Feline Infectious Peritonitis (FIP): No Longer a Death Sentence

What is FIP?

Feline Infectious Peritonitis (FIP) is a serious viral disease caused by a mutation of a very common feline coronavirus called feline enteric coronavirus (FECV). Most cats are exposed to this coronavirus at some point in their lives, between 75 and 100%, especially in multi-cat households, shelters, or breeding environments. In the vast majority of cats, the virus causes either mild gastrointestinal upset or no symptoms at all.

In a small number of cats, however, the virus mutates into a more aggressive form that can spread throughout the body and trigger widespread inflammation. This mutated form is what causes FIP. 

FIP is most commonly seen in:

• Cats under 2 years of age
• Purebred cats
• Cats experiencing stress (rehoming, surgery, illness, etc.)
• Cats from shelters or multi-cat environments

How Does FIP Affect the Body?

Once the virus mutates, it begins living inside immune cells called macrophages. These infected cells travel throughout the body and cause inflammation of blood vessels and organs. FIP can appear in two general forms or a mixture of both forms:

1. “Wet” (Effusive) FIP: This form causes fluid buildup in the abdomen, chest, or occasionally around the heart. Cats may develop:

• A swollen or distended belly
• Difficulty breathing
• Weight loss and muscle wasting
• Fever
• Lethargy and decreased appetite 

2.  “Dry” (Non-Effusive) FIP: This form causes inflammatory lesions within organs (e.g., brain, eyes, lymph nodes, heart)  rather than fluid accumulation. Signs may include: 

• Weight loss
• Poor appetite
• Persistent fever
• Neurologic changes (wobbliness, seizures, weakness)
• Eye inflammation or vision changes

Diagnosing FIP: A “Brick-by-Brick” Process

Unfortunately, there is no single, simple blood test that can definitively diagnose FIP in a living cat. Instead, your feline veterinarian will build the diagnosis piece by piece using history, physical exam findings, imaging, laboratory work, and sometimes fluid or tissue testing.

• Bloodwork Findings
• Fluid Analysis (Very Important for “Wet” FIP)
• PCR Testing
• Radiographs (chest and abdomen)
• Abdominal Ultrasound
• Echocardiogram (heart ultrasound)
• Biopsy and Special Tissue Testing

Many other illnesses can mimic FIP, which is one reason diagnosis can be challenging. Depending on the symptoms, veterinarians may also consider:

• Cancer (especially lymphoma)
• Heart disease
• Toxoplasmosis
• Fungal infections
• Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV)
• Severe liver disease
• Other inflammatory or infectious diseases

Is FIP Treatable?

Historically, FIP was considered fatal. Thankfully, legal treatment options have changed dramatically since May 2024, and many cats now recover successfully with antiviral therapy. There are several antiviral treatment options available for FIP, and your veterinarian will help determine which medication and treatment plan is most appropriate for your cat.

1. GS-441524 is currently the most widely used treatment for FIP and has shown excellent success rates, often exceeding 90% when treatment is started early and monitored appropriately.
2. Remdesivir is a closely related injectable antiviral medication. It is most commonly used in cats that are very sick, hospitalized, or unable to tolerate oral medications initially. Once the cat is stable enough, treatment is often transitioned to oral GS-441524.
3. Molnupiravir is another antiviral option that may be considered in certain situations, particularly for cats that relapse or do not respond as expected to other medications. Your feline veterinarian will help determine if this medication is appropriate based on your cat’s individual case and treatment response.

Lastly, FIP can cause significant inflammation and discomfort; many cats also benefit from supportive medications (not supplements) during treatment. Depending on the individual cat’s symptoms, this may include:

• Anti-inflammatory medications to reduce inflammation
• Anti-nausea medications to control nausea or vomiting
• Appetite stimulants to encourage eating. 

Ultimately, treatment plans are tailored to each cat’s specific needs and clinical signs. Your feline veterinarian will work with you to create a plan. 

Monitoring FIP: During and After Treatment

Regular recheck exams and bloodwork are important parts of monitoring cats during FIP antiviral treatment. There are recommended monitoring timelines that your feline veterinarian will discuss with you during treatment. 

At each visit, your veterinarian will perform a full physical exam, which may include a neurological and eye exam, along with a point-of-care ultrasound when needed.

Bloodwork is usually rechecked at certain weeks after finishing treatment to ensure continued improvement and stability.

What is the Prognosis for FIP? 

The outlook for cats with FIP has improved tremendously in recent years. Early recognition, appropriate diagnostics, and timely antiviral treatment can significantly improve outcomes.  

Many cats are now able to achieve long-term remission and return to a happy, normal quality of life.

The Latest on FIP Research 

We still do not fully understand how feline coronavirus mutates into FIP, but ongoing research in this area is very promising. Here are some of the ongoing studies: 

There are possible FIP comorbidities that veterinarians should be aware of. For instance, some cats with FIP can develop inflammation of the heart muscle, a condition called myocarditis. This may lead to changes in heart function, abnormal heart rhythms, or enlargement of parts of the heart.

Emerging evidence indicates that shorter treatment regimens of 42 days may be as effective as the traditional 84-day (12-week) courses.

Ongoing studies are evaluating the combination of antivirals with protease inhibitors, like GC376, for treating naturally occurring FIP in cats and whether this approach could shorten treatment time while maintaining high success rates.

There are unlicensed or “black market” versions of GS-441524 available, but studies have shown that these products can vary widely in their actual drug concentration and quality. This means the amount of active medication may not always match what is listed on the label, which can affect safety and treatment success. For this reason, veterinary-supervised and reliably sourced formulations are strongly preferred.