Feline Infectious Peritonitis (FIP) is a serious and often life-threatening viral disease caused by a mutation of feline coronavirus (FCoV). Although many cats are exposed to FCoV during their lives, only a small percentage—approximately 1–2%—develop FIP.

Clinically, FIP is commonly classified into two main forms:

• Wet FIP (Effusive FIP)

• Dry FIP (Non-effusive FIP)

Understanding the differences between these forms is critical for early recognition, accurate diagnosis, and effective treatment.

1. What Causes Wet vs. Dry FIP?

Both forms are caused by the same underlying virus, but the body’s immune response determines how the disease presents.

• Wet FIP develops when the immune response is weak and uncontrolled inflammation leads to fluid leakage into body cavities.

• Dry FIP occurs when the immune response is partially effective, leading to granulomas (inflammatory lesions) forming in organs rather than fluid accumulation.

In short:

2. Wet FIP: Symptoms

Wet FIP is often easier to recognize because of visible fluid buildup.

Common signs include:

• Swollen abdomen (ascites)

• Difficulty breathing (if fluid accumulates around lungs)

• Lethargy and weakness

• Loss of appetite

• Fever that does not respond to antibiotics

• Weight loss

• Pale gums or jaundice in some cases

The fluid is typically yellowish and protein-rich, often described by veterinarians as “straw-colored.”

3. Dry FIP: Symptoms

Dry FIP is more challenging to diagnose because it lacks obvious fluid buildup.

Common signs include:

• Chronic weight loss

• Persistent fever

• Reduced appetite

• Enlarged lymph nodes

• Organ dysfunction (liver, kidneys, intestines)

• Eye problems (uveitis, cloudiness, retinal changes)

• Neurological symptoms in some cases, such as:

  • Wobbly walking

  • Head tilt

  • Seizures

  • Behavioral changes

Because these symptoms overlap with many other diseases, dry FIP is often misdiagnosed initially.

4. Key Differences at a Glance

5. How Veterinarians Diagnose FIP

There is no single “perfect” test for FIP. Diagnosis is usually based on a combination of:

Step 1 — Clinical signs

Veterinarians evaluate symptoms such as fever, weight loss, and abdominal distension.

Step 2 — Blood tests

Common findings may include:

• High total protein

• Low albumin-to-globulin ratio

• Elevated inflammatory markers

• Anemia

Step 3 — Imaging (Ultrasound/X-ray)

Used to detect:

• Fluid accumulation (wet FIP)

• Organ abnormalities (dry FIP)

Step 4 — Fluid analysis (if wet FIP suspected)

The fluid is tested for protein levels and inflammatory cells.

Step 5 — Advanced testing (if needed)

In some cases, PCR or biopsy may be used to support diagnosis.

6. Treatment Differences: Wet vs. Dry FIP

With modern antivirals such as GS-441524 (Pronidesivir), both forms of FIP are treatable—but dosing may differ.

Typical reference dosing:

• Wet/Dry FIP: 15 mg/kg per day

• Ocular/Neurological FIP: 30 mg/kg per day

• Standard treatment course: 84 days

Cats should be weighed weekly and dosing adjusted accordingly.

7. Monitoring During Treatment

Regardless of type, ongoing monitoring is essential:

• Weekly weight checks

• Periodic blood tests

• Observation of appetite and energy levels

• Monitoring of liver and kidney function

Early improvements may include:

• Reduced fever within 1–2 weeks

• Improved appetite

• Gradual weight gain

• Decreased fluid (wet FIP)

8. Prognosis: What to Expect

Before antiviral therapy, survival rates for FIP were extremely low. With GS-441524 treatment:

• Many cats achieve remission

• Wet FIP often responds quickly

• Dry and neurological FIP may take longer but can still recover with appropriate dosing

Early diagnosis + correct dosing + full treatment course = best outcome.

9. Care at Home: Supporting Recovery

Cat owners can help by:

• Ensuring consistent daily dosing

• Providing high-quality nutrition

• Minimizing stress

• Maintaining hydration

• Following veterinary guidance strictly

Supportive supplements (liver, kidney, probiotics) may be recommended depending on the case.

10. Summary

The key message:

FIP—whether wet or dry—is no longer a death sentence. With modern antiviral therapy, recovery is possible.

References

1. Pedersen NC et al. Efficacy of GS-441524 for treating FIP in cats. JFMS, 2019.

2. Hartmann K. Feline Infectious Peritonitis. Vet Clinics of North America.

3. Addie DD et al. Diagnosis and management of FIP. Veterinary Journal.

4. UC Davis FIP Research Program.