Abstract
Most veterinarians recognize the cat that presents with thoracic or abdominal effusion as possibly having feline infectious peritonitis (FIP). However, what about those cats only presenting with uveitis, seizures or with non-specific signs? The diagnosis of FIP in these cases becomes a lot more complicated and can be frustrating.
The image on the right highlights how the presumptive diagnosis of FIP is made ‘brick by brick’, with each additional brick increasing the index of suspicion for the disease. Typically, there is not one single laboratory test or clinical sign that definitively diagnoses the disease. For most cases, the minimum database consists of serum biochemistry, a complete blood count and urinalysis, along with feline retrovirus testing. However, these test results may or may not be helpful in the case of FIP. One of the factors that makes FIP unique is that running analyses on blood samples is unhelpful in most instances since the virus is typically present only in low amounts in the blood, and is often not detectable. This characteristic requires the clinician to ‘think outside the box’: what diagnostic plan will have the greatest potential for providing enough individual bricks to point toward FIP as the underlying cause for the presenting clinical picture?
A presumptive diagnosis of FIP is made ‘brick by brick’, with each additional brick increasing the index of suspicion for the disease. Graphic designed by Vicki Thayer and Susan Gogolski, based on Melissa Kennedy’s diagnostic brick wall
Deciding which samples, such as effusion, tissue (eg, obtained via fine-needle aspiration or Tru-cut biopsy), cerebrospinal fluid, aqueous humour or serum, should be obtained and analyzed will depend upon the clinical signs present and the overall diagnostic accuracy of the test. Further, the various diagnostic tests offered by reference laboratories can differ in that one PCR assay may not be the same as another performed at a different laboratory. While PCR assays ultimately identify very small amounts of the FIP-associated virus, each assay is designed to identify a certain mutation. If that specific mutation is not present in that particular FIP-associated virus, the test is falsely negative. This is a challenge when dealing with a virus that is prone to mutations every time it replicates.
Our knowledge of the virus is continuing to evolve through ongoing research. But, a further complication in understanding FIP is that while we know feline coronavirus mutates, this does not necessarily mean the mutated virus will cause FIP. Other factors are instrumental – notably the mutated virus changes cellular tropism and is able to efficiently replicate, multiplying in monocytes and macrophages, allowing for rapid systemic spread throughout the body. This is followed by activation of FIP-associated feline coronavirus-infected monocytes/macrophages, leading to pronounced cytokine production and immune system activation. The resulting viral load may still, however, be undetectable using current assays.
Invaluable guidance for clinicians when dealing with the varied clinical presentations of FIP and idiosyncrasies of the available diagnostic tests is provided in the 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines, which appear on pages 905–933 of this issue. Compiled by a Task Force of experts, they contain a range of graphics and complementary resources, as outlined in the box below. While the gold standard of a single diagnostic test remains elusive, these guidelines offer the best means currently available for diagnosing FIP in your patients.
