EXPRESS: Identification of within-host deletions in domain 0 of the spike gene of pathogenic feline coronavirus type 2 from the United States.

FCoV-2 spike gene deletions correlate with prolonged FIP disease and within-host pathogenic evolution.

This retrospective case series examined three U.S. domestic cats infected with feline coronavirus genotype 2 (FCoV-2) to characterize viral mutations associated with feline infectious peritonitis (FIP). Using whole-genome sequencing and phylogenetic analysis, researchers identified distinct deletions in domain 0 of the spike gene in two cats (#344 and #597) that experienced prolonged clinical signs consistent with FIP lasting at least two months. The third cat (#346), offspring of cat #344, displayed acute clinical signs and possessed an intact spike gene without deletions. Viral RNA quantification revealed low titers of the deletion-containing FCoV-2 variant in fecal samples. The findings support a model where intact spike variants are transmitted between cats, while internal deletions develop within individual hosts during chronic infection, enabling rapid systemic dissemination. These results contribute to understanding FCoV-2 pathogenesis and align with emerging evidence that spike gene deletions, similar to those observed in the highly pathogenic FCoV-23 variant from Cyprus, correlate with increased virulence and FIP development.

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