Approximately 20% of U.S. beef cattle receive prophylactic in-feed administration of chlortetracycline (CTC) to reduce bovine respiratory disease (BRD) incidence during the transition into feedlots. To determine the impact of prophylaxis on selected antimicrobial resistance genes (ARGs), 300 beef cattle were placed into 10 pens (30 head/pen). Five "CTC group" pens received in-feed CTC (10 mg/lb of body weight/d) from the fifth to ninth day after feedlot arrival, whereas the five "Control group" pens received no CTC. Fecal swabs and pen surface materials were collected for metagenomic DNA isolation on five sample occasions: arrival at the feedlot, 5 d posttreatment (dpt), and 27, 75, and 117 dpt. For each sample occasion, fecal samples and pen surface material samples were pooled by pen. Quantitative polymerase chain reaction was used to determine the abundances of 10 ARGs. Due to low detection percentages (%D) and quantification percentages (%Q), the abundances of five ARGs were not analyzed: aac(6')-Ie-aph(2'') (%D = 43%, %Q = 4%), blaCMY-2 (%D = 41%, %Q = 0%), blaCTX-M (%D = 0%, %Q = 0%), blaKPC-2 (%D = 21%, %Q = 16%), and mecA (%D = 4%, %Q = 0%). The %D and %Q for the ARGs aadA1, erm(B), tet(A), tet(B), and tet(M) were ≥98% and ≥90%, respectively. The abundances of aadA1, erm(B), tet(A), tet(B), and tet(M) resistance genes did not differ (p > 0.05) between the CTC and control groups at any sampling time for feces or pen surface material. Although only 10 ARGs were examined in this study, the results suggest that a single 5-d in-feed CTC prophylaxis of beef cattle to prevent BRD has a negligible impact on the abundances of ARGs.
Keywords: antimicrobial resistance; beef cattle; chlortetracycline; in-feed; prophylaxis; qPCR.