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Article Excerpt
In 1969, the Joint Committee on the Use of Antibiotics in Animal Husbandry and Veterinary Medicine in the United Kingdom warned that uncontrolled use of similar antimicrobials in humans and food animals might promote the emergence of resistant strains of foodborne bacteria that could endanger human health and compromise the effectiveness of antimicrobial therapies in human patients (Swann 1969). The Animal Health Institute (AHI) and its member companies collaborated with Cox Associates, an operations research consulting company, to develop and apply new, practical, quantitative risk assessment (QRA) modeling methods to assess the previously impossible-to-quantify risks (and benefits) to human health from continued use of animal antimicrobials. We came to some surprising conclusions that were robust to many uncertainties. Among these were that antimicrobials that benefit animal health may benefit human health, while regulatory interventions that seek to reduce antimicrobial resistance in animals may unintentionally increase illness rates (and hence antimicrobial use and resistance rates) in humans. These new QRA models and methods enable industry and regulatory decision makers to quantify and compare the probable human health consequences of alternative animal antimicrobial use plans and to design more effective approaches to protect human and animal health.
Key words: decision analysis: risk; health care: epidemiology.
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For more than half a century, the agriculture industry has used antimicrobials to prevent bacterial diseases and to promote healthy growth in food animals, such as cattle, poultry, and pigs. Natural selection favors the bacteria in the intestines of these food animals that resist these antimicrobials. Levels of resistance found in practice vary widely both among specific antimicrobial-bacterium (drug-bug) pairs and across countries.
In the United States, the Center for Veterinary Medicine (CVM) of the Food and Drug Administration (FDA) must decide what uses of animal antimicrobials to approve and for what conditions of use. Growers typically use animal antimicrobials as feed additives, mixing them into feed or water at low (subtherapeutic) doses to prevent illness or promote healthy growth or to treat and cure illnesses, such as airsacculitis (a fatal respiratory disease of chickens and turkeys) and necrotic enteritis (NE, a bacterial disease that causes harmful or fatal lesions in the intestines of poultry). They spend approximately 100 million dollars per year on animal antimicrobials used as feed additives in the United States. The Animal Health Institute (AHI) represents the companies that manufacture and sell animal antibiotics and other medications that veterinarians and food producers use. Both the CVM and the AHI are therefore interested in understanding the potential effects of animal antimicrobials on animal and human health, including possible risks from the natural selection of resistant strains of bacteria.
Since the 1960s, scientists, political authorities, and activists in the United States and worldwide have become concerned that animal antimicrobial-resistant bacteria from food animals might reach and infect humans--especially, those with weak immune systems--via undercooked meats, cross-contamination of other foods in the kitchen, or other (possibly unknown) environmental pathways, and cause illnesses that would resist treatment with similar or identical human antimicrobials. Such illnesses might last a long time and perhaps even cause death in rare, severe cases. We use the term treatment failure to describe an antimicrobial treatment that fails to alleviate symptoms as quickly or completely as it would in normally responsive patients. (Treatment failures can occur for reasons other than antimicrobial resistance, such as a patient's intolerance to an antimicrobial drug.) Because of the possibility of resistance-related treatment failures, many people have proposed banning the use in food animals of those classes of antimicrobials used in human medicine.
Industry stakeholders have been unable to reach consensus on the size of this potential threat and to agree on effective risk-management approaches because they lack empirical data demonstrating a causal relation between animal antimicrobial use and adverse effects on public health. Regulatory scientists have noted that "the debate regarding antimicrobial use in animals and subsequent human health implications has been going on for over 30 years, beginning with the release of the Swann report (Swann 1969) in the United Kingdom. The latest report released by the National Research Council (1998) confirmed that there were substantial information gaps that contribute to the difficulty of assessing potential detrimental effects of antimicrobials in food animals on human health" (McDermott et al. 2002, p. 71). Industry scientists are more blunt: "Even though antibiotics have been fed for nearly 50 years to literally billions of animals, there is still no convincing evidence of unfavorable health effects in humans that can be directly linked to the feeding of subtherapeutic levels of antibiotics to swine or other animals" (Cromwell 2002, p. 7). "There has been renewed concern in recent years about the use of antibiotics in food animal production and the potential risk it may pose to public health due to transfer of antibiotic resistance factors via the food supply ... yet there is still no documented case of human treatment failure due to antibiotic resistant bacteria acquired from USDA (US Department of Agriculture) inspected meat and poultry" (Cummings 2006, p. 209).
