To the Editor: Streptococcus suis is an important zoonotic pathogen for swine and humans. Among 33 sero-types, serotype 2 is more frequently isolated from diseased pigs than other serotypes (1). However, not all sero-type 2 strains are virulent, and degree of virulence varies among strains (2). Previous studies have reported sev-eral S. suis putative virulence factors, including the polysaccharide capsule, the muramidase-released protein, the extracellular factor, and suilysin (3–5). Some of these factors have been used as virulence-associated markers, and the association of the factors of S. suis isolates with virulence or clinical back-ground has been suggested in Europe (2,5). However, because many viru-lent isolates lacking these factors have also been isolated from clinical cases in Canada (6), they cannot be used as virulence markers in North America.Recent analysis of S. suis iso-lates by multilocus sequence typing (MLST) suggested the association of some clonal groups with particular clinical manifestations. That is, most invasive isolates belonged to the se-quence type (ST) 1 complex, while the ST27 and ST87 complexes were found to include a higher proportion of lung isolates (7). Although S. suis has been prevalent worldwide, the geographic location of the isolates used so far was mainly Europe, North America, and East Asia (7–9). Moreover, the clonal association with virulence of S. suis has been discussed mainly on the ba-sis of clinical and experimental data in swine (7). In this report, to broaden understanding of the population struc-ture of S. suis as a zoonotic agent, we characterize 20 S. suis isolates (Table) recovered from humans in Thailand in 1998–2002
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