The use of antimicrobial agents in swine production at subtherapeutic concentrations for the purpose of growth promotion remains controversial due to the potential impact on public health. Beginning at weaning (3 wk), pigs received either non-medicated feed or feed supplemented with subtherapeutic levels of either tylosin (11 to 44 ppm) or chlortetracycline (5.5 ppm). After only 3 weeks, pigs given feed supplemented with tylosin had significantly higher levels of tylosin-resistant anaerobes (p < 0.0001) compared to the control group, increasing from 11.8% to 89.6%, a level which was stable for the duration of the study, even after a two-week withdrawal prior to slaughter. Tylosin-fed pigs had a higher incidence of detection for erm(A), erm(F), and erm(G), as well as significantly (p < 0.001) higher concentrations of erm(B) in their feces. The continuous administration of chlortetracycline-supplemented feed, however, had no significant effect on the population of chlortetracycline-resistant anaerobes in comparison with non-treated pigs (p > 0.05). The resistance genes tet(O), tet(Q), and erm(B) were detected in all pigs at each sampling time, while tet(G), tet(L), and tet(M) were also frequently detected. Neither chlortetracycline nor tylosin increased the growth rate of pigs.
Holman DB, Chénier MR; Impact of Subtherapeutic Administration of Tylosin and Chlortetracycline on Antimicrobial Resistance in Farrow-to-Finish Swine; FEMS Microbiol Ecol. 2013 Feb 11.
doi: 10.1111/1574-6941.12093. [Epub ahead of print]
Source: Department of Animal Science, McGill University, Ste-Anne-de-Bellevue, QC, Canada, H9X 3V9.