Emergence of a Highly Pathogenic Leptospira Serogroup 56601 Revealed by 15-Year Surveillance and Experimental Validation in a Rural Area of Zhejiang Province, China.

BACKGROUND

Leptospirosis is a globally important zoonosis maintained by chronically infected animal reservoirs, yet its long-term eco-epidemiological dynamics remain poorly understood. Southeast China has historically been an endemic region, but changing climate and land use may be reshaping Leptospira transmission.

METHODS

We conducted 15-year longitudinal surveillance (2009-2024) in Pan'an County, Zhejiang Province, integrating field sampling of rodents and other hosts, serogroup identification, multilocus sequence typing, and experimental mouse infection. Annual rainfall, host data, and molecular analyses were combined to assess temporal trends, serogroup turnover, and virulence differences between classical and emerging lineages.

RESULTS

Rodent Leptospira positivity persisted annually (1.2-18%) with peaks during heavy-rainfall years, while human cases declined after 2015. The host range narrowed from multispecies detection to rodent dominance after 2020. Serological and phylogenetic analyses revealed a post-2020 emergence of serogroup 56601, which replaced historical O1/O6 lineages and formed a genetically compact cluster, suggesting recent clonal expansion. In mice, the 56601 strain caused higher renal bacterial loads, more severe tubular injury, and stronger cytokine responses than O1.

CONCLUSIONS

Long-term surveillance demonstrates rapid ecological and genetic turnover of Leptospira and identifies 56601 as an emerging lineage with increased pathogenic potential in endemic regions.