| Summary: | Carbadox is an antibiotic used to control dysentery and promote growth in swine in the United States; however, the drug also causes tumors and birth defects in laboratory animals. Despite this and because the drug has no analogs in human medicine, it is not considered “medically important” and can be used in livestock without veterinarian oversight. In their recent study, T. A. Johnson et al. (mBio 8:e00709-17, 2017, https://doi.org/10.1128/mBio.00709-17) demonstrated that carbadox has profound effects on the swine gut microbiome, including the induction of transducing phage carrying tetracycline, aminoglycoside, and beta-lactam resistance genes. In swine production, carbadox can be used in conjunction with other antibiotics (e.g., oxytetracycline) that could fuel the emergence of strains carrying phage-encoded resistance determinants. Johnson et al.’s findings underscore the potential unforeseen consequences of using antibiotics in livestock production and call into question our current methods for classifying whether or not a veterinary drug has relevance to human health.
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