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Mann, Sabine (2011): Antimicrobial Susceptibility of Fecal Escherichia coli Isolates in Dairy Cows Following Systemic Treatment with Ceftiofur or Penicillin. Dissertation, LMU München: Faculty of Veterinary Medicine
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Abstract

The existence of resistance mechanisms of bacteria against the action of antimicrobial drugs is a well documented fact and has complicated treatment of bacterial disease since the beginning of antimicrobial therapy in the last century. Drug resistance is a problem of veterinary and human medicine alike, especially when considering infections with food-borne pathogens such as Salmonella. Fecal E. coli isolates were used as a model to measure the