Antibiotic resistance as a result of salmon aquaculture in Chile

Antibiotic use in salmon aquaculture produces resistant bacteria with potential to transfer resistance to human pathogens

Chile is the world’s second largest producer of farmed salmon, after Norway. Pathogenic infections are a frequent source of mortality in intensive salmon aquaculture. As stocking densities increase in an effort to increase yield, increases in epizootic diseases are often the limiting factor of productivity. Unlike it Europe where the use of antibiotics in aquaculture is banned, where vaccination and hygienic animal husbandry are the most used methods of disease control, Chile’s primary method of disease control is the excessive use of antimicrobials. The emergence of antimicrobial-resistant pathogens and new pathogens has stimulated even greater use of antimicrobials.

Antimicrobials are placed within medicated feed. Antimicrobials in feed that is not consumed, along with unmetabolised antimicrobials in fish urine and faeces, end up in the surrounding environment, where they select for resistant bacteria and may also encourage horizontal gene transfer of antimicrobial-resistance genes (ARG). There is a potential flow of ARG between environmental bacteria and the pathogens of fish and humans.  

To find evidence of these potential effects of excessive antimicrobial use in aquaculture, this study investigated the antimicrobial resistance of 200 bacterial species sampled from aquaculture areas and non-aquaculture areas, searched within them for ARGs, and tested their ability to horizontally transfer ARGs to the human pathogen E.coli via conjugation.

They found that of the 200 bacterial isolates, 81% displayed antimicrobial resistance to at least one of the eight antimicrobials used. Resistance to the three most commonly used antimicrobials in Chile was 32%, 26% and 53% at the aquaculture site and 22%, 25% and 45% at the non-aquaculture site. There was no significant difference in antimicrobial resistance between the sites. They concluded from this that the effects of aquaculture antimicrobial use are far reaching (the ‘non-aquaculture site’ was located 8km from the aquaculture site).  I think that without additional samples from a much further distance from the aquaculture site, it is hard to say whether there is not just a background level of antimicrobial resistance in the environment. Specific resistance determinant DNA probes were used to identify ARGs within the bacterial isolates, 42 ARGs were confirmed in bacteria from the aquaculture site, whereas only 14 were confirmed in the non-aquaculture site bacteria. 9 multi-resistant bacteria, 6 from the aquaculture site and 3 from the non-aquaculture site were tested for the ability to horizontally transfer ARGs to E.coli, 2 bacteria from the aquaculture site successfully transferred the genes via conjugation.

The implications of this study are profound. They have found that levels of resistant bacteria are much higher in isolates from Chilean marine sediments than in other aquaculture-active countries around the world. They have found that resistance to specific antimicrobials can be as high as 58%, both in aquaculture and non-aquaculture sites. Comparison to other studies shows the severity of Chile’s situation, Chilean isolates showed 32% and 45% resistance to tetracycline and amoxicillin, whereas Washington isolates show 3-9% and 6-14% resistance respectively, and a Danish study showed 4.8% and 14.5% resistance respectively. Horizontal gene transfer of ARGs to E.coli is particularly hard hitting, as it highlights how the excessive use of antimicrobials in aquaculture can have a direct impact on human health. Hopefully the results of this study, which is yet to be published, will have an impact on the use of antimicrobials in Chilean aquaculture. It certainly backs up the EU’s restriction on their use.

Shah, S. Q. A., Cabello, F. C., L'Abée‐Lund, T. M., Tomova, A., Godfrey, H. P., Buschmann, A. H., & Sørum, H. (2014). Antimicrobial resistance and antimicrobial resistance genes in marine bacteria from salmon aquaculture and non‐aquaculture sites. Environmental microbiology.