Bacterial resistance to antimicrobial agents used in fish farming: A critical evaluation of method and meaning

The use of antimicrobial agents in aquaculture has resulted in the increase in the frequency of strains resistant to these agents. Potentially these resistant strains can have an impact on the therapy of fish diseases, the therapy of human diseases, or the environment of the fish farms. Analysis of the extent of these impacts is hindered by the limited information available and the variation in methods that have been used. There is, for example, considerable variation in the methods used to measure the sensitivity of strains and in the criteria used to determine the clinical significance of these laboratory data. It is important that some standardisation of sensitivity testing methods is attempted. The available data on the frequency of resistance in fish pathogens suggest that the use of antimicrobial agents in aquaculture has significantly reduced the therapeutic options for treating fish diseases. The data available to assess the impact of the use of these agents in aquaculture on the therapeutic options for the treatment of human infections are incomplete. At present, no quantitative assessment of this risk can be attempted. Considerations of the data on the impact of the veterinary use of these agents on the therapy of human diseases would suggest that the extent of the risk represented by their use in aquaculture is small. The epidemiology of the human pathogens that have been associated with fish would tend to confirm this assessment. There is little data pertaining to the ecology of R plasmids in the natural environment. The significance of these plasmids in transferring resistance determinants from the aquatic compartment to the human compartment can, at present, only be assessed at a theoretical level. However, such a theoretical analysis suggests that the contribution of R plasmids, selected in the aquatic environment, to the frequency of resistance in human pathogens is probably very small. Fish farmers will have to develop methods of husbandry that limit the rate at which resistant strains emerge. Without these changes in husbandry, fish farming will rapidly enter the preantibiotic era. It is probable that these changes will also have the effect of reducing any impact of antimicrobial agents used in aquaculture on the environment outside the fish farm.