Clint Lichty from South West Ontario Veterinary Services, Strep suis: Getting The Right Sample!

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Streptococcus suis (S. suis) continues to be ranked  as  one  of  the  most  common diseases  on  pig  farms  on  a  global  basis. Efforts  to  reduce  antimicrobial  usage  as part  of  a  more  comprehensive  plan  to  limit the development of antimicrobial resistance has spurred investment  in  S.  suis  research.  Understanding  the ecology of S. suis is important as disease prevention and control strategies are developed. There is a wide range of disease causing abilities within this species. S.  suis  is  a  bacterium  that  may  be  classified  into pathogenic, opportunistic or commensal strains.

  • Pathogenic strains of S. suis are associated with clinical  cases  in  all  ages  of  growing  pigs  and breeding  stock  and  isolates  are  retrieved  from pigs  with  meningitis,  arthritis,  endocarditis, epicarditis, polyserositis and septicemia.
  • Possibly  opportunistic  strains  of  S.  suis  are isolated from the lung of individual pigs that are otherwise  free  of  neurological  or  systemic disease.
  • Commensal strains of S. suis are isolated from laryngeal,  tonsil,  or  nasal  samples  from  farms with  no  known  history  or  current  control methods for S. suis on that farm.

Serotyping  and  Multilocus  Sequence  Typing (MLST) are the primary diagnostic methods used to differentiate  strains.  Serotyping  is  the  best  known and  the  more  traditional  diagnostic  technique. Multilocus  sequence  typing  (MLST)  is  a  common research  diagnostic  approach  that  is  used  in  many species  of  bacterial  pathogens.  US,  Canadian  and Mexican  researchers  wanted  to  characterize  the diversity  of  208  S.  suis  isolates  collected  between 2014  and  2017  across  North  America  by  using serotyping and MLST. They also wanted to further investigate  associations  between  subtype  and pathotype  classifications  (pathogenic,  possibly opportunistic,  and  commensal),  based  on  clinical information  and  site  of  isolation.

The  researchers found  the  following  in  these  208  North  American isolates:

  • 20   serotypes   were   identified   with   the predominant  serotypes  being  1/2  (n=54)  and  7 (n=23).
  • serotypes 1, 1/2, 2, 7, 14, and 23 were most commonly pathogenic
  • 14  of  those  20  serotypes  ranged  from 56‐100% that were classed as pathogenic
  • 6  of  those  20  serotypes  had  0%  that  were classed as pathogenic
  • serotypes  21  and  31  were  most  commonly commensals
  • 58 sequence types (STs) were identified with the predominant STs being ST28 (n=52) and ST94 (n=18).
  • ST1,  ST13,  ST25,  ST28,  ST29,  ST94, ST108,  ST117,  ST225,  ST373,  ST961,  and ST977  were most  commonly  classified  as pathogenic
  • ST750,  and  ST821  were  most  commonly classified as commensals

Take Home Message

  • Streptococcus suis isolates were classified into 20  different  serotypes  and  58  different Sequence types (ST)s.
  • Sequence  Types  (ST)s  rather  than  the  more traditional  serotype  appear  to  be  a  more effective  predictor  of  disease  causing  ability for any particular isolate.
  • This research supports the findings of previous studies that indicate that within serotypes there can  be  a  variety  of  disease  causing  abilities. Some  serotypes  are  rarely  if  ever  associated with clinical disease.
  • These  findings  are  important  in  guiding  how we think about diagnostic sample selection for S. suis autogenous vaccines.

Submitted by Clint Lichty, DVM

Ref: April A. Estrada, Marcelo Gottschalk, Stephanie Rossow, Aaron Rendahl,  Connie  Gebhart,  Douglas  G.  Marthaler  Serotype  and Genotype (Multilocus Sequence Type) of Streptococcus suis Isolates from the United States Serve as Predictors of Pathotype Journal of Clinical Microbiology Aug 2019, 57 (9) e00377-19; DOI: 10.1128/JCM.00377-19

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