Algorithm: Diagnosing and Treating Feline Infectious Peritonitis

Diagnosis of feline infectious peritonitis (FIP) can present clinical challenges, but obtaining a timely and precise diagnosis is imperative to treatment success. The widespread availability of antiviral medications for FIP makes successful patient outcomes common when cats are diagnosed accurately and treatment is started early.

Before prioritizing diagnostic tests for FIP, it is critical to adequately rule out diagnostic differentials that may mimic FIP. It is inappropriate to use antivirals for suspected FIP without having adequately assessed for the presence and risk of other differentials. Once such differentials have been ruled out, treatment trials with antiviral medications may be considered. Of note, treatment trials may be needed in cases with high suspicion for, but without confirmed diagnosis of, FIP (e.g., by reverse transcription PCR testing). In such cases, a recheck of the cat’s response to the antiviral therapy is recommended within a few days; if there is no improvement, other differentials need to be considered and tests performed to investigate them.

Using expert-developed scoring systems to gauge the likelihood of FIP can be helpful. The Brisbane score can increase confidence in suspicion of FIP by providing quantitative diagnostic criteria for assessing an individual patient.¹ The European Advisory Board on Cat Diseases has also developed excellent diagnostic scoring systems that can help when FIP is suspected.²

After a definitive diagnosis has been obtained, or if there is a strong suspicion for FIP in a patient with appropriate clinicopathologic findings, treatment should be started as quickly as possible. The 2 antiviral classes currently used to treat this disease are nucleoside analogues (remdesivir and its active metabolite GS-441524 and molnupiravir and its active metabolite EIDD-1931) and protease inhibitors (nirmatrelvir–ritonavir and GC376 [not commercially available]). Nucleoside analogues are the most well-studied and have proven safety and efficacy for FIP.^3-5 Protease inhibitors may have some efficacy as add-on treatments or as treatment for refractory cases,⁶ but more research regarding their use is needed.

This algorithm focuses on FIP diagnosis and when and how to treat this disease. However, because the presentation of this disease is variable, readers are encouraged to explore the extensive reviews available online,^2,7 which provide more detail.