The Effects on Human Health of Subtherapeutic Use of Antimicrobials in Animal Feeds (1980) / Chapter Skim
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1. The Use of Antimicrobial Agents
1. The Use of Antimicrobial Agents
Pages 1-11
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are isolated from microorganisms, strains of some microbial species have predictably evolved the capacity to inactivate them or became impermeable to them, i.e., these strains have developed resistance to these antibiotics. Resistance to synthetic antimicrobial agents arises from the variation normally displayed by individual microorganisms within species.
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However, the continuous use of subtherapeutic levels of antimicrobials in animal feeds for growth promotion, improvement of feed efficiency, and disease prophylaxis has been criticized as posing dangers to human health by making an important quantitative contribution to the pool of antimicrobial-resistant bacteria that may be transferred to the human population. Possible "qualitative" effects of the selection pressure imposed by subtherapeutic usage on resistance profiles or transfer mechanisms are discussed below and by O'Brien (Appendix I)
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Such a shift in the composition of the enteric flora may facilitate infection by a pathogen (Seelig, 1966~. Another important consideration in the evaluation of possible effects on human health is the possibility of "qualitative" as well as quantitative changes i n resistance brought about by the continuing selection pressure exerted by subtherapeutic levels of antimicrobials in animal feeds.
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A thorough assessment of the significance for human health of various uses of antimicrobials requires knowledge of the consequences of the selection pressures imposed by intermittent, therapeutic doses versus subtherapeutic continuous feeding and of different routes of administration. Unfortunately, there are insufficient data comparing These regimens.
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Thus, R plasmids are endowed with genes that increase the probability of survival of host cells in the presence of combinations of antimicrobials. The continued spread among bacteria of resistance to more than one antimicrobial and the further acquisition of additional resistance genes by individual R plasmids results from the selection pressure imposed by the use of antimicrobials.
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estimated from dispensed prescription data and FDA certification records that approximately 190 million prescriptions for the major ant~microbials were filled for ambulatory patients in the United States in 1977. This is nearly one course of treatment per year for each person in the United States and includes approximately 43 million prescriptions for tetracycline.
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This is not to say that resistant _ cold of animal and poultry origin cannot reach the human population: clearly they can and do (Lipton et al., 1977~. And some resistant salmonellae of animal origin certainly seem to have caused serious human epidemic disease (Anderson, 1968[b]
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8 TABLE 1 Antibiotic Production from 1950 to 1978 (millions of kg~a~b Medicinal Use in Added to Animal Feed and Year Total Humans and Animals Other Applications 1978 11.66 6.08 5.58 1977 10.48 6.35 4.58 1976 9.30 4.
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From page 9... ...
Since most herds and flocks receive antimicrobials somewhere in the production chain either for growth promotion, prophylaxis, or therapy, it is difficult to identify slaughtered livestock that have not been given antimicrobials or have not been exposed to animals that had. DEF INITION OF HUMAN HEALTH HAZARD The difficulty of determining whether human health is affected by the subtherapeutic use of antimicrobials in feeds is compounded by the diversity of opinions concerning the definition of a hazard to human health.
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There are no data from which to predict quantitatively the change in the morbidity and mortality of humans that might result from an increased prevalence of resistant bacteria in animals or from the transfer of these organisms to humans. Although a measurable risk to human health cannot be ascribed to these phenonema, they remain plausible potential hazards since some of the individual steps in the transmission chain to humans have been independently demonstrated but not quantified (see Chapter 3~.
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. Although pertinent information is limited, the committee concurs with Adkinson's conclusion that "there is little reason to believe that foodstuffs obtained from animals fattened with antibiotic-supplemented feed impose a significant risk to human health by contributing to antib~otic-induced allergic reactions." Adkinson indicated that further investigations in several areas could provide information that would be useful in clinical situations.
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