Controversies in Science and Technology: From Maize to Menopause

4 Antibiotics in Animal Agriculture: An Ecosystem Dilemma

4 Antibiotics in Animal Agriculture An Ecosystem Dilemma Randall S. Singer Introduction The increasing rate of development of bacterial resistance to antimicrobials has been well documented (Levin et al. 1998; Levy 1997; Levy 1998; Salyers and Amabile-Cuevas 1997). The development of this resistance has resulted in human and animal bacterial pathogens that are unresponsive to many forms of treatment currently available. Bacterial resistance to antibiotics has also become a high-profile, highly politicized health concern. With worries that the ability to treat bacterial infections might soon return to the pre-antibiotic era, researchers are attempting to identify new classes of antimicrobials as well as alternative therapies and prevention measures. When trying to solve this extremely complex problem of increasing rates of resistance, researchers naturally look for those activities that are major contributors and whose alteration or elimination would slow the loss of antibiotic efficacy. Antibiotic use is the major selection pressure influencing this situation, and because large amounts of antibiotics are used in animal agriculture, this practice has been labeled an overuse, and occasionally an abuse, of a valuable therapy for human health. Most researchers and health care professionals will agree that the considerable misuse of antibiotics in human medicine has resulted in the high 67 prevalence of resistant bacterial pathogens affecting human health today. However, there should be little argument that the use of antibiotics in animal agriculture provides a pressure that results in the selection of resistant bacteria and a pool of resistance genes. Because it is well established that bacteria, both resistant and susceptible, can be transferred from animals to humans and subsequently cause disease, the use of antibiotics in animals has some effect on human health. Several high-profile examples have highlighted the potential risk to human health posed by a resistant bacterial isolate that is transferred through the food chain (Fey et al. 2000; Molbak et al. 1999). Although these studies lacked the data to support the notion that antibiotic use in these animals caused the bacterial isolate in question to become resistant, it is clear that any use of antimicrobials will exert a selective pressure that can lead to the creation, amplification, dissemination , or persistence of antibiotic resistance mechanisms. This effect of antimicrobials occurs in many different populations and settings, including animals, plants, humans, and the environment. All these will add to the selective pressures exerted on bacteria; therefore, discerning why an organism has become resistant to an antibiotic or tracing the origin of an antibiotic resistance mechanism is extremely difficult. Why has the use of antibiotics in animals been identified as a major component of the apparently escalating prevalence of resistant bacteria? If the use of antibiotics in animals has this effect, why are antibiotics still used in animal agriculture? Arguments about the use of antibiotics in animals persist not because the question is whether this use results in increasing resistance. The real questions that need resolution are how much does animal agricultural use affect resistance? How significantly does animal antibiotic use affect human health? What are the repercussions to human and animal health of the removal of antibiotics from animals? This chapter focuses on several specific topics that need clarification in order to resolve this debate. First, the importance of the background level of bacterial resistance must be recognized in the context of its relevance to assigning cause-and-effect relationships. Second, we must have a better understanding of risk in order to reach consensus on strategies that will minimize the human health effects associated with bacterial resistance. Third, different types of animal antimicrobial uses must be clarified in the context of the political and scientific policies that exist to regulate these uses. Finally, the ability to generalize the findings of individual studies to a national policy needs to be addressed in the context of U.S. agriculture. 68 s i n g e r For each of these topics, I will make suggestions about the types of studies that need to be conducted in order to make rational and scientifically sound decisions related to this complex issue. Background Levels of Resistance The current threat of antimicrobial resistance cannot be adequately determined strictly through a surveillance of bacterial pathogens. The majority of bacteria in the ecosystem of humans and animals are nonclinical and often exist in a commensal relationship with their host, meaning they are always...