Diagnosis of feline infectious peritonitis

Abstract

Feline infectious peritonitis (FIP), a fatal immune-mediated disease, is caused by feline infectious peritonitis virus (FIPV), the high-virulence pathotype of Feline coronavirus (FCoV). It is believed that the FIPV is an accumulation of mutations of the low-virulence feline enteric coronavirus (FECV) favored by extensive viral replication and transmission in a multi-cat environment. However, the complete extent of such etiological mutations has remained unestablished. The absence of FIP-specific clinical signs and laboratory parameters further aggravates the diagnosis of FIP. Currently, an accurate FIP ante-mortem diagnostic assay is unavailable. In this study, we aimed to establish a highly sensitive FIP diagnostic PCR assay. In the course of the investigation, we developed several quantitative real-time PCRs (qRT-PCR) targeting different genomic regions of FCoV, detecting FCoV via either mRNA or genomic RNA (FIP M gene mRNA qRT-PCR, FIP N gene mRNA qRT-PCR, and FIP MN gene qRT-PCR). We subsequently evaluated the performance of the PCRs by the use of the developed dual-labeling immunofluorescence (IF) assay as the gold standard. We concluded that the FIP MN gene qRT-PCR is the most accurate FIP diagnostic assay, with 84% sensitivity at 100% specificity, resulting in 100% positive and 47% negative predictive value. Upon discovering a symmetrical distribution of the results in the MN gene qRT-PCR and the IF assay, we evaluated the performance of the IF assay by using the MN gene qRT-PCR as the gold standard. The IF assay generated a 44% sensitivity at 100% specificity, resulting in 100% positive and 21% negative predictive value. Additionally, we pinpointed a serum albumin to globulin ratio of ≤0.5 was the most FIP-pertinent clinical correlate. We also determined that mutations of the furin cleavage motif within the FCoV spike protein that largely abrogate furin cleavage are associated with 58% of cases of FECV to FIPV conversion. On the basis of this study, we established the FIP MN gene qRT-PCR as a robust single assay that greatly overcomes the ante-mortem diagnostic challenge of FIP. By demonstrating the co-localization of FIP viral vesicles with macrophages, the high resolution IF assay can be used as confirmatory FIP assay with 100% specificity.