U.S. industrial animal agriculture routinely incorporates low-dose concentrations of antimicrobials into the feed or water of healthy production animals for the purposes of growth promotion and feed efficiency, an application approved by the U.S. Food and Drug Administration (FDA). This practice selects for resistance among bacteria exposed to antimicrobials, and there has been concern that such resistance could negatively impact public health. Considerable evidence is accumulating that these resistant organisms (and/or antimicrobial residues) move beyond the food animal production operation via 1. food products, 2. soils (upon which animal wastes are applied), 3. water (waste runoff into surface streams and seepage into underground aquifers), 4. crops (antimicrobial uptake from soil), 5. air (blown out of animal holding facilities by industrial tunnel fans), 6. insect carriage (e.g., flies), 7. rodent carriage and 8. human carriage (e.g., farm personnel).

During a time when bacterial resistance to an array of antimicrobials is increasing, renewed attention has been directed toward the threat that resistance arising from low-dose use of antimicrobials on food animal production farms could pose for human and veterinary pharmaceuticals, particularly with fewer novel antimicrobials reaching the market. We now know that resistance to antimicrobials can develop rapidly, extend to other antimicrobials in the same or a different class, and be shared among bacteria through multiple genetic exchange mechanisms within or between genera, culminating in multi-drug resistance in some organisms. While the FDA has recognized the threat that resistance might present, regulatory action has been slow to evolve on this problem, particularly in an atmosphere of industry pushback. Nevertheless, discontinued use of antimicrobials for non-therapeutic use has been called for by the World Health Organization, the World Organization for Animal Health, the Food and Agricultural Organization, the American Medical Association, the American Public Health Association, the American College of Preventive Medicine, the Council of State and Territorial Epidemiologists, the Infectious Diseases Society of America and others.

The American Veterinary Medical Association (AVMA) has a significant role in counseling animal agriculture on issues of animal and public health, and has taken an important step on the resistance question by creating the Antimicrobial Use Task Force, charged with reviewing the judicious use of veterinary antimicrobials, including the use of these compounds for growth promotion and feed efficiency. The report from the task force was due in April, and a decision by the AVMA on this issue is expected this summer. The AVMA is a keen advocate for veterinarians as well as public health and, while the association has thus far supported the position of industry in the use of low-dose antimicrobials (pending results from the task force), I am hopeful, as a veterinarian and AVMA member, that my professional association will reformulate its position to conscientiously guide the veterinary community toward improved management strategies that will protect the efficacy of the antimicrobials we depend on daily for therapeutic use in sick animals and reduce the risks of bacterial resistance. This can be accomplished by disallowing the use of antimicrobials as growth promoters and requiring that antimicrobials be available only through veterinary prescription for use in unhealthy animals or in animals at immediate risk of contagious exposure to diseased animals.

The AVMA, as stated in its response to the recent Report of the Pew Commission of Industrial Farm Animal Production, is fully aware of the selection for resistance in bacteria exposed to antimicrobials, but believes that the use of these compounds at low-doses for growth promotion is also effective against inapparent, subclinical disease that could otherwise lead to costly, unsafe outbreaks. Industrial agriculture and the AVMA maintain that the use of low-dose levels of antimicrobials in healthy animals is ethically correct because concerns over food safety (requiring low-dose antimicrobials in their view) outweigh potential harm from resistant bacteria, while enabling efficient production. The AVMA position likely has some validity in the context of current industrial animal production environments (Confined Animal Feeding Operations – CAFOs) where disease risks can be heightened and growth rate performance reduced by stressors such as poor ventilation and hygiene, inadequate temperature regulation and animal crowding interfering with natural behaviors. Elevated risks have led to a dependence on low-dose antimicrobials to compensate for these suboptimal husbandry practices made worse by large numbers of animals producing large quantities of untreated wastes that often trigger respiratory distress in a microbially rich environment.

Production animal operations in Denmark have been cited by the AVMA as demonstrating the need for continuous antimicrobials at low doses in feed or water since bans on antimicrobials as growth promoters were followed in some cases by an increase in disease levels, especially in weaner pigs. However, a fuller examination of the data from the Scandinavian countries reveals that these disease spikes did not always occur, and when they did, could be controlled by evidence-based management protocols that reduced antimicrobial resistance. That is, with feed restrictions that had lower protein content, strict sanitation protocols, more humane treatment and the use of antimicrobials by prescription as needed for sick animals, animal production did not suffer following growth promoter bans, nor was there increased mortality. While believing that animal health and welfare are threatened by the ban, the AVMA does acknowledge that the Denmark data do “show that swine production, average annual number of piglets per sow, and weaned and finishing (just prior to slaughter) pig average daily weight gains have increased and weaned pig mortality (death rate) has drastically decreased in recent years”. By encouraging industry toward more sophisticated, time-tested husbandry practices, combined with the use of antimicrobials as needed by veterinarians for sick animals, the animal production industry can operate efficiently while addressing the root causes of disease and bacterial resistance that will simultaneously eliminate the need for antimicrobials as growth promoters or as deterrents to subclinical disease, while reducing public health risks.

Antimicrobials are critical for contemporary human and veterinary medicine, and all interventions should be considered that protect and conserve their value. If use of low-dose antimicrobials for growth promotion can be safely discontinued by adopting other strategies for disease prevention, not only will the expense of these antimicrobials be recovered by the producer, but the levels of resistant organisms escaping from the farm environment will be mitigated. By making antimicrobials available for farm use only through veterinary prescription, prudent and transparent application of these valuable pharmaceuticals will be better assured, while the reduction of resistant bacteria achieved by withdrawing their low-dose use as growth promoters will help preserve their efficacy.

Raymond Tarpley is a veterinarian in College Station, TX, retired from the faculty at Texas A&M, with an interest in acquainting veterinary students, veterinarians and biologists with the emerging field of Conservation Medicine.  Conservation Medicine promotes an interdisciplinary, systemic approach to sustainable global wellness by investigating the links between human, animal and ecosystem health.  He conducts summer workshops to introduce this theme to students and professionals in the context of marine and coastal ecosystems. He is currently enrolled in the MPH program at the Johns Hopkins Bloomberg School of Public Health.