How to Interpret Abdominal Ultrasound Findings in Suspected FIP

Feline Infectious Peritonitis (FIP) remains one of the most challenging diseases in feline medicine due to its complex pathogenesis and variable clinical presentation. While definitive diagnosis often requires histopathology or laboratory testing, abdominal ultrasound has become an invaluable non-invasive tool to guide clinicians toward a presumptive diagnosis. Proper interpretation of ultrasound findings can significantly impact treatment decisions and prognosis assessment. This article discusses the key ultrasound features associated with FIP and offers guidance on their clinical significance.

Understanding FIP Pathophysiology and Ultrasound Correlates

FIP originates from a mutation of feline coronavirus (FCoV) that leads to an aberrant immune response, resulting in severe vasculitis and granulomatous inflammation throughout the abdomen. The disease commonly involves the serosal surfaces, mesenteric lymph nodes, kidneys, liver, and intestines. These pathological changes are reflected vividly in ultrasound imaging, enabling clinicians to detect characteristic alterations before more invasive procedures.

Key Ultrasound Features in FIP

1. Mesenteric and Abdominal Lymphadenopathy

Enlarged lymph nodes are a hallmark ultrasound finding in FIP. These nodes often appear enlarged (greater than 1.5–2 times normal size) with a rounded shape and hypoechoic or mixed echogenicity. Their borders may appear poorly defined due to infiltration or edema.

Differential diagnoses include lymphadenitis and neoplastic processes, but in conjunction with other FIP signs, significant lymphadenopathy supports suspicion of the disease.

2. Peritoneal and Pleural Effusions

Ascites is frequently observed in both "wet" and "dry" forms of FIP. Ultrasound shows anechoic or hypoechoic fluid accumulation in the abdominal cavity, sometimes with fibrin strands evident as echogenic material within the fluid.

Fluid analysis can reveal high protein content and low cellularity, consistent with inflammatory exudate. Thoracic effusions, if present, are similarly characterized and contribute to diagnosis.

3. Gross Changes in Organs

Liver and kidney abnormalities are common. The liver may exhibit diffuse or patchy hypoechoic areas, sometimes with irregular borders or a mottled appearance. The kidneys may show cortical swelling or hypoechoic regions reflecting granulomatous infiltration.

Other organs, such as the spleen, jejunum, and omentum, may display thickening, hypoechoic nodules, or diffuse haziness.

4. Focal and Diffuse Abdominal Lesions

Hypoechoic or mixed echogenicity nodules or masses within the abdominal cavity may be encountered, representing granulomatous inflammation, pyogranulomas, or fibrin deposits.

These findings alone are non-specific but, combined with other features, support the suspicion of FIP.

Differential Diagnosis Considerations

Ultrasound findings in FIP often overlap with other conditions, such as lymphomas, infectious diseases like toxoplasmosis, or hepatic abscesses. Therefore, ultrasound should be integrated with clinical signs, laboratory tests (e.g., elevated globulins, RT-PCR), and serology to enhance diagnostic accuracy.

Limitations and Challenges

While ultrasound provides valuable clues, it is not definitive. Variability in ultrasound features among different FIP cases necessitates cautious interpretation. Operator expertise and equipment quality also influence image resolution and diagnostic confidence.

Practical Approach to Ultrasound in Suspected FIP

Conduct a thorough abdominal scan, focusing on lymph nodes, liver, kidneys, and peritoneal cavity.

Note the presence, distribution, and characteristics of effusions.

Identify organ-specific abnormalities such as hypoechoic lesions or irregular margins.

Correlate ultrasound findings with clinical signs and laboratory results.

Complementary Diagnostic Modalities

Abdominal fluid analysis for protein levels, cellularity, and cytology.

Serological testing and molecular diagnostics like RT-PCR for coronavirus.

Histopathology remains the gold standard when feasible.

Conclusion

Abdominal ultrasound plays a crucial role in the presumptive diagnosis of FIP by revealing characteristic features such as lymphadenopathy, effusions, and organ alterations. Recognizing these patterns, understanding their significance, and integrating ultrasound findings with other diagnostic data enable clinicians to make more informed decisions, ultimately improving patient management and prognostic assessments.

References

1. Addie, D. D., & Jarrett, O. (1992). Feline infectious peritonitis. In Practice, 14(6), 296–301.

2. Pedersen, N. C. (2014). An update on feline infectious peritonitis: Diagnosis and treatment. The Veterinary Journal, 201(3), 133–138.

3. Kip, A. E., et al. (2020). Ultrasound features of feline infectious peritonitis in cats: A retrospective study. Veterinary Radiology & Ultrasound, 61(3), 301–309.

4. Addie, D. D., et al. (2010). Feline infectious peritonitis: ABCD guidelines on prevention and management. Journal of Feline Medicine and Surgery, 12(7), 593–599.

5. Hartmann, K., et al. (2019). Feline infectious peritonitis: Update on diagnosis and treatment. Veterinary Clinics of North America: Small Animal Practice, 49(2), 239–253.