What Tests Should Be Done First if a Cat Is Suspected of Having FIP
Introduction
Feline Infectious Peritonitis (FIP) is a devastating viral disease affecting domestic cats, often leading to fatal outcomes if left undiagnosed and untreated. Caused by certain mutations of feline coronavirus (FCoV), FIP primarily impacts young kittens and cats living in multi-cat environments, but any feline can develop this condition. There are two classical forms: the effusive (wet) form, typified by fluid accumulation in body cavities, and the non-effusive (dry) form, which presents with more subtle symptoms related to granulomatous lesions in organs. Suspecting FIP can be distressing for both the veterinarian and the pet owner, given that clinical signs often mimic those of other diseases. Therefore, a logical, stepwise diagnostic approach is essential for an accurate diagnosis.
Understanding FIP and Its Challenges
Diagnosing FIP remains one of the biggest challenges in feline medicine since no single test can definitively confirm the disease in all cases. Many symptoms such as lethargy, weight loss, fever, and fluid buildup can be indicative of other conditions, so veterinarians rely on a combination of clinical findings, laboratory tests, and disease context. Proper determination requires a systematic methodology that considers the cat’s age, environment, medical history, and presenting signs.
History and Physical Examination
The diagnostic journey for suspected FIP starts with taking a thorough medical history. Important factors include the cat’s age, breeding environment, exposure to other cats, past illness episodes, and ongoing symptoms. FIP commonly affects kittens and young adults under two years. Cats from shelters, catteries, or households with many cats have increased risk due to higher FCoV exposure.
A physical examination may reveal signs such as persistent fever unresponsive to antibiotics, abdominal distension, difficulty breathing, jaundice, neurological issues (especially in dry FIP), or palpable abdominal masses. These findings establish clinical suspicion and inform subsequent testing steps.
Initial Laboratory Evaluations
Complete Blood Count (CBC) and Biochemistry Panel
The first diagnostic step is a routine blood test including a complete blood count (CBC) and biochemical profile. Characteristic findings may include:
Lymphopenia (low lymphocyte count), reflecting immune suppression
Neutrophilia or increased white blood cell count
Non-regenerative anemia
Elevated total protein, especially due to increased globulins
Decreased albumin-to-globulin ratio (A:G ratio); values below 0.8 heighten suspicion of FIP
Liver enzyme elevations or signs of kidney involvement, depending on the affected organs
These non-specific results are not diagnostic of FIP alone but help differentiate this disease from others.
Serum Protein Electrophoresis
Further detailed assessment of blood proteins may show a polyclonal gammopathy (elevated globulin levels), which is typical but not exclusive to FIP.
Imaging Studies
Radiographs and ultrasound are vital in suspected wet FIP cases, allowing evaluation for effusion in the abdomen or chest. Imaging can also reveal organ enlargement, lymphadenopathy, or masses that hint at granulomatous changes, often seen in dry FIP.
Ultrasound-guided assessment can determine the volume and nature of abdominal or thoracic fluids, providing insight for subsequent fluid sampling.
Effusion Analysis: Cytology and Chemistry
When a cat presents with fluid accumulation (pleural or abdominal effusion), fluid analysis is crucial.
Fluid is obtained via centesis and subjected to several basic tests:
Appearance: FIP effusions are classically viscous, yellow-tinged, and clear to slightly cloudy.
Protein Content: FIP fluid protein concentration often exceeds 3.5 g/dL.
Cellularity: Typically low to moderate cell count, with predominance of neutrophils and macrophages.
Rivalta Test: A centuries-old, inexpensive test to help differentiate FIP effusions from other causes. A positive Rivalta test occurs when a drop of effusion forms a gelatinous cloud in an acetic acid solution, highly suggestive of FIP (especially in wet cases), but rarely definitive.
Serology and Feline Coronavirus Antibody Testing
Because the virus responsible for FIP originates from the feline coronavirus, another step is detection of antibodies against FCoV.
High FCoV titers indicate exposure but do not confirm FIP. Many healthy cats have high titers.
Interpretation must consider both clinical symptoms and lab values, as seropositivity alone is insufficient for diagnosis.
PCR Testing for Feline Coronavirus
Reverse-transcriptase PCR (RT-PCR) tests detect viral RNA, offering greater sensitivity than antibody titres.
