For over 50 years, antibiotics have been widely prescribed to treat bacterial infections in humans. Many antibiotics commonly used in humans have also been used in animals for therapeutic and other purposes, including growth promotion. Resistance to penicillin, which was the first broadly used antibiotic, started to emerge soon after its widespread introduction. Since that time, resistance to other antibiotics has emerged, and antibiotic resistance has become an increasing public health problem worldwide. Antibiotics kill most, if not all, of the susceptible bacteria that are causing an infection, but leave behind—or select, in biologic terms—the bacteria that have developed resistance, which can then multiply and thrive. Infection-causing bacteria that were formerly susceptible to an antibiotic can develop resistance through changes in their genetic material, or deoxyribonucleic acid (DNA). These changes can include the transfer of DNA from resistant bacteria, as well as spontaneous changes, or mutations, in a bacterium’s own DNA. The DNA coding for antibiotic resistance is located on the chromosome or plasmid of a bacterium. Plasmid-based resistance is transferred more readily than chromosomal-based resistance. Once acquired, the genetically determined antibiotic resistance is passed on to future generations and sometimes to other bacterial species. The dose of antibiotic and length of time bacteria are exposed to the antibiotic are major factors affecting whether the resistant bacteria population will dominate. Low doses of antibiotics administered over long periods of time to large groups of animals, such as doses used for growth promotion in animals, favor the emergence of resistant bacteria. To investigate the impact on human health of antibiotic use in animals, researchers have used both epidemiologic studies alone and epidemiologic studies combined with molecular subtyping of bacterial isolates. Epidemiologic studies examine patterns of health or disease in a population and the factors that influence these patterns. These studies help to identify the cause of a disease and the factors that influence a person’s risk of infection. Many studies investigating antibiotic-resistant bacteria and their impact on human health combine epidemiologic studies with molecular subtyping—also called “DNA fingerprinting”—a technique that translates bacteria’s genetic material into a “bar code” that can be used to identify specific pathogens and link them with disease outbreaks. For example, following an outbreak of a diarrheal disease among people in a community, an epidemiologic study would determine all the common exposures among the people with the disease, and molecular subtyping of bacterial isolates could determine what pathogens were responsible for the disease. While the use of antibiotics in animals poses potential human health risk, it is also an integral part of intensive animal production in which large numbers of poultry, swine, and cattle are raised in confinement facilities. (See fig. 1.) Antibiotics are used in animals to treat disease; to control the spread of a disease in a group of animals when disease is present in some of the animals; to prevent diseases that are known to occur during high-risk periods, such as after transport, when the animals are stressed; and to promote growth—that is, to allow animals to grow at a faster rate while requiring less feed per pound of weight gain. This use of antibiotics is commonly referred to as growth promotion and generally entails using low doses of antibiotics over long periods of time in large groups of animals. Many animal producers believe the use of antibiotics for growth promotion also prevents disease. Antibiotics are generally administered by injection to individual animals and in feed or water to groups of animals. Figure 2 shows how antibiotic-resistant bacteria that develop in animals can possibly be transferred to humans, who may then develop a foodborne illness, such as a salmonella infection, that is resistant to antibiotic treatment. Once the resistant bacteria develop in animals, they may be passed to humans through the consumption or handling of contaminated meat. An animal or human may carry antibiotic-resistant bacteria but show no signs or symptoms of an illness. Resistant bacteria may also be spread to fruits, vegetables, and fish products through soil, well water, and water runoff contaminated by waste material from animals harboring these bacteria, although such routes are beyond the focus of this report. Researchers in human medicine have debated the public health impact of antibiotic use in agriculture for many years. In the United States the debate intensified before FDA approved the first fluoroquinolone antibiotic for use in animals in 1995. At that time, drugs from the fluoroquinolone class had already been used for humans for nearly a decade. Debate focused on whether development of resistance to the drug approved for use in animals could, through cross-resistance, compromise the effectiveness of other drugs in the fluoroquinolone class that were valuable in treating human diseases. Efforts have been made to address the spread of antibiotic resistance by providing education to change behaviors of physicians and the public, but researchers differ on whether changes in agricultural practices are also needed. CDC has undertaken educational efforts aimed at physicians and the public. CDC is encouraging physicians to reduce prescribing antibiotics for infections commonly caused by viruses, such as ear and sinus infections. Patients are being