Streptococcus suis serotype 2 strains isolated in Argentina (South America) are different from those recovered in North America and present a higher risk for humans

Abstract

Introduction: Streptococcus suis serotype 2 is an important swine pathogen and emerging zoonotic agent causing meningitis and septicemia/septic shock. Strains are usually virulent (Eurasia) or of intermediate/low virulence (North America). Very few data regarding human and swine isolates from South America are available.

Case presentation: Seventeen new human S. suis cases in Argentina (16 serotype 2 strains and a serotype 5 strain) are reported. Alongside, 14 isolates from pigs are analyzed: 12 from systemic disease, one from lungs and one from tonsils of a healthy animal. All human serotype 2 strains and most swine isolates are sequence type (ST) 1, as determined by multilocus sequence typing and present a mrp⁺/epf⁺/sly⁺ genotype typical of virulent Eurasian ST1 strains. The remaining two strains (recovered from swine lungs and tonsils) are ST28 and possess a mrp⁺/epf ^- /sly^- genotype typical of low virulence North American strains. Representative human ST1 strains as well as one swine ST28 strain were analyzed by whole-genome sequencing and compared with genomes from GenBank. ST1 strains clustered together with three strains from Vietnam and this cluster is close to another one composed of 11 strains from the United Kingdom.

Conclusion: Close contact with pigs/pork products, a good surveillance system, and the presence of potentially virulent Eurasian-like serotype 2 strains in Argentina may be an important factor contributing to the higher number of human cases observed. In fact, Argentina is now fifth among Western countries regarding the number of reported human cases after the Netherlands, France, the UK and Poland.

Keywords: Streptococcus suis serotype 2; multilocus sequence typing; sequencing; swine; virulence factors; zoonosis.

Fig. 1.

Phylogenetic relationship of ST1 strains. (a) Phylogenetic tree of all strains (including sequence type) used in this study. The ST1 cluster was compressed and is indicated by a triangle. (b) Phylogenetic tree of ST1 strains included in the present study compared with available data from GenBank. The numbers on the branches correspond to the bootstrap values. Strains from different areas are represented by different colors: Asia (black), Europe (blue) and Argentina (red). Geographical origins and year of isolation are included, in parentheses, after the strain name.