Novel ST557 Streptococcus equi subsp. zooepidemicus lineage associated with a human case of septic arthritis

Background: Streptococcus equi subsp.zooepidemicus (SEZ) is an opportunistic zoonotic pathogen that is sporadically implicated in invasive human infections.The genomic diversity of human SEZ isolates remains poorly characterized, limiting insight into emerging lineages, antimicrobial resistance (AMR), and virulence traits.Methods: We describe a case of septic arthritis caused by SEZ in an elderly patient from Costa Rica and characterize the isolate using a combination of phenotypic and in silico analyses.Phylogenomic analyses were performed to contextualize the strain within the global humananimal SEZ population. Results:The isolate showed phenotypic resistance to clindamycin and susceptibility to erythromycin.Multilocus sequence typing assigned the isolate to a novel sequence type, ST557.Phylogenetic analysis revealed close relatedness of ST557 to animal-derived strains from different geographical regions.Whole-genome sequencing (WGS) revealed a defined set of virulence-associated genes, including szM and szP, along with capsule biosynthesis genes (hasAB) and the AMR determinant lsa(C).Conclusions: This study reports the first case of SEZ septic arthritis in Costa Rica and identifies a novel sequence type within a globally distributed zoonotic lineage.These findings underscore the value of integrating genomic surveillance within a One Health framework for zoonotic streptococci in the region.