How FIP is Diagnosed: A Comprehensive Guide for Oceania

Feline infectious peritonitis (FIP) remains one of the most misunderstood and misdiagnosed feline diseases. Caused by a mutation of the feline coronavirus (FCoV), FIP triggers a severe inflammatory response that can affect multiple systems in the body. While treatment breakthroughs like GS-441524 have transformed outcomes, accurate diagnosis remains the most critical step in helping cats recover.

In Australia and New Zealand, many veterinarians are becoming more familiar with the signs of FIP, but diagnosing it remains complex due to overlapping symptoms and the absence of a single definitive test in most cases. This guide walks through the key tools and signs that lead to a presumptive or definitive FIP diagnosis—from initial suspicion to laboratory confirmation.

Why Diagnosing FIP is Challenging

Unlike diseases with a clear biomarker or positive/negative test result, FIP is a diagnosis of exclusion. This means veterinarians often need to rule out other illnesses first, especially since FIP mimics many other feline diseases, including lymphoma, toxoplasmosis, and bacterial peritonitis.

Key Challenges Include:

• Lack of a single definitive test for all forms of FIP

• Variable presentation: wet (effusive), dry (non-effusive), ocular, or neurological

• Overlap of symptoms with more common, less fatal illnesses

• Low sensitivity/specificity of many traditional tests

For this reason, diagnosing FIP often involves a combination of clinical signs, laboratory tests, imaging, and advanced molecular diagnostics.

Initial Clinical Signs and Suspicion

Early suspicion of FIP arises from a combination of symptoms and history, especially in cats under 2 years old or those from multi-cat environments (e.g., shelters or batteries).

Common signs that raise suspicion:

• Persistent fever unresponsive to antibiotics

• Weight loss and poor appetite

• Lethargy

• Swollen abdomen or chest effusion (wet FIP)

• Neurological signs like ataxia or seizures

• Eye inflammation (uveitis, retinal lesions)

A cat showing several of these signs, especially if they are young and previously exposed to feline coronavirus, should prompt further investigation.

Bloodwork and What It Reveals

Routine blood tests are often the first diagnostic step after clinical suspicion. While no result is FIP-specific, patterns in bloodwork can strongly support a presumptive diagnosis.

Blood test indicators that support FIP:

• Lymphopenia: low lymphocyte count

• Non-regenerative anemia

• Hyperglobulinemia: elevated globulin levels

• Hypoalbuminemia: low albumin levels

• Low A:G ratio (<0.4): a hallmark of FIP

• Mild elevations in liver enzymes or bilirubin

• Elevated total protein

A low albumin-to-globulin (A:G) ratio is particularly suspicious when other signs align with FIP. However, none of these alone is conclusive.

Rivalta Test: Purpose and Limitations

The Rivalta test is commonly used when effusion fluid (from the chest or abdomen) is present. It helps distinguish exudative (inflammatory) fluid typical of FIP from transudate caused by heart failure or cancer.

How it works:

A drop of effusion fluid is added to a test solution. If the drop retains its shape or slowly floats down, the result is positive, suggesting FIP.

Limitations:

• Not definitive: other diseases can yield a positive result

• Only useful for wet FIP

• Variable accuracy depending on test handling and sample quality

In Australia and New Zealand, it is a cost-effective screening tool, particularly in general practice settings, but should always be used alongside other diagnostics.

PCR, IHC, and Effusion Analysis

To approach a definitive diagnosis, more advanced testing may be needed. These methods are often sent to reference laboratories in New Zealand or Australia, such as Gribbles Veterinary Pathology, IDEXX, or university labs.

1. PCR (Polymerase Chain Reaction)

Detects FCoV RNA in tissues or effusion fluid. It can confirm viral presence but cannot always distinguish between mutated (FIP-causing) and benign strains.

• RT-PCR targeting the spike gene mutations offers more specific results

• A positive PCR in effusion fluid is strongly supportive of FIP

• Negative PCR does not rule out FIP, especially in dry cases
  

2. IHC (Immunohistochemistry)

This involves biopsy and staining of tissue samples (e.g., from lymph nodes or intestines) to detect FCoV antigens within macrophages—a gold standard for confirmation.

• Highly specific but requires invasive sampling

• Mostly performed in referral hospitals or post-mortem facilities
  

3. Effusion Analysis

Fluid analysis includes cytology, protein levels, and cell counts. FIP fluid typically appears:

• Straw-colored

• High in protein (>3.5 g/dL)

• Low in cellularity

• Rich in macrophages and neutrophils
  

Definitive vs. Presumptive Diagnosis

Given the lack of a universally accessible "yes or no" test, most cases are diagnosed presumptively—a strong clinical picture backed by supportive lab data and imaging.

Presumptive Diagnosis:

• Based on symptoms, bloodwork, imaging, and possible Rivalta or PCR

• Sufficient for starting antiviral treatment, especially if early intervention is needed
  

Definitive Diagnosis:

• Confirmed via IHC, RT-PCR with spike gene mutation, or viral culture from tissue

• More likely pursued in complex or referral cases

In the Oceania veterinary setting, most FIP diagnoses are presumptive due to cost and accessibility, but that has not hindered successful treatment outcomes when therapy is initiated promptly.

The Importance of a Holistic Diagnostic Approach

No single test confirms or rules out FIP. That’s why experienced veterinarians in Australia and New Zealand rely on a multi-faceted diagnostic approach, combining:

• Thorough history and clinical exam

• Strategic use of bloodwork and fluid analysis

• Judicious use of PCR and imaging

• Monitoring of response to antiviral therapy

Early treatment with antivirals like GS-441524 significantly improves survival rates, especially when diagnosis occurs before neurological involvement.

At Cure FIP Oceania, we encourage pet owners and veterinarians to act early when suspicion is high. Our team supports diagnosis, dosing guidance, and post-treatment monitoring throughout New Zealand and Australia.

Final Takeaway

Diagnosing FIP is not simple, but it is increasingly possible. Through careful observation, strategic testing, and awareness of the disease’s evolving presentations, cat owners and vets in Oceania are saving lives.

If your cat is showing suspicious symptoms or if you’re a vet unsure how to proceed, Cure FIP Oceania is here to help. Our mission is to provide access to life-saving treatment and up-to-date information based on trusted veterinary science.