Abstract
Streptococcus suis is a significant bacterial pathogen in the swine industry and represents a zoonotic threat to human health. This study aimed to investigate the prevalence, serotype distribution, and pathotypic characteristics of S. suis isolates obtained from nasopharyngeal swabs of slaughtered pigs in the high-density swine farming region of Thailand. Among 322 swab samples, 194 samples (60.25%) were found to harbor S. suis. The most prevalent serotype was serotype 8 (7.98%), followed by 19 (7.56%), 29 (6.72%), 3 (5.88%), and 2 (5.04%), with 39.92% of isolates classified as non-typeable. Molecular characterization revealed the presence of various clonal complexes (CCs), with CC221/234 being the most prevalent (19.15%). Human-associated clades (HAC) were identified in 29.79% of isolates, including serotypes 2 (two isolates), 1/2 (two isolates), and 4 (four isolates) in CC233/379. Additionally, several isolates exhibited a high potential for zoonotic transmission, particularly within the CC233/379 clade, which emerged exclusively in Thailand. Molecular pathotyping uncovered challenges in differentiating pathogenic and commensal S. suis strains in healthy pigs. Despite this, surveillance and monitoring of S. suis populations are essential to track dynamics and mitigate the risk of human infections.