A Precautionary Response in Europe
Absence of proof of harm is not proof of absence of harm. Harm to human health that is too small to be detected may still be real. Real or not, suspected harm may be unacceptable to regulatory and political decision makers and their constituents. Indeed, in studies of risk perception and communication, researchers have repeatedly found that public concern is largely associated with nonquantitative aspects of perceived risks, including unfamiliarity, scientifically unknown or uncertain risks, mechanisms or processes that are not understood, involuntary and personally uncontrollable exposures (for example, to resistant bacteria in commercially prepared foods), irreversible effects (such as loss of efficacy of current antimicrobials due to the emergence of antimicrobial resistance) caused by human actions, threats specifically to children, and media attention. The perceived risks to human health from antimicrobial use in food animals satisfy most of these conditions for creating high public concern, regardless of what the quantified risks may be. High public concern can readily translate to outrage and calls for legislative and regulatory action if the public does not perceive human health benefits from animal antimicrobial use and if it perceives continued use of animal antimicrobials as letting companies increase their profits in producing food animals while exposing members of the public to needless health risks.
In 1998, the European Union moved from concern to action, banning five classes of animal antimicrobials used to promote growth and to prevent animal bacterial diseases from further use in food animals. It banned the use of the remaining antimicrobials in 2006. The bans were widely viewed as an application of the precautionary principle (Pugh 2002), which may be roughly paraphrased as avoid activities judged to have the potential to cause major, possibly irreversible harm, even if causality has not been fully established. This approach is often applied to biological hazards that cannot be contained easily.
Advocates hailed the banning as a successful act of "political will" that circumvented the deliberate data collection, rational analysis, and causal modeling typical of quantitative risk assessment (QRA) emphasized in the United States. Many advocates, such as the Union of Concerned Scientists, the Alliance for Prudent Use of Antibiotics (APUA), and the activist coalition Keep Antibiotics Working (KAW), urged the United States (and specifically regulatory decision makers at CVM) to drop reliance on QRA and follow Europe's example.
Uncertainty Creates a Risk-Management Dilemma
Discontinuing animal antimicrobial use is not necessarily risk free. A year after the 1998 bans in Europe, the authors of a United States National Academy of Sciences (NAS) study of the potential human-health risks and benefits of animal antimicrobials summarized the scientific situation:
The benefit to human health in the proper use of antibiotics
in food animals is related to the ability of these
drugs to combat infectious bacteria that can be transferred
to humans through direct contact with the sick
animal, through consumption of food contaminated
with pathogens, or through proliferation in the environment
(National Academies of Science 1999, p. 73).
... Some groups have argued for a substantial reduction
in the use of antibiotic drugs in food-animal production.
Others contend that microbial contamination
of animal-food products would increase without the
use of these drugs. The following summaries of data
and studies suggest that antibiotic use in farm animals
is largely beneficial .... [But] The risk [of animal-to-human
transfer of resistance] is greater than zero,
but basically incalculable, and the threat is perceived
to be significant. The use of perceived here is stressed.
The threat might be real, and case studies have shown
that the passage of resistant organisms from animals
to humans can occur and be perpetuated and amplified
through food. The question remains, how likely
is that to happen? The answer is not available and
can be addressed only with the development of the
proper database and effective risk analysis (National
Academies of Science 1999, p. 78).
Thus, as of 1999, regulators and public health officials in the United States and in much of the rest of the world outside Europe were in the uncomfortable position of having identified a potential risk to human health of unknown size--a risk that might or might not be real, that had not been quantified, and that had concerned politicians, scientists, medical practitioners, and activists. These concerned groups advocated using the precautionary principle to risk management--to ban first and resolve uncertainties later. At the time, no one knew the consequences of such bans for human health, and health analysts could not easily predict the number of treatment failures the bans would prevent (by reducing resistant bacteria) nor the number of new illnesses they might cause (by no longer reducing susceptible bacteria). In 1999, it was also unclear how existing data could fill in the gaps and inform effective risk-management decision making and policy making (National Academies of Science 1999).
In the absence of data, assumptions were being used to support calls for...
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