Distribution of pathogenic Streptococcus suis in the US

In this article written for the National Hog Farmer, PhD candidate April Estrada describes the high diversity of Streptococcus suis (S.suis) isolates.

There have been several reports of increased mortality in piglets due to S. suis. Different strains have various degree of pathogenicity and selection for vaccine development can be challenging partly due to inconsistent cross-protection between strains.

Streptococcus suis subtypes

Subtyping can be done by serotyping or multilocus sequence typing (MLST). 35 serotypes are know whereas 1,170 different S.suis sequence type profiles have been identified by MLST. ST25 and ST28 seem to be common in North America whereas ST1 is more prevalent in Europe.

Characterization of US isolates

208 isolates were obtained from the UMN and Kansas State Veterinary Diagnostic Laboratories and were classified as follows:

  • pathogenic if the strain was isolated from neurologic or systemic tissues of a diseased pig
  • opportunistic if the strain was isolated from lung tissue
  • commensal if the strain came from a healthy pig

Overall 19 serotypes and 58 MLST were identified from samples originating from 19 states. Most of the samples in this study came from Minnesota, Missouri, Illinois, and Kansas.

Predominant pathogenic strains were from serotypes 1/2, 2 and 7 and were from sequence types (ST) 1, ST28, ST94, and ST108. Other serotypes and sequence types were identified using an odds ratio and can be seen in the table below.

This study showed that sequence types seemed to be a better predictor of pathogenicity than serotypes but further research is needed to understand the diversity of S.suis.

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