Clinical Classification of FIP Types

Feline Infectious Peritonitis (FIP) is a fatal disease caused by a mutated form of the feline coronavirus (FCoV), known as feline infectious peritonitis virus (FIPV). Based on clinical presentation and pathological characteristics, FIP is categorized into several types, which is crucial for diagnosis and treatment decisions.

According to Professor Pedersen (2009) and the guidelines of the Advisory Board on Cat Diseases (ABCD), FIP is commonly classified into four clinical types: Wet (Effusive), Dry (Non-effusive), Neurological, and Ocular【Pedersen NC, 2009】【Addie et al., 2015】【Riemer et al., 2016】.

1. Wet FIP (Effusive FIP)

Characteristics:

• Accumulation of protein-rich exudate in the abdominal or thoracic cavity (ascites or pleural effusion).

• The fluid is typically yellow, viscous, and rich in fibrin.

• Causes abdominal distension, dyspnea, and fluid wave sensation upon palpation.

Epidemiology:

• Accounts for approximately 60–70% of FIP cases.

• More common in kittens and immunocompromised cats.

Clinical Significance:

• Easier to diagnose due to the presence of effusion; Rivalta test often positive; fluid analysis is highly valuable.

2. Dry FIP (Non-effusive FIP)

Characteristics:

• No visible effusion.

• Granulomatous lesions form in organs such as the liver, kidneys, spleen, or lymph nodes.

• Symptoms include fever, anorexia, and progressive weight loss.

Laboratory Findings:

• Low albumin/globulin (A:G) ratio (<0.4), increased globulin and α1-acid glycoprotein.

Diagnostic Challenge:

• May be misdiagnosed as lymphoma or chronic inflammatory disease.

• Requires a combination of ultrasound, lab markers, and clinical evaluation.

3. Neurological FIP

Characteristics:

• Involves the central nervous system and is a subtype of dry FIP.

• Presents with ataxia, seizures, nystagmus, abnormal gait, and altered consciousness.

Epidemiology:

• Occurs in 5–10% of FIP cases.

Diagnostic Tools:

• MRI, cerebrospinal fluid (CSF) analysis, and increased inflammatory proteins in the brain (e.g., IgG index).

4. Ocular FIP

Characteristics:

• Uveitis, anterior chamber fibrin, iris hyperemia or discoloration, vitreous opacity.

• May lead to blindness and abnormal or asynchronous pupillary reflexes.

Diagnosis:

• Requires ophthalmic examination including slit-lamp and fundoscopic evaluation.

• Often coexists with dry FIP.

5. Overlap and Diagnostic Approach

• Many cases exhibit overlapping features, such as Neurological + Dry FIP or Ocular + Wet FIP.

• These types are not strictly isolated, but reflect varying degrees of viral dissemination and immune response.

• A comprehensive approach combining clinical signs, lab work, and imaging is essential for accurate classification.

Summary

Accurate classification of FIP types is key to diagnosis and targeted treatment. Veterinarians should assess fluid accumulation, organ-specific symptoms, neurologic signs, and ocular manifestations to guide therapy. With the availability of antiviral drugs like GS-441524, including approved oral formulations such as NeoFipronis®, all types of FIP can now be treated with appropriate dosing strategies.

Key References

• Pedersen NC. A review of feline infectious peritonitis virus infection: 1963–2008. J Feline Med Surg. 2009;11(4):225–258.

• Addie DD, et al. Feline infectious peritonitis: ABCD guidelines on prevention and management. J Feline Med Surg. 2015;17(7):570–582.

• Riemer F, et al. Clinical and laboratory features of cats with feline infectious peritonitis – a retrospective study of 231 confirmed cases. Vet Microbiol. 2016;183:183–190.