MICROBIOLOGICAL HAZARDS FOR HUMANS OF ANTIMICROBIAL GROWTH PROMOTER USE IN ANIMAL PRODUCTION

Antimicrobial Growth Promoters (AGPs) are allowed as feed additives, i n the European Union. AGPs use increases farms productivity, and poses few toxicological problems, except microbiological ones. AGPs modify the animal gut flora, in a way that might be harmful for us. First, AG Ps may select antibiotic resistant bacteria in the animal commensal fl ora (e.g., E. coli, Enterococci sp.). This enlarged resistance reservo ir increases the chance (i) of human contamination, (ii) of resistance -plasmids transfer to pathogens, and (iii) of emergence of a new resis tant determinant. Second, AGPs may increase the enteric pathogens excr etion by animals. An enhanced shedding time and/or fecal density of pa thogens would increase the risk of human contamination (e.g., Salmonel la, Campylobacter, Listeria sp.). In the assessment of risks, it is ve ry difficult to distinguish the effects of an AGP from those of other influences operating simultaneously (therapy, contaminations). Risk mu st be assessed in sequential steps: (1) Preliminary in vitro experimen ts, (2) In vivo basic studies (controlled trials, e.g., gnotobiotic an imal models, and experimental farms), (3) Field epidemiology: comparis on between farms, area and periods with and without the AGP exposure, in retrospective and prospective studies (monitoring). Five documented examples are published, suggesting that AGPs use be hazardous for hum ans. Recently was shown the selection of vancomycin resistant enteroco cci by avoparcin. In most cases, however, the antibiotics were not use d according to European regulation on AGPs, and the evidence that anti biotic use in animals was the cause of the hazard was lacking or circu mstantial. Other published studies suggest that AGPs allowed in Europe are not a threat to consumers, bur evidence is also largely circumsta ntial. To conclude, genetic resistance to specific AGPs exists, it may be carried on plasmids, and may transfer from animals to humans. Thus , risks are identified. They are not, however, quantified. The hazard of AGPs to humans has not yet (and may never) be proven or disproved. We may, I think, go on using some AGPs in Europe, provided we remain v igilant, by monitoring prospectively resistance in both animals and hu mans.