Kaneene

Interventions for Controlling Antimicrobial Resistance to Salmonella and Campylobacter in Dairy Cattle

As public health problems, Campylobacter, Salmonella, and E. coli are the most common infectious causes of foodborne illness in the U.S., and the presence of antimicrobial resistance in these bacteria has wide-ranging implications for human health. Foods of animal origin (meat, milk, and eggs) are a common source of these bacteria: while these organisms can cause severe disease in humans, food animals are able to carry these bacteria with no ill effects. On dairy farms, bacteria may acquire resistance to antimicrobial drugs in three major ways: 1) the use of antimicrobial drugs only allows drug-resistant bacteria to survive and proliferate (selection pressure); 2) introduction of cattle that already carry drug-resistant bacteria; and 3) animals on the farm being exposed to drug-resistant bacteria introduced by other means, such as from wildlife or farm workers. If selection pressure from antimicrobial drug use on the farm occurs, then stopping antimicrobial drug use should increase bacterial susceptibility to drugs, which would result in a decrease in bacterial drug resistance. This study proposes to test the hypothesis that Salmonella and Campylobacter isolated from dairy calves will increase in susceptibility to tetracycline when medicated milk replacer is no longer fed. Five matched pairs of dairy farms from Michigan and New York will be recruited. Half of the farms will continue feeding medicated milk replacer, while the other half will begin feeding milk replacer without tetracycline. Samples will be taken from calves (fecal samples) and their housing environments (calving pen and calf housing). Data about herd management and antimicrobial use will be collected. These samples will be processed to identify Campylobacter, Salmonella, and E. coli, and antimicrobial susceptibility testing for tetracycline and 16 other drugs will be conducted using an automated system to determine the minimum level of a given drug needed to inhibit bacterial growth (the minimum inhibitory concentration, or MIC). A subset of Campylobacter isolates will be tested by agar dilution, an alternative method for susceptibility testing, to compare the results between tests. The median MIC for tetracycline in Salmonella, Campylobacter, and E. coli will be determined for each herd, and the effect of not feeding medicated milk replacer on changes in median MICs over time will be analyzed. Regression models will be used to determine the likelihood of bacterial isolates becoming resistant to tetracycline. The findings of this study will provide information about herd management practices that may influence the levels of antimicrobial resistance on dairy farms, which will be invaluable in the dairy industry's efforts to ensure a safer food supply. These data can be used to develop herd management practices targeted at reducing antimicrobial resistance on the farm, and will be provided to dairy farmers, agricultural extension agents, and veterinarians through conferences, informational pamphlets, and a project website. Dairy farmers enrolled in the study will be provided with results of bacterial testing for their herds and for the rest of the project herds.