The development and dissemination of bacterial resistance to 3rd generation cephalosporin antimicrobials is of significant importance to public health and domestic livestock production alike. The World Health Organization recognizes the extended-spectrum cephalosporin antimicrobials as “critically important” and their use in food animal production is coming under increasing scrutiny (1). Reports of recovery of blaCMY-2 and blaCTX-M resistance genes in bacterial isolates from U.S. livestock, fresh retail meat, and human diagnostic samples represent a potential pathway for the selection and distribution of cephalosporin-resistant pathogens and subsequent zoonotic infection of humans (2-5).
There is widespread concern that modern commercial food animal production practices are contributing significantly to this public health issue by maintaining a reservoir for resistance genes and providing multiple potential zoonotic transmissions of these genes to humans including foodborne, direct contact, and environmental routes of infection (6). Public health research in this area frequently cites the mass administration of antimicrobials to animals in Concentrated Animal Feeding Operations (CAFOs) as an important factor in the selection, propagation, and dissemination of resistant bacteria. The common assertion in both the scientific and popular literature is that the majority of antimicrobial use in CAFOs is for purposes of growth promotion and prophylaxis of diseases resultant from high-density food animal production, such as respiratory and enteric conditions, which can cause significant production losses at sub-clinical levels. The annual reports of total tonnage of antimicrobials used in U.S. livestock production are frequently cited as supporting evidence of this assertion (Figure 1). This data is collected by the Food and Drug Administration (FDA) under a 2008 amendment to the Animal Drug Fee User Act (7). While these annual reports are currently the best available measure of antimicrobial consumption by the U.S. food animal production industry, several limitations of the use of this data exist (7). The annual report only contains information on total amount of antimicrobials sold each year and does not describe food animal species, indication for treatment, dosage, or route of administration (7). Further, a significant portion of the annual reported tonnage (28-30%) is made up of the ionophore antimicrobial class, which is not utilized in human medicine and is not likely to significantly contribute to antimicrobial resistance of bacteria of public health concern (8). The FDA reported sales of cephalosporin antimicrobials as 41,328 kg, 24588 kg, and 26,611 kg for the years 2009, 2010, and 2011 respectively (7).
While the understanding of the contribution of antimicrobial use in CAFOs to resistant pathogens in human and animal infections is still developing, the precautionary principle is frequently cited in public...