One Health Trust researcher Thomas P. Van Boeckel was involved in a study that investigated the origin, geographic dispersion pathways, and host transitions of human group B Streptococcus agalactiae sequence type 283 (GBS ST283), which has recently been associated with freshwater fish handling and consumption in Asia. The researchers performed Bayesian phylodynamic and phylogeographic analyses of 328 genomes from ST283 isolates collected between 1998 and 2021 across four continents. Whole genome sequences of ST283 were also assembled to screen for genes conferring antimicrobial resistance and virulence.

Across ST283’s evolutionary history, phylogeographic analysis revealed a posterior mean of 35 independent transition events between countries. Following its emergence in 1981, ST283 dispersed greatly within the Asian continent, for example, from Thailand to Hong Kong and Laos, Singapore to Laos, and from Hong Kong to Laos and Vietnam. Thailand notably accounted for 62.1 percent of all transition events between 1981 and 2001. The earliest intercontinental transition event originated in Vietnam and spread to the United States, and Vietnam has since remained a source of international ST283 dissemination. 

After accounting for heterogeneous sampling bias, phylogenetic analysis showed that more human-to-fish transitions (median = 9; IQR: 8-9) occurred than fish-to-human transitions (median = 2; IQR: 1-3). This finding was consistent even after controlling for the year of sampling and phylogenetic diversity within each host type (humans and fish), indicating the potential role that humans play as a host of GBS ST283, particularly in areas that lack sufficient hygiene and wastewater treatment practices and allow for environmental transmission of the pathogen from humans to freshwater fish.

Seven antibiotic resistance genes (ARGs) were detected upon whole genome sequencing. These genes were associated with resistance to aminoglycosides, beta-lactams, dihydrofolate reductase inhibitors (i.e., trimethoprim), macrolides, and tetracyclines. The genes conferring resistance to macrolides (mre(A)) and to tetracyclines (tet(M)) were most commonly identified in 97.9 and 29.3 percent of isolates respectively. These two genes were more often lost than gained over ST283’s evolutionary history. Similarly, there was an observed loss of human-associated virulence factor genes during the human-to-fish transition, possibly limiting the potential for “spillback” of ST283 from fish to humans.

Read the full article, published in PLOS Global Public Health, here.