Dose Imprecision and Resistance: Free-Choice Medicated Feeds in Industrial Food Animal Production in the United States

Article excerpt

BACKGROUND: Industrial food animal production employs many of the same antibiotics or classes of antibiotics that are used in human medicine. These drugs can be administered to food animals in the form of free-choice medicated feeds (FCMF), where animals choose how much feed to consume. Routine administration of these drugs to livestock selects for microorganisms that are resistant to medications critical to the treatment of clinical infections in humans.

OBJECTIVES: In this commentary, we discuss the history of medicated feeds, the nature of FCMF use with regard to dose delivery, and U.S. policies that address antimicrobial drug use in food animals.

DISCUSSION: FCMF makes delivering a predictable, accurate, and intended dose difficult. Overdosing can lead to animal toxicity; underdosing or inconsistent dosing can result in a failure to resolve animal diseases and in the development of antimicrobial-resistant microorganisms.

CONCLUSIONS: The delivery of antibiotics to food animals for reasons other than the treatment of clinically diagnosed disease, especially via free-choice feeding methods, should be reconsidered.

KEY WORDS: antibiotic resistance, antibiotics, antimicrobials, cow, feed blocks, industrial food animal production, livestock, medicated feed supplements, poultry, swine. Environ Health Perspect 119:279-283 (2011). doi:10.1289/ehp. 1002625 [Online 28 October 2010]

Animal feed is a broad term that, for animals produced for-human consumption (e.g., dairy and beef cattle, hogs, layer hens, broiler chickens, turkeys), includes a diet that is specific to an animal's species, age, and production stage, and may vary according to time of year and plant species grown (Forbes 2007). Feeds are often provided to food animals on a "free-choice" basis, which means that animals elect whether or not to eat the feed and how much of it to ingest. Feeds containing medically active ingredients, such as antibiocic and antiparasitic drugs or insecticides, that are fed on a "free-choice" basis, are designated free-choice medicated feeds (FCMF) by the U.S. Food and Drug Administration (FDA 201 Oh). The FDA has approved 685 drugs for medicated feed, some of which are consumed on a free-choice basis (for produce list in a searchable database, see FDA 2010d).

The FDA reported that 13.1 million kg of antimicrobial drugs were sold or distributed for use in food-producing animals in 2009 (FDA 20101). Many of the same antibiotics or classes of antibiotics used to treat clinical infections in humans are also used in industrial food animal production (IFAP) (McEwen and Fedorka-Cray 2002).

The use of FCMF in food animals has been associated with imprecise drug intake, leading to under- or overadministration of drugs (Figure 1) (Bogan and Marriner 1983; Hall 2000; Toutain et al. 2010). Overadministration of drugs may lead to animal toxicity (Guardabassi and Kruse 2008; Hall 2000) and the presence of drug residues in meat or milk, although this is rare (Burch et al. 2008). Underadministration or inconsistent administration of drugs may lead to animal treatment failure (Burch et al. 2008) or the emergence of antibiotic-resistant strains of microorganisms in food animals (Guardabassi and Kruse 2008; Khachatourians 1998; Lees et al. 2006). Antibiotic-resistant commensal and environmental bacteria can contribute to maintaining or perpetuating a reservoir of resistance genes (Chee-Sanford et al. 2009; Silbergeld et al. 2008; Wright 2007), and these bacteria can share genes for antibiotic resistance with pathogenic bacteria via horizontal gene transfer (Andremont 2000). Multiple resistance genes travel on the same mobile genetic element (e.g., plasmid), allowing one pharmaceutical to select for microorganisms that are resistant to multiple classes of antibiotics (Wright 2007). In addition to natural selection and horizontal gene transfer as mechanisms for resistance, sublethal bactericidal antibiotic use at doses below those expected to provide overt selective pressure induces mutations in bacterial genomes that may confer antibiotic resistance (Kohanski et al. …