Streptococcus parasuis: An underestimated zoonotic pathogen with a potentially distinct pathobiological profile

Streptococcus parasuis , a species formally delineated in 2015 from former Streptococcus suis serotypes 20, 22 and 26, has recently been recognized as a potential opportunistic zoonotic pathogen after long-term taxonomic misclassification. For decades, this bacterium was misclassified within the serological framework of its closely related congener, as phenotypic similarities prevented accurate identification by conventional diagnostic methods. With the development of genomic epidemiology and increased clinical case reporting, at least eleven human infections have now been documented across China. The true disease burden remains uncertain and may be underestimated due to persistent diagnostic limitations. This Review synthesizes current knowledge on S. parasuis , examining its taxonomic revision from serologically classified strains to a genomically defined species, its complex population structure comprising two major phylogenetic lineages and 12 minimum core genome clusters, and its characterized pathobiological features including resistance to multiple antibiotic classes in some isolates, an “open” pan-genome indicative of genetic plasticity, and a potentially distinct tissue tropism that preferentially targets peripheral organs rather than the central nervous system. We propose that S. parasuis raises questions about host specificity and virulence evolution within streptococci, providing a model for investigating divergent pathobiological profiles among closely related bacterial species. Critically, a recent human case with no documented livestock contact suggests a possible environmental exposure route, which requires further confirmation and may expand risk groups beyond traditional occupational populations. Looking forward, we identify priority areas for collaborative research: (i) clinical laboratories should deploy species-specific diagnostic tools to reduce ongoing misidentification; (ii) genomic surveillance networks should characterize the global population structure of S. parasuis to detect emerging clones; (iii) functional studies are needed to validate candidate virulence determinants and explain its distinct tropism; and (iv) environmental investigations should assess the survival and transmission of this pathogen outside animal hosts. Addressing these priorities will support improved risk management of this potentially underrecognized pathogen.