Slaughterhouse pigs are a major reservoir of Streptococcus suis serotype 2 capable of causing human infection in southern Vietnam

Abstract

Streptococcus suis is a pathogen of major economic significance to the swine industry and is increasingly recognized as an emerging zoonotic agent in Asia. In Vietnam, S. suis is the leading cause of bacterial meningitis in adult humans. Zoonotic transmission is most frequently associated with serotype 2 strains and occupational exposure to pigs or consumption of infected pork. To gain insight into the role of pigs for human consumption as a reservoir for zoonotic infection in southern Vietnam, we determined the prevalence and diversity of S. suis carriage in healthy slaughterhouse pigs. Nasopharyngeal tonsils were sampled from pigs at slaughterhouses serving six provinces in southern Vietnam and Ho Chi Minh City area from September 2006 to November 2007. Samples were screened by bacterial culture. Isolates of S. suis were serotyped and characterized by multi locus sequence typing (MLST) and pulse field gel electrophoresis (PFGE). Antibiotic susceptibility profiles and associated genetic resistance determinants, and the presence of putative virulence factors were determined. 41% (222/542) of pigs carried S. suis of one or multiple serotypes. 8% (45/542) carried S. suis serotype 2 which was the most common serotype found (45/317 strains, 14%). 80% of serotype 2 strains belonged to the MLST clonal complex 1,which was previously associated with meningitis cases in Vietnam and outbreaks of severe disease in China in 1998 and 2005. These strains clustered with representative strains isolated from patients with meningitis in PFGE analysis, and showed similar antimicrobial resistance and virulence factor profiles. Slaughterhouse pigs are a major reservoir of S. suis serotype 2 capable of causing human infection in southern Vietnam. Strict hygiene at processing facilities, and health education programs addressing food safety and proper handling of pork should be encouraged.

Figure 1. Provinces of Vietnam.

Provinces (coloured) from which sampled pig originated are located in the south of Vietnam and surrounding Ho Chi Minh City.

Figure 2. Pulse field gel electrophoresis (PFGE) of Streptococcus suis serotype 2 strains isolated from tonsils of healthy pigs.

Six S. suis serotype 2 strains isolated from patients with meningitis in southern Vietnam whose PFGE patterns represent the dominant PFGE patterns across S. suis serotype 2 strains isolated from humans (labeled A–F), were included for comparison purpose. A dendrogram was generated by Dice analysis (band tolerance, 1.3%) and cluster analysis with unweighted pair group method with arithmetic mean, using Bionumerics software (Applied Maths, Belgium). Bars indicate 95% confidence intervals. Columns: Cls: PFGE cluster; Pro: province of pig's origin or patients' residence; Date: date of tonsil collection or patients' admission; SH: slaughterhouse ID; WS: Whole sale seller ID; ST: sequence type, Res: tetracycline and erythromycin resistance gene detected by PCR and sequencing, ef: epf or epf* gene detected by PCR. CC: Cu Chi- HCMC; LA: Long An; HM: Hoc Mon – HCMC; HCM: Ho Chi Minh City; BD: Binh Duong; BTr: Ben Tre; DN: Dong Nai; BTh: Binh Thuan. O: amplicons of tet(O) gene detected. M: amplicons of tet(M) gene detected. B: amplicons of erm(B) gene detected. L: amplicons of tet(L) gene detected *: amplicons of mosaic tetracyclin resistance encoding gene tet(O/W/32/O) detected in these strains. #: no amplicons of erm(A), erm(B) or mef(A) were detected in these erythormycine resistant strains. 0: no amplicons of epf or epf* gene detected. 1: amplicons of epf gene detected. 2: amplicons of epf* gene detected.