Association of Group B Streptococcus colonization and bovine exposure: a prospective cross-sectional cohort study

Abstract

Background: While Group B Streptococcus (GBS) human colonization and infection has long been suspected as originating from cows, several investigators have suggested that ongoing interspecies GBS transmission is unlikely due to genotyping data demonstrating that human and bovine-derived GBS strains represent mostly distinct populations. The possibility of ongoing transmission between humans and their livestock has not been systematically examined.

Methodology/principal findings: To examine ongoing interspecies transmission, we conducted a prospective cross-sectional cohort study of 68 families and their livestock. Stool specimens were collected from 154 people and 115 livestock; GBS was detected in 19 (12.3%) humans and 2 (1.7%) animals (bovine and sheep). Application of multilocus sequence typing (MLST) identified 8 sequence types (STs or clones), with STs 1 and 23 predominating. There were 11 families in which two members submitted stools and at least one had GBS colonization. In 3 of these families, both members (consisting of couples) were colonized, yielding a co-colonization rate of 27% (95% CI: 7%-61%). Two of these couples had strains with identical MLST, capsule (cps) genotype, susceptibility, and RAPD profiles. One couple co-colonized with ST-1 (cps5) strains also had a bovine colonized with the identical strain type. On multivariate analysis of questionnaire data, cattle exposure was a predictor of GBS colonization, with each unit increase in days of cattle exposure increasing the odds of colonization by 20% (P = 0.02). These results support interspecies transmission with additional evidence for transmission provided by the epidemiological association with cattle exposure.

Conclusions/significance: Although GBS uncommonly colonizes livestock stools, increased frequency of cattle exposure was significantly associated with human colonization and one couple shared the same GBS strains as their bovine suggesting intraspecies transmission. These results set the framework for GBS as a possible zoonotic infection, which has significant public health implications.

Figure 1. Neighbor joining phylogeny of 21 group B Streptococcus (GBS) strains.

Phylogeny is based on concatenated multilocus sequence typing data for seven housekeeping genes and the capsule (cps) genotypes are provided.

Figure 2. RAPD patterns of GBS strains from epidemiologically linked co-colonized participants.

The OPB17 primers, previously described by Martinez et al.⁹ and two additional primer sets⁹ demonstrated similar banding patterns between the married couple and their bovine (Lanes 2–4) and the other co-colonized couple (Lanes 5–6). Lanes 7 and 8 represent additional strains isolated from two unrelated participants, and lanes 1 and 9 are a 1kb plus ladder.

Figure 3. Hypothetical mode of group B Streptococcus (GBS) transmission between bovines and humans.

It is possible that the diversification of GBS occurs independently in both humans and bovines, and only a subset of those strains can cause human and bovine disease. It is also possible that there is occasional transmission of GBS to other animal species (e.g, sheep, goats, etc.), particularly in a farm environment.