The One Health paradigm holds that the health of humans, animals, plants and the environment are closely interrelated. The negative implications of antimicrobial overuse in terrestrial agriculture for human and animal health are widely accepted. The implications of antimicrobial overuse in aquaculture are less well appreciated. Large amounts of tetracyclines, florfenicol and quinolones are used in marine salmon aquaculture in Chile to prevent and treat bacterial infections, most commonly infections caused by the opportunistic pathogen Piscirickettsia salmonis. We found antimicrobial residues in sediments and wild free-ranging fish in waters surrounding aquaculture sites, and increased levels of resistance to these antimicrobials in bacteria cultured from sediments both close to and at a distance from these sites. These culturable marine bacteria contained many antimicrobial resistance genes and transmissible genetic structures including plasmids and integrons.
We have also demonstrated horizontal transfer of some of these antimicrobial resistance genes and transmissible genetic elements between salmonid aquaculture-related marine bacteria and clinical isolates of human urinary pathogens in coastal areas adjacent to salmon aquaculture. Other investigators have documented emergence of new antimicrobial-resistant salmon pathogens, including resistant P. salmonis. They have also demonstrated the presence of antimicrobial resistance genes and their transmissible genetic elements in the intestinal microbiome of antimicrobial-treated salmon and salmon pathogens that are identical or closely related to resistance genes encountered in the microbiome of pathogens of terrestrial animals and humans. Even more worrisome, our and others' research indicates that the marine resistome potentially contains new and undescribed mechanisms of antimicrobial resistance.
Excessive use of antimicrobials in the salmon aquaculture ecosystem thus results in generation of a hotspot for selection, recombination and dissemination of old and new antimicrobial resistance genes that can negatively affect human and animal health. Given the variable pathogenicity of P. salmonis for salmon under aquaculture, the One Health paradigm suggests that improvements in the conditions of aquaculture including changes in fish density and biomass would be associated with decreases in exposure of humans, animals and the environment to antimicrobial-resistant bacteria, antimicrobial resistance genes and antimicrobial residues.