Use of antimicrobial agents in animal feeds: implications for human health.

The inclusion of subtherapeutic doses of antimicrobial agents in animal feed is credited for having contributed to lower costs of meat, milk, and eggs. The practice often is associated with the acquisition of resistant enteric flora by the involved animals, a phenomenon that in turn may contribute to the human reservoir of coliforms and salmonellae resistant to antimicrobial agents. Farm workers may transiently acquire resistant intestinal flora and on rare occasions develop salmonellosis. Although irrefutable evidence of the growth-promoting properties of antibiotics in animal feed was provided 30-40 years ago, additional studies--with a focus on mechanisms of the effect--are presently needed. It may be possible to identify factors effective in promoting growth without deleterious effects on the intestinal flora. A national program of surveillance of antimicrobial administration (in both subtherapeutic and therapeutic doses) to food-producing animals should be established. Molecular epidemiologic research efforts must be undertaken to determine whether genetic information of animal origin contributes significantly to the human environmental pool of antimicrobial resistance. In the meantime, it does not appear that the banning of drugs as feed additives, with concomitant unrestricted use of these agents for the treatment of both animals and people, would favorably influence the problems of antimicrobial resistance and salmonellosis in human populations.