Antibiotics can be administered orally or parenterally in swine production, which may influence antimicrobial resistance (AMR) development in gut bacteria. A total of 40 barrows and 40 gilts were used to determine the effects of tylosin administration route on growth performance and fecal enterococcal AMR. The antibiotic treatments followed Food and Drug Administration label directions and were as follows: (1) no antibiotic (CON), (2) 110 mg tylosin per kg feed for 21 d (IN-FEED), (3) 8.82 mg tylosin per kg body weight through intramuscular injection twice daily for the first 3 d of each week for 3 weeks (IM), and (4) 66 mg tylosin per liter of drinking water (IN-WATER). Antibiotics were administered during d 0 to 21 and all pigs were then fed the CON diet from d 21 to 35. Fecal samples were collected on d 0, 21, and 35. Antimicrobial susceptibility was determined by microbroth dilution method. No evidence of route × sex interaction (p > 0.55) was observed for growth performance. From d 0 to 21, pigs receiving CON and IN-FEED had greater (p < 0.05) average daily gain (ADG) than those receiving IM, with the IN-WATER group showing intermediate ADG. Pigs receiving CON had greater (p < 0.05) gain-to-feed ratio (G:F) than IM and IN-WATER, but were not different from pigs receiving IN-FEED. Overall, enterococcal isolates collected from pigs receiving IN-FEED or IM were more resistant (p < 0.05) to erythromycin and tylosin than CON and IN-WATER groups. Regardless of administration route, the estimated probability of AMR to these two antibiotics was greater on d 21 and 35 than on d 0. In summary, IM tylosin decreased ADG and G:F in finishing pigs, which may be because of a response to the handling during injection administration. Tylosin administration through injection and feed resulted in greater probability of enterococcal AMR to erythromycin and tylosin compared with in-water treatment.
Keywords: administration route; antimicrobial resistance; fecal enterococci; finishing pig; growth performance; tylosin.