Controversies in Science and Technology: From Maize to Menopause

5 The Impact of Antibiotic Use in Agriculture on Human Health and the Appropriate Public Policy Response

5 The Impact of Antibiotic Use in Agriculture on Human Health and the Appropriate Public Policy Response Tamar Barlam The discovery of antibiotics was one of the most important advances in medical treatments. However, the prevalence of antibiotic-resistant infections is increasing. Although resistance to antibiotics will develop even with appropriate use,1 the inappropriate prescribing by physicians and poor infection-control practices accelerate the problem. In addition, the growing scientific consensus is that antibiotic use in the production of food animals is an important factor. As described in previous chapters, antibiotics are given in animal agriculture therapeutically to animals that are sick or those closely exposed to sick animals in a flock or herd. Antibiotics also are given nontherapeutically , often as a feed or water additive, to healthy animals for purposes of growth promotion (antibiotics are thought to improve feed efficiency, allowing an animal to grow more rapidly on less food) and disease prevention . Disease is anticipated because the animals are raised in crowded, often stressful, conditions and frequently become ill. Most antibiotics used in agriculture are given without the advice of a veterinarian and are available to farmers as medicated feed additives or over the counter. 83 The importance of agricultural antibiotic use in promoting antibioticresistant infections in people is controversial. There are three potential pathways by which antibiotic-resistant bacteria that originate in animals can reach people to cause disease (see figure 5.1). First, people who handle or eat improperly prepared food that is contaminated with bacteria can become ill. Second, the bacteria directly infect farmworkers, who may then in turn infect family members and eventually the community. And, finally, the bacteria and undigested antimicrobials can leach from farm waste into the environment and cause a public health risk. In this chapter, each of these pathways is discussed, followed by a discussion of an appropriate public health response. The Food Pathway Figure 5.2 describes the steps by which antibiotic use in livestock and poultry leads to antibiotic-resistant infections in people. First, antibiotics are administered to animals. In human medicine, it is generally believed that the greater the use, the greater the risk. Therefore, if large amounts of antibiotics are used in agriculture, that would likely pose a significant health risk. Second, the antibiotics select for antibiotic-resistant bacteria. Third, those antibiotic-resistant bacteria contaminate meat and poultry products. Finally, antibiotic-resistant bacteria appear in human bodies. To 84 ba r l a m Figure 5.1 Possible routes of exposure/transfer. Antibiotics Poultry Cattle Pigs Food Environment Consumers Consumers Farm workers establish this final step, evidence of increasing antibiotic resistance in those bacteria most closely linked to animals must be found in affected humans. Although bacteria not carried by food may also acquire antibiotic resistance indirectly from antibiotic use in food animals through plasmid transfer of resistance elements, the focus of this chapter is on bacteria directly linked to animals—Salmonella, Campylobacter, E. coli, and enterococci. Antibiotic Use in Food Animals Three major sources have estimated the amount of antibiotics given to livestock and poultry (Animal Health Institute 2002; Institute of Medicine 1989; Mellon, Benbrook, and Benbrook 2001). In 1989, the Institute of Medicine estimated that almost thirty-two million pounds of antibiotics were used in animals in 1985 and approximately half of that amount was for growth promotion. The Animal Health Institute, which represents the pharmaceutical companies that produce antibiotics for animals, estimated that 18.2 million pounds were used in 2001 for treatment and prevention of disease and 3.7 million pounds were used for growth promotion. The estimate included all animals, including companion animals. The Union of Concerned Scientists (UCS) estimated that 24.6 million pounds of antibiotics were used nontherapeutically in food animals, and more than half Impact of Antibiotic Use in Agriculture 85 Figure 5.2 Antibiotic use in animal agriculture and its impact on human infections: the food pathway. Commensal Bacteria Pathogenic Bacteria AntibioticResistant Infections in People Antibiotic Use in Livestock and Poultry AntibioticResistant Bacteria on Meat and Poultry AntibioticResistant Bacteria in Livestock and Poultry of the antibiotics were closely related or identical to antibiotics used by people (see table 5.1). In addition, UCS estimated that three million pounds were used in human medicine. Although the three estimates differ in many respects, all show that vast quantities of antimicrobial agents are being used in the agricultural sector. Antibiotic-resistant Bacteria in Animals...