowly. Symptoms may include the general signs mentioned above, as well as neurological signs such as seizures and ataxia (uncoordinated movements). Ocular signs, like inflammation in the eyes or changes in eye color, can also occur with dry FIP.
The effusive form of FIP is characterized by a more rapid onset and progression of symptoms. In addition to the nonspecific signs, cats with wet FIP develop fluid accumulation in body cavities, most commonly in the abdomen and chest. Abdominal fluid buildup can cause a distended abdomen or a “pot-bellied” appearance. If fluid accumulates in the chest cavity, it can cause difficulty breathing. The fluid associated with wet FIP is typically yellowish and protein-rich.
FIP Diagnosis: Navigating the Challenges
Unfortunately, diagnosing FIP definitively remains a challenge. While coronavirus antibody levels (titers) can be measured, they cannot differentiate between FeCV and FIPV exposure. A positive antibody test only indicates past exposure to some form of feline coronavirus, not necessarily the FIPV strain that causes FIP. Despite this limitation, young cats with persistent fever unresponsive to antibiotics and high coronavirus titers are often presumptively diagnosed with FIP, and this diagnosis is frequently accurate. This is especially true if characteristic fluid accumulation (yellowish, protein-rich fluid with high white blood cell count) is present in body cavities. However, a healthy cat with high coronavirus titers is not inherently more likely to develop FIP or carry FIPV than a cat with low titers. In cats with weakened immune systems, FIPV infection might not even result in elevated coronavirus titers because their immune system may be unable to produce sufficient antibodies.
Other tests aim to detect the virus directly. The immunoperoxidase test can identify viral proteins in infected white blood cells within tissue samples obtained through biopsy. The immunofluorescence test can detect viral proteins in infected cells from tissue or body fluids. Polymerase chain reaction (PCR) technology can detect viral genetic material in tissue or fluid samples. While these tests can be helpful, none are completely accurate and each has limitations that can lead to false negative or false positive results. PCR tests on fluid are becoming more refined and can be supportive, but a negative PCR does not rule out FIP.
In most cases, a presumptive FIP diagnosis is reached by considering a combination of factors: the cat’s history, physical examination findings, and laboratory test results. Veterinarians assess the cat’s age, clinical signs, and conduct bloodwork and fluid analysis (if fluid is present) to strengthen or weaken the suspicion of FIP. Ultimately, diagnosing FIP often requires a degree of clinical judgment based on the overall picture.
FIP Treatment: A New Era of Hope
Until recently, FIP was considered untreatable and almost invariably fatal. However, the landscape of FIP treatment has been revolutionized by the development of antiviral drugs. While ongoing research continues to refine our understanding of these medications – including long-term effectiveness, optimal dosage, potential for drug resistance, and precise mechanisms of action – studies in both laboratory settings and in cats with naturally occurring FIP have demonstrated the efficacy and safety of GS-441524, a leading antiviral treatment. GS-441524 has been available in Australia and the UK for several years and has recently become available in the United States as an orally compounded medication, requiring a veterinary prescription.
Before this recent regulated availability in the US, and still to some extent today, numerous unregulated or “black market” sources offered GS-441524 for sale. Importantly, reports indicate significant variability in the quality and concentration accuracy of products from these sources. This makes the availability of a reliably produced oral formulation in the US a crucial advancement. If you are considering GS-441524 therapy for a cat with FIP, a thorough discussion of the risks and benefits with your veterinarian is essential, as a veterinary prescription is necessary to obtain this potentially life-saving medication. Other antiviral drugs, such as molnupiravir, are also being explored for FIP treatment.
Supportive care remains an important aspect of managing cats with FIP. This may include fluid therapy to combat dehydration, drainage of accumulated fluids to improve comfort and breathing, and blood transfusions in some cases to address anemia. While supportive care alone cannot cure FIP, it can improve a cat’s quality of life and may be used in conjunction with antiviral therapy.
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 in high-density cat populations like shelters and catteries. Reducing housing density to three or fewer cats per room is recommended to minimize stress associated with overcrowding. While FeCV is highly contagious, spreading through feces and saliva (primarily infecting cats orally), FIPV itself is not believed to be directly contagious, pending further research. Instead, FIP is thought to develop in individual cats after FeCV infection and subsequent viral mutation into FIPV. FIPV is not believed to be shed in feces or secretions, although ongoing research may refine this understanding.
Maintaining optimal cat health, including preventing infections from other viruses like feline leukemia virus (FeLV) and feline calicivirus through appropriate vaccination, can help reduce the likelihood of FIP development. Stress management is also key, as stress can weaken the immune system. Cleanliness is crucial; litter boxes should be kept clean and located away from food and water bowls to minimize fecal-oral transmission of FeCV. Some sources suggest isolating newly acquired cats or cats suspected of FeCV infection from other cats, but the effectiveness of this strategy is debatable given the ubiquity of FeCV.
An FIP vaccine is available, but its effectiveness in preventing FIP is questionable, and the American Association of Feline Practitioners Feline Vaccine Advisory Panel does not routinely recommend it. While the vaccine appears safe, the risks and benefits should be carefully considered in consultation with your veterinarian to determine if vaccination is appropriate for your cat.
Updated 2024
