Advancements in FIP Diagnosis and Treatment for Cats

Feline infectious peritonitis (FIP), once considered nearly uniformly fatal, can now be treated with antiviral medications. However, veterinarians still lack a reliable antemortem test that can confirm or rule out the disease for every patient, creating significant uncertainty where a definitive diagnosis could facilitate immediate treatment.

"FIP is extremely diagnostically challenging, especially in the noneffusive form," said Samantha J.M. Evans, DVM, PhD, DACVP, DACVM, an associate professor of clinical pathology at Colorado State University (CSU) and lead author of a new study on FIP diagnostics. "It's crucial to get a diagnosis now that effective treatments are available."

In their study, Dr. Evans and her team assessed multiplex fluorescent immunocytochemistry (MF-ICC) in cats showing signs of FIP. This method detects feline coronavirus antigens within macrophages, key cells affected by the disease, without needing surgical biopsies.

The MF-ICC test was found to be the most precise of the four diagnostic methods evaluated, although Evans cautioned against overestimating its efficacy. "It is not a panacea," she stated. "We must consider a combination of clinical signs and various diagnostic tests to build a case for FIP rather than relying on a single test."

Feline coronavirus is prevalent, especially in multi-cat environments, yet most exposed cats do not go on to develop FIP. This complicates tests that can signal exposure without clarifying if the virus is causing systemic issues. Other tests, such as RT-PCR and serology, detect viral RNA or the cat's antibody response but do not conclusively diagnose FIP.

Histopathology, combined with immunohistochemistry, can confirm diagnoses but may require invasive tissue collection. MF-ICC improves this process via the use of fluorescent antibodies to locate both coronavirus antigens and vimentin, a marker helping to identify macrophages. Superimposing images taken at various wavelengths determines if both markers are in the same cell.

In a clinical trial that included 84 cats—58 diagnosed with FIP and 26 with other diseases—MF-ICC demonstrated 77 percent sensitivity and 81 percent specificity, with an overall accuracy of 78 percent. Results showed that a single cell positive for both markers could provide significant diagnostic information. This was noteworthy since it indicated that minimal material could still yield clinically meaningful results.

The study revealed variability in results based on sample types. While MF-ICC was more sensitive in effusion fluid, it remained a stronger indicator in tissue samples from cats with non-effusive FIP. Despite promising results, Evans noted this does not categorically prove MF-ICC's superiority over existing tests. Instances occurred where different tests identified different FIP cases, which emphasizes the value of using both MF-ICC and PCR together for a better diagnostic outcome.

A positive MF-ICC result can strongly support an FIP diagnosis, but a negative result cannot definitively rule it out. Key factors such as sample selection and the patient's specific symptoms play a crucial role in diagnosis. Quick diagnosis is essential, as severely affected cats may have limited time for effective treatment.

With effective antivirals, uncertainty from inconclusive test results can delay necessary treatment and mislead owners regarding their cats' conditions. "Having a rapid turnaround is vital for many cats," Dr. Benjamin Curtis, a study co-author, noted, emphasizing the importance of swift diagnostics.

Dr. Petra Černá, another co-author, highlighted the need for fast and accurate tests, particularly to avoid misdiagnosing cats that do not have FIP while they are treated with antivirals. For general practitioners, MF-ICC could prove useful when other samples have been collected, as it adds another valuable method in the diagnostic toolbox.

While MF-ICC is not an in-clinic test due to the need for specialized equipment and expertise, CSU plans to offer the assay through its Veterinary Diagnostic Laboratory. The study aims to develop a comprehensive diagnostic approach, including MF-ICC, PCR, and other relevant tests for a more reliable diagnosis of FIP.

Ultimately, while progress has been made in FIP diagnosis and treatment, further research is necessary to assist cats with severe complications and improve outcomes for all affected patients. Evans concluded that no single test suffices, and combining techniques will help in forming a thorough diagnostic picture for FIP.