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Antibiotic Usage and Risk Factors for Antimicrobial Resistance in Pork Production


We propose a cross-sectional epidemiological study to determine if a dose-response relationship in antibiotic resistance exists among Esherichia coli, Salmonella, and Campylobacter isolated from swine operations that make regular use of antibiotics and organic farms that use virtually no antibiotics. We will also compare management variables and measures of production efficiency between organic and conventional farms.

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Antibiotic use in farm animals has been implicated as contributing to the alarming increase in antibiotic resistance in bacteria that are pathogenic to humans. Many countries, including the United States, are seriously considering imposing complete bans on the use of antibiotics in farm animals. The livestock industry is concerned that such bans will lead to an enormous increase in the cost of food production due to an increase in disease and a reduced growth efficiency. It needs to be determined if a reduction in the use of antibiotics used for food animals could eventually help reverse the trend toward ever increasing rates of antimicrobial resistance.
The long-term objectives for our research are to <ol>
<li>Determine the occurrence of Salmonella spp. (including Salmonella typhimurium DT104) and Campylobacter spp. (including Campylobacter jejuni and coli) on organic (no use of antibiotics) and conventional farms in Midwestern farms producing slaughter pigs,
<li>Understand the epidemiological features of shedding and prevalence of these bacteria,
<li>Understand the importance of antibiotic use as a determinant of antibiotic susceptibility,
<li>Identify risk factors related to increased antimicrobial resistance in Salmonella spp. and Campylobacter spp., and those management factors used to replace the need for antibiotics,
<li>Investigate production efficiency in organic farms as compared to conventional farms, and 6) educate the swine production industry regarding the need for prudent use of antmicrobials. </ol>
A total of 70 conveniently selected swine herds (35 organic farms and 35 conventional farms) will be used for this study. To be included in the study, farms must market at least 2000 slaughter pigs per annum. Organic farms must have adhered to the rules and regulations of their organic association such that no antibiotics were used on the farm for at least 3 years. Each herd will be visited once in the summer and once in the winter. At each visit, 15 randomly identified pigs will be selected for sampling from the group of pigs that is closest to go to slaughter. These older pigs have had the greatest potential exposure to antibiotics. Also, the older animals are closer to enter the human food chain where they can expose people to foodborne pathogens. Data will be collected from all farms regarding management factors such as nutrition, disease control methods, physical facilities, farm personnel, animal isolation and quarantine, reproduction, animal density, and measures of herd productivity and efficiency. Questions will focus on those procedures and practices which are used to replace the need for antimicrobials. Once received in the laboratory, samples will be processed in accordance with standard diagnostic microbiological procedures for each animal species. Fecal samples will be enriched in Tetrathionate broth and then plated onto Brilliant Green and XLT4 agar plates. Fecal samples will also be inoculated into Campy medium and incubated under micro-aerophilic conditions. The bacteria will be identified using conventional identification schemes. The identification scheme used will be Gram stain dependent and may include, but not be limited to, Gram staining, catalase, coagulase, API 20E, API 20NE, TSI, MIO, and Urea. Once the bacteria have been identified, they will be sub-cultured for in vitro susceptibility testing using the broth microbroth dilution as described in the National Committee for Clinical Laboratory Standards (NCCLS) M31-T document. Once the testing is complete, multiple colonies from fresh subcultures will be suspended in sterile defibrinated sheep blood and frozen at -70 degrees Celsius. In vitro antimicrobial susceptibility testing for Salmonella and E. coli will be determined using a broth dilution breakpoint method in which the antimicrobials are prepared commercially in a microtitre plate format (Sensititre Ltd, Crawley, UK). In vitro susceptibility testing for Campylobacter will be determined using the 'Etest' for Antimicrobial Resistance Testing' (AB Biodisk North America Inc., New Jersey, U.S.A.).
Microbial and clinical data show that increasing bacterial resistance is making it more difficult to treat serious bacterial infections in both animals and humans. Reports suggest that the use of antibacterial agents in animals is a cause of this problem. This study is designed to determine an association between the use of antimicrobial agents in swine production and the presence of antimicrobial resistance in human foodborne pathogens isolated from slaughter pigs.

Norby, Brandon
Michigan State University
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