Growth Performance and the Development of Antibiotic Resistant Bacteria in Swine Fed Growth-promoting Antimicrobials

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Title: Growth Performance and the Development of Antibiotic Resistant Bacteria in Swine Fed Growth-promoting Antimicrobials
Author: Holt, Jonathan Paul
Advisors: Eric van Heugten, Committee Co-Chair
Abstract: Three experiments were conducted to study the impact of growth-promoting levels of various antimicrobials on performance and the development of antibiotic-resistant commensal bacteria in swine. In experiment one, feeding subtherapeutic levels of antibiotics, or high levels of zinc and copper did not improve growth performance of nursery pigs housed in a biosecure facility that were farrowed from sows not previously exposed to antibiotics. In experiment two, feeding nursery pigs obtained from a commercial source high levels of zinc resulted in increased ADG and ADFI compared to pigs fed a diet containing no antimicrobials or high levels of copper. In experiment three, subtherapeutic levels of chlortetracycline improved ADG, ADFI, and G:F for only the first week after weaning. Performance improvements were not observed for finishing pigs fed subtherapeutic levels of virginiamycin compared to pigs fed no antibiotics. High levels of zinc may be as effective as subtherapeutic antibiotics at improving nursery pig performance. The use of antimicrobial growth-promoters in clean, well-managed facilities with high labor inputs will not improve growth performance of swine. Feeding various antimicrobials were ineffective at reducing the total number of commensal bacteria isolated from pigs in any study. Although pigs in experiment one were farrowed from sows not exposed to antibiotics for over 33 years, commensal bacteria isolated from these pigs exhibited resistance to multiple antibiotics. E. coli resistant to neomycin were isolated only from pigs fed diets containing subtherapeutic antibiotics in experiment one. In experiment two, feeding pigs diets containing growth-promoting levels of antibiotics, zinc, or copper resulted in increased percent of E. coli resistant to tylosin, erythromycin, and copper. Feeding a diet with subtherapeutic levels of tylosin and sulfamethazine also increased the percent of Enterococcus resistant to tylosin, sulfamethazine, and erythromycin compared to pigs fed no antimicrobials or high levels of zinc. While fluctuations in antibiotic resistance were observed between treatments in experiment two, there were no differences between treatments in percent of bacteria resistant to any antibiotics at the conclusion of the trial. In experiment three, percent of E. coli and Enterococcus isolated from pigs that were resistant to chlortetracycline and virginiamycin fluctuated when monitored for a 19 week grow⁄finish study, however no differences in percent of resistant bacteria were observed between pigs fed diets containing no antibiotics or growth-promoting levels of antibiotics at the conclusion of the trial. In the fourth experiment, manure from pigs receiving growth-promoting antibiotics was spread on soil boxes and subjected to a simulated rainfall event to measure antibiotic resistance in water runoff. A greater amount of antibiotic resistant bacteria were recovered from water runoff at the initial sampling period compared to sampling 30 min after runoff began. Percent of antibiotic resistance isolated from swine can fluctuate over time. Development of antibiotic-resistant bacteria may be increased by the use of antimicrobial growth-promoters, however, resistant bacteria are present in swine facilities regardless of the use of their use. These resistant bacteria can potentially enter the environment through land application of manure. Antibiotic resistance will not be easily reversed by removal of antibiotics from livestock feeds.
Date: 2008-04-28
Degree: PhD
Discipline: Animal Science
Poultry Science

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