To our knowledge, no comprehensive risk assessment of agricultural uses of antimicrobial agents has been published. The published risk assessments of antimicrobial use in farm settings are all subject to multiple, serious limitations in scope, including (1) limitation to one species of microorganism; (2) limitation to one or a very few related antimicrobial agents; (3) limitation to a single outcome (death, hospital days, number of illnesses, etc.); (4) limitation to one species of farm animal

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... e.g., chicken or swine); and (5) limitation to therapeutic use, despite reason for concern about misstated, off-label, or illegal use. In addition, all of the risk assessments reviewed overlooked important issues by accepting 2 further limitations: (6) limiting the scope of the analysis to what has already happened and ignoring the effects of continuing the practices of recent years; and (7) examining only the effects on the species of microorganism that was initially affected and ignoring the cross-species spread of resistance by plasmid transfer. After our review of the risk assessments now available, we propose a comprehensive scheme for organizing existing knowledge and dealing with critical gaps. NEED FOR A MODEL TO EVALUATE RISK Antimicrobials are used extensively in both veterinary and human medicine, and the problem of emerging resistance to these important drugs thus substantially affects both fields. The emergence of antimicrobial resistance in human populations is a public health problem of continually growing importance, and the emergence of antimicrobial resistance in animal populations has important economic implications. The association between animals and humans suggests that resistance does not arise separately in these 2 groups. The prudent use of antimicrobial agents in both groups requires an ability to relate the risk of emerging antimicrobial resistant pathogens to antimicrobial use.