Effects of In-Feed Chlortetracycline Prophylaxis in Beef Cattle on Animal Health and Antimicrobial-Resistant Escherichia coli

Abstract

ABSTRACT Concerns have been raised that in-feed chlortetracycline (CTC) may increase antimicrobial resistance (AMR), specifically tetracycline-resistant (TET r ) Escherichia coli and third-generation cephalosporin-resistant (3GC r ) E. coli . We evaluated the impact of a 5-day in-feed CTC prophylaxis on animal health, TET r E. coli , and 3GC r E. coli . A control group of cattle ( n = 150) received no CTC, while a CTC group ( n = 150) received in-feed CTC (10 mg/lb of body weight/day) from the 5th to the 9th day after feedlot arrival. Over 25% (38/150) of the animals in the control group developed illnesses requiring therapeutic treatment with antimicrobials critically important to human medicine. Only two animals (1.3%) in the CTC group required such treatments. Fecal swab and pen surface occurrences of generic E. coli (isolated on media that did not contain antimicrobials of interest and were not isolated based on any specific resistance), TET r E. coli , and 3GC r E. coli were determined on five sampling occasions: arrival at the feedlot, 5 days posttreatment (5 dpt), 27 dpt, 75 dpt, and 117 dpt. On 5 dpt, TET r E. coli concentrations were higher for the CTC group than the control group ( P < 0.01). On 27 dpt, 75 dpt, and 117 dpt, TET r E. coli concentrations did not differ between groups. 3GC r E. coli occurrences did not differ between control and CTC groups on any sampling occasion. For both groups, generic, TET r , and 3GC r E. coli occurrences were highest on 75 dpt and 117 dpt, suggesting that factors other than in-feed CTC contributed more significantly to antimicrobial-resistant E. coli occurrence. IMPORTANCE The occurrence of human bacterial infections resistant to antimicrobial therapy has been increasing. It has been postulated that antimicrobial resistance was inevitable, but the life span of the antimicrobial era has been prematurely compromised due to the misuse of antimicrobials in clinical and agricultural practices. Direct evidence relating the use of antimicrobials in livestock production to diminished human health outcomes due to antimicrobial resistance is lacking, and the U.S. Food and Drug Administration has taken an approach to maximize therapeutic efficacy and minimize the selection of resistant microorganisms through judicious use of antimicrobials. This study demonstrated that prophylactic in-feed treatment of chlortetracycline administered for 5 days to calves entering feedlots is judicious, as this therapy reduced animal morbidity, reduced the use of antimicrobials more critical to human health, and had no long-term impact on the occurrence of antimicrobial-resistant E. coli .