Genetic diversity, pathogenicity, and serological cross-neutralization characteristics of contemporary feline calicivirus isolates from China.

Feline calicivirus (FCV) shows extensive genetic and antigenic diversity and typically causes oral-respiratory disease (ORD), although some isolates are associated with virulent systemic disease (VSD). The molecular basis of FCV-associated VSD remains unclear and cannot be attributed to a single VP1 residue, phylogenetic lineage, or recombination event. Here, we comparatively characterized three VSD-associated isolates (XA13_VSD, GZ_VSD, and AH1_VSD) and one ORD-associated isolate (SH2022_ORD) by integrating phylogenetic background, modular genomic variation, in vitro replication, experimental pathogenicity, tissue viral RNA distribution, and serological cross-neutralization. All four isolates belonged to genogroup GI, but the VSD-associated isolates were distributed across different GI sublineages. RDP5 analysis of 253 complete genomes identified 90 recombination events, including 27 spanning the Region A/Region B boundary; AH1_VSD showed a representative modular recombination pattern near the ORF1/ORF2 junction. Within this four-isolate panel, the three VSD-associated isolates showed higher in vitro replication levels and caused more severe disease than SH2022_ORD in experimentally infected cats, accompanied by greater viral RNA shedding, whole-blood FCV RNA detection at 3-9 dpi, broader tissue viral RNA distribution, and more severe histopathological lesions. After homologous neutralizing titers were standardized to 2 ¹ ⁰, AH1_VSD antiserum displayed the broadest cross-neutralizing profile within the tested panel. Collectively, these findings support a multifactorial view of FCV-associated systemic disease involving genomic background, replication and pathogenicity traits, viral dissemination, tissue involvement, and capsid antigenic variation. The comparatively broad cross-neutralization profile of AH1_VSD provides a rationale for its further evaluation as a potential parental candidate for attenuation and vaccine development.