Gaining on FIP Diagnostic Dilemma

Feline infectious peritonitis (FIP) is caused by feline coronavirus (FCoV) and is typically described as having an “effusive” or “noneffusive (dry)” form. In vivo diagnosis can be difficult (especially in noneffusive cases) and is based on history, clinical signs, and clinicopathologic findings, all of which can sometimes be misleading. A number of diagnostic tests can aid in the diagnosis of FIP, including analysis of effusions, serum protein electrophoresis (SPE), alpha1- acid glycoprotein (AGP), anti-FCoV serology, and immunohistochemistry (IHC). Histopathology with IHC is considered the gold standard for diagnosis but is not always feasible. In this retrospective study, the authors compared the usefulness of these tests in 12 challenging clinical cases in which FIP was ultimately ruled out (n = 4) or diagnosed (n = 8). Among the FIP group, 3 of 8 cats had an atypical clinical presentation (mediastinal mass cytologically consistent with lymphoma, hemorrhagic syndrome with abdominal effusion, and absence of clinical signs apart from “stunted growth”). Five of 8 cats with FIP had a clinical presentation partially consistent with FIP but atypical postmortem findings that included mesenteric fibrotic lymphadenopathy without additional lesions (n = 2), intestinal pyogranulomatous lesions with tissue periodic acid-Schiff stain (PAS)–positive for rods and fungal hyphae (n = 1), segmental thickening of the intestinal wall characterized by severe fibrosis (n = 1), and a mediastinal cyst containing fluid consistent with FIP (n = 1). Intralesional FCoV was detected by IHC in 8 of 8 cases of FIP. In the non-FIP group, the clinical features and effusions (when present) were consistent with FIP. Serology and SPE were occasionally consistent with FIP. Postmortem examinations or histopathology, when performed, and serum AGP concentrations were inconsistent with FIP. AGP concentrations were in the upper reference range for the laboratory, however. In the FIP group, clinical signs (3 of 8), analysis of effusion (3 of 6), SPE (5 of 8), serology or polymerase chain reaction (PCR) (4 of 5), and postmortem examination (5 of 8) were all poorly predictive of FIP. All cats with FIP had increased AGP values. In this study, AGP was the only diagnostic test in complete concordance with IHC.

Commentary: This study attempted to address what is often a challenging scenario—the clinical diagnosis of FIP. Stemming from the ubiquitous FCoV, FIP develops into a fatal disease. Diagnostic tests must differentiate FIP from benign exposure to FCoV at any time in the cat’s life. Unfortunately, none of the diagnostic tests in this study performed particularly well against the current gold standard, IHC performed on biopsy samples. The best test seemed to be AGP. However, there are two caveats associated with AGP: The first, which the authors acknowledge, is that AGP is elevated in any inflammatory condition in cats. The sensitivity and specificity of AGP for FIP versus other inflammatory disorders need to be studied before AGP can be used in the clinical setting. The second difficulty is that AGP is not widely available and to my knowledge is found only in research laboratories. Therefore, while the results of this paper are intriguing, more work must be done to find a better tool for diagnosingFIP.—Elizabeth Thomovsky, DVM, MS, Diplomate ACVECC

Performances of different diagnostic tests for feline infectious peritonitis in challenging clinical cases. Giori L, Giordano A, Giudice C, et al. J SMALL ANIM PRACT 52:152-157, 2011.