Deciphering Viral Replication Dynamics in Feline Infectious Peritonitis: A Quantitative Approach

Feline infectious peritonitis (FIP) is a complex immune-mediated disease caused by feline coronavirus (FCoV). Despite advancements in understanding its pathogenesis, challenges persist in elucidating viral factors related to virion composition and replication. This study examined FCoV-infected cats with and without FIP for potential associations with or variation in expression ratios for different viral genes. We analyzed tissue samples with FIP lesions from 46 cats with FIP and mesenteric lymph nodes from 10 FCoV-infected cats (7b RT-qPCR positive) without FIP with three RT-qPCR assays, targeting (sub)-genomic RNAs of the RNA-dependent RNA polymerase (RdRp) and envelope (E) genes. In cats with FIP, the RdRp mRNA assay yielded the highest copy numbers, followed by the combined RdRp gRNA and mRNA assay; the E mRNA assay yielded the lowest copy numbers. In cats without FIP, significantly fewer viral RNA copies were detected regardless of the assay. Viral gene expression was not detected in six and was observed only at low levels in one or more assays in four samples. The observed correlation between assays and the intragroup correlation assay indicate consistent transcription of both the structural E protein and RdRp genes within FIP lesions in cats with FIP but not in cats with systemic FCoV infection alone.