FIP begins with a common and usually harmless virus called feline enteric coronavirus (FCoV). Most cats who contract FCoV experience mild diarrhea or no symptoms at all and clear the virus without incident. However, in a small percentage of cats, the virus mutates within the body into a form that triggers a devastating immune response. It is this mutated form that causes FIP.

The mutation is unpredictable, and researchers still do not fully understand why it happens in some cats but not others. What we do know is that certain factors increase risk:

• Age: Kittens and young cats (under two years) are most vulnerable, as their immune systems are still maturing.
• Stress: Events like rehoming, surgery, or introduction to new cats can trigger the mutation in cats carrying FCoV.
• Immune status: Cats with weakened immune systems, including those with FeLV, are at higher risk.
• Genetics: Some purebred lines appear more susceptible, though any cat can develop FIP.
• Multi-cat environments: Shelters, catteries, and multi-cat households have higher rates of FCoV circulation.

It is important to understand that FIP itself is not contagious. While the underlying coronavirus spreads easily between cats, the mutation that causes FIP occurs within an individual cat's body. A cat with FIP cannot directly give another cat FIP.

FIP presents in two main forms, and recognizing the symptoms early is essential for the best chance at successful treatment.

Wet (effusive) FIP is characterized by fluid accumulation in the abdomen or chest. Symptoms include:

• A visibly swollen, fluid-filled abdomen that develops over days to weeks
• Difficulty breathing if fluid accumulates around the lungs
• Persistent fever that does not respond to antibiotics
• Loss of appetite and progressive weight loss
• Lethargy and withdrawal from normal activities

Dry (non-effusive) FIP is harder to diagnose because it does not produce obvious fluid buildup. Instead, it causes inflammatory lesions on organs including the kidneys, liver, eyes, and brain. Symptoms vary depending on which organs are affected:

• Chronic, fluctuating fever
• Gradual weight loss and poor appetite
• Eye changes including cloudiness, uneven pupil size, or visible inflammation
• Neurological signs such as wobbliness, seizures, or personality changes
• Jaundice (yellowing of the gums, ears, or whites of the eyes)

Diagnosing FIP can be challenging. There is no single definitive test. Veterinarians rely on a combination of clinical signs, bloodwork (elevated globulin levels, low albumin-to-globulin ratio), fluid analysis if present, and sometimes tissue biopsy. The Rivalta test on abdominal fluid can provide a quick preliminary indication, and PCR testing of fluid or tissue samples can detect coronavirus genetic material.

For decades, FIP was considered a death sentence. That has changed dramatically in recent years thanks to antiviral medications that target the virus directly.

GS-441524 is the most widely used antiviral treatment for FIP. It works by inhibiting viral replication, allowing the cat's immune system to regain control. Treatment typically lasts 84 days (12 weeks) and involves daily injections or oral tablets. Success rates are remarkably high, with published studies showing remission rates above 80% for wet FIP and slightly lower for dry and neurological forms.

Molnupiravir is another antiviral that has shown promise in treating FIP, particularly the neurological form. It is available as an oral medication, which can be easier to administer than injections. Research is ongoing, but early results are encouraging.

Treatment considerations include:

• Early treatment yields the best results. If you suspect FIP, do not wait.
• Treatment can be expensive, often ranging from $2,000 to $6,000 depending on the cat's weight and form of FIP.
• Cats must complete the full treatment course. Stopping early significantly increases the risk of relapse.
• Regular bloodwork monitoring during treatment is essential to track progress.
• After completing treatment, a three-month observation period determines whether the cat has achieved lasting remission.

The availability of these treatments varies by country due to regulatory differences. Some veterinarians can prescribe or compound these medications directly, while in other regions, owners may need to source them through specialty compounding pharmacies. Your veterinarian can guide you through the options available in your area.

Receiving a FIP diagnosis for your cat is devastating, but it is no longer the hopeless situation it once was. If treatment is an option, the journey will require commitment, both financially and emotionally, but the possibility of a full recovery is real.

During treatment, focus on keeping your cat comfortable and reducing stress. Provide a warm, quiet space, high-quality food (kitten food is often recommended for the extra calories), and plenty of gentle attention. Many cats show noticeable improvement within the first one to two weeks of antiviral treatment, with fevers resolving and appetites returning.

For cats where treatment is not possible, whether due to how advanced the disease is, financial limitations, or other factors, palliative care can provide comfort in their remaining time. Anti-inflammatory medications, appetite stimulants, fluid drainage if needed, and pain management can all help maintain quality of life.

Whatever path you find yourself on, know that your love and advocacy for your cat through this illness is deeply meaningful. Whether the outcome is recovery or saying goodbye, the care you provide defines the bond you share.