PCR on blood, tissue, or effusion fluid may distinguish systemic FCoV infection.
Detection of mutated FCoV (associated with FIP) is possible using specialized PCR assays, which can be particularly useful if performed on effusion fluid or tissues.
Limitations exist: PCR positivity only indicates virus presence, not that FIP mutation has occurred.
Immunohistochemistry
Immunohistochemistry (IHC) involves the detection of viral antigens within macrophages of diseased tissues or effusion cells.
IHC is considered one of the most definitive ante-mortem tests for FIP if performed on biopsy samples or effusion cells showing characteristic histopathology.
This process is more invasive and reserved for cases where diagnosis remains unclear after standard non-invasive testing.
Stepwise Approach: Integrating Findings
No test in isolation can definitively diagnose FIP in every case. FIP diagnosis is often made after ruling out other causes and integrating results from history, clinical exam, laboratory tests, imaging, and, where fluid is present, effusion analysis. The typical diagnostic steps used by veterinarians in the US may follow this pathway:
1. Obtain a detailed history and conduct a physical exam.
2. Order CBC and biochemistry panel, emphasizing protein profile and A:G ratio.
3. Conduct imaging studies for evidence of effusions or organ changes.
4. If effusion is present, collect and analyze fluid (cytology, chemistry, and Rivalta test).
5. Evaluate FCoV antibody titres; run RT-PCR and, if required, mutation-specific FIP PCR on tissue/fluid samples.
6. If diagnosis remains uncertain and the cat's condition allows, obtain tissue biopsy for IHC.
Special Considerations
Cats with dry FIP pose the greatest diagnostic challenge:
Fluid accumulation may be absent or minimal, limiting effusion analysis.
Symptoms are often more subtle—neurological signs (ataxia, seizures), ocular involvement, or non-specific organ dysfunction predominate.
Advanced imaging and tissue sampling assume greater importance, sometimes necessitating referral to specialists.
While PCR and IHC improve diagnostic confidence, their collection is more complex, emphasizing the need for owner discussion and consent.
Current Limitations and Future Directions
The quest for a single, rapid, and definitive FIP test continues, but for now, diagnosis is a combination of exclusion and inclusion. Newer molecular tools like mutation-specific PCR or next-generation sequencing hold promise in distinguishing pathogenic FCoV strains. Furthermore, continued advancement of minimally invasive sampling and improved markers (e.g., acute phase proteins, cytokine profiles) will likely shape future diagnostic paradigms.
References
1. Hartmann, K. (2022). Feline infectious peritonitis: Update on pathogenesis, diagnosis, and treatment. Veterinary Clinics: Small Animal Practice, 52(1), 65–86.
2. Pedersen, N. C. (2020). Feline infectious peritonitis and feline enteric coronavirus infections. In: Greene, C. E. (Ed.), Infectious Diseases of the Dog and Cat (5th ed., pp. 213-234). Elsevier.
3. Felten, S., & Hartmann, K. (2019). Diagnosis of feline infectious peritonitis: A review of the current literature. Viruses, 11(11), 1068.
4. Kipar, A., & Meli, M. L. (2014). Feline infectious peritonitis: Still an enigma? Veterinary Pathology, 51(2), 505–526.
5. Addie, D. D., et al. (2015). Feline coronavirus antibodies in cerebrospinal fluid of cats with neurological signs. Veterinary Record, 176(24), 626.
6. Stranieri, A., et al. (2018). Diagnostic accuracy of the rivalta test for feline infectious peritonitis. Journal of Feline Medicine and Surgery, 20(9), 844-851.
7. Porter, E., Tasker, S., & Day, M. J. (2020). Diagnostic approach to feline infectious peritonitis. Journal of Feline Medicine and Surgery, 22(10), 913–922.
8. Barker, E. N., Tasker, S., & Gruffydd-Jones, T. J. (2017). Diagnostic challenges of feline infectious peritonitis: A review. Veterinary Journal, 219, 40–48.
9. Chang, H. W., et al. (2010). Mutations in the spike gene of feline coronavirus correlate with feline infectious peritonitis. Virus Research, 153(2), 217–223.
10. Dempsey, S. M., & Ewing, P. J. (2011). A review of the pathogenesis and diagnosis of feline infectious peritonitis. Journal of Feline Medicine and Surgery, 13(9), 619–630.