taught that antibiotics are only for bacterial infections, not viral infections. Many researchers contend that efforts to reduce the use of antibiotics in animals are also needed to preserve the effectiveness of antibiotics necessary for treatment of bacterial diseases in humans and animals and to decrease the pool of resistant bacteria in the environment. However, agricultural industry officials argue that antibiotic use in animals is essential to maintaining the health of animals and therefore the safety of food. Professional organizations and associations differ on the use of antibiotics in animals. Many professional organizations that have studied the human health implications of antibiotic use in animals—including WHO and, in the United States, the Institute of Medicine of the National Academy of Sciences and the Alliance for the Prudent Use of Antibiotics—have recommended either limiting or discontinuing the use of antibiotic growth promoters. Many of the professional associations for human medicine— such as the American Medical Association, the American College of Preventive Medicine, the American Public Health Association, and the Council of State and Territorial Epidemiologists—have position statements for limiting antibiotic use in animals for nontherapeutic purposes, such as growth promotion, for antibiotics that are important for both human and animal health. Many of the professional associations for veterinary medicine—such as the American Veterinary Medical Association and the American Association of Swine Practitioners—agree on the goal of reducing the use of antibiotics in animals but differ on the means to achieve this goal. These associations are calling for veterinarians to work with owners of animals to implement judicious use guidelines. While limiting the use of antibiotics in animals for growth promotion may reduce the human health risk associated with antibiotic-resistant bacteria, such restrictions also may increase the cost of producing animals and the prices consumers pay for animal products. For example, a 1999 economic study estimated that a hypothetical ban on all antibiotic use in feed in swine production would increase U.S. consumers’ costs by more than $700 million per year. However, the increase in consumer costs would be much smaller if—as the Institute of Medicine proposed in 2003—producers were allowed to continue to use some antibiotics for growth promotion and only antibiotics that are used in humans were banned for growth promotion. Moreover, in other animal species, such as beef cattle or chickens, the economic impacts of growth promotion restrictions would likely be smaller than in swine because antibiotic use for growth promotion is less prevalent in the production of these other species. Appendix II summarizes studies of the economic effects of banning antibiotic use for growth promotion and other proposed restrictions on antibiotic uses in animals. The three federal agencies responsible for protecting Americans from health risk associated with drug use in animals are FDA, CDC, and USDA. These agencies have a variety of responsibilities related to surveillance, research, and regulation. All three agencies collaborate on surveillance activities, such as the National Antimicrobial Resistance Monitoring System—Enteric Bacteria (NARMS), which was initiated in 1996 because of public health concerns associated with the use of antibiotics in animals. In addition, FDA’s primary responsibilities as a regulatory body focus on human health and animal drug safety. CDC primarily conducts research and education that focus on human health. USDA oversees the retail meat trade, including related farm and slaughter operations. USDA activities may include studies of healthy farm animals, evaluations of diagnostic data involving sick animals, and biological sampling from slaughter and meat processing plants. USDA also conducts research and education related to antibiotic resistance. In addition, FDA approves for sale and regulates the manufacture and distribution of drugs used in veterinary medicine, including drugs given to animals from which human foods are derived. Prior to approving a new animal drug application, FDA must determine that the drug is safe and effective for its intended use in the animal. It must also determine that the new drug intended for animals is safe with regard to human health. FDA considers a new animal antibiotic to be safe if it concludes that there is reasonable certainty of no harm to human health from the proposed use of the drug in animals. FDA may also take action to withdraw an animal drug from the market when the drug is no longer shown to be safe. These three agencies also participate in the federal Interagency Task Force on Antimicrobial Resistance. Task force activities focus on antibiotic resistance from use of antibiotics in animals, as well as the human use of antibiotics. In January 2001, the task force developed an action plan based on advice from consultants from state and local health agencies, universities, professional societies, pharmaceutical companies, health care delivery organizations, agricultural producers, consumer groups, and other members of the public. The action plan includes 84 action items, 13 of which have been designated as top-priority items and cover issues of surveillance, prevention and control, research, and product development. A federal agency (or agencies) is designated as the lead for each action item. The United States is one of the world’s leading exporters of meat. In 2002, U.S. meat exports accounted for about $7 billion.