Salmon aquaculture, Piscirickettsia salmonis virulence, and one health: Dealing with harmful synergies between heavy antimicrobial use and piscine and human health

Abstract In two recently published articles in Aquaculture , Mardones et al. (2018) discuss the emergent fish pathogen and cause of salmonid rickettsial syndrome (SRS), Piscirickettsia salmonis , while Avendano-Herrera (2018) discusses the heavy use of antimicrobials in Chile to prevent and treat salmonid bacterial infections including SRS. Mardones et al. put forward a research agenda that focuses largely on P. salmonis and its relation to the biology of SRS. We believe a broader view of the problem would be beneficial. An extended examination of the scientific literature suggests that P. salmonis is likely an opportunistic environmental pathogen with low levels of virulence and pathogenicity and/or an endogenous pathobiont colonizing the normal salmonid microbiome. Multiplication of this bacterium and its ability to produce SRS is at least in part triggered by shortcomings in piscine husbandry that negatively affect salmonid health and well-being. This explanation is consistent with the limited efficacy of vaccines and antimicrobials in preventing and modifying the course of SRS, and suggests that efforts focused on geographical concentration of aquaculture sites; high densities of cultured fish at these sites; levels of nutrients and eutrophication; co-infections with bacteria, viruses and sea lice; excessive use of antimicrobials and other treatments for these infections; and climate change would deserve increased research consideration in the future. In their analyses and proposed research agendas, neither of these articles takes into account the negative effects of excessive and perhaps unnecessary antimicrobial use on piscine, human and environmental health as understood by the One Health paradigm.