Effect of simultaneous exposure of pigs to Streptococcus suis serotypes 2 and 9 on colonization and transmission of these serotypes, and on mortality

Introduction

Streptococcus suis (S. suis) is a major pathogen in pigs worldwide, causing meningitis, septicemia, arthritis, endocarditis, and mortality. S. suis in humans is considered as an emerging life-threatening infection, especially in Asia. Main risk factor for human infection is direct contact with infected pigs or their products.In pigs, various serotypes of S. suis have been identified as cause of clinical infection. Comparable to e.g. Streptococcus pneumoniae in human, the presence of S. suis serotypes in pigs, however, differs between geographical areas, and varies over time. In several European countries, including The Netherlands, there has been a shift in predominant serotype from serotype 2 towards serotype 9 in the last two decades.We hypothesize a relation with one serotype affecting the other in colonization, transmission and invasion. The aim of this study was to evaluate whether simultaneous exposure of pigs to serotypes 2 and 9 affects the colonization and transmission of each serotype, and affects mortality.

Methods

Thirty-six caesarean-derived/colostrum-deprived piglets were randomly assigned to three groups, and there housed pair-wise. At 6 weeks-of-age one pig per pair was inoculated intranasally with either one (serotype 2 or 9; ‘mono-group’) or two serotypes simultaneously (‘dual-group’). Pigs in the mono-groups received 1x10⁹ CFU serotype 2 or 9, and in the dual-group a mixture of 1x10⁹ CFU serotype 2 and 1x10⁹ CFU serotype 9 (i.e. 2x10⁹ CFU S. suis/pig). The other pig of each pair was contact-exposed. Tonsillar brushing samples were collected from all pigs during three weeks post inoculation. Bacterial loads in the samples were quantified using multiplex real-time PCR. Transmission rates of the serotypes among pigs were estimated using a mathematical SI-model.

Results

The transmission rates for serotype 9 were 67/day (95%CI: 0-∞) and 4.1/day (95%CI: 1.6-10.6), for the mono- and dual-group, respectively (P=0.99). The transmission rates for serotype 2 were estimated at 29.4/day (95%CI: 0-∞) in the mono-group, and 2.9/day (95%CI: 1.2-6.9) in the dual-group (P=0.99). Bacterial loads did not differ significantly between serotypes (P=0.99). In the dual-group the average serotype 2 load in tonsillar samples from contact pigs was 1.4-1.8 10LogCFU/sample (i.e. 25-40 fold) reduced on days 1 to 4 and 6, in comparison to the mono-group (P<0.01).Simultaneous exposure to the serotypes reduced the mortality hazard 6.3 times (95%CI: 2.0-19.8) compared to exposure to serotype 2 only, and increased it 6.6 times (95%CI: 1.4–30.9) compared to exposure to serotype 9 only.

Conclusions

Transmission rates for serotype 2 did neither differ significantly between serotypes, nor for a single serotype between the mono- and dual-group. This implies that simultaneous exposure to serotypes 2 and 9 does not affect the relative transmission rates of each serotype.Natural contact exposure to serotypes 2 and 9 simultaneously affects the clinical outcome of an infection of a particular serotype in a population, possibly by affecting the mucosal load. This might have contributed to the observed shift in distribution of clinical isolates in the field from serotype 2 to 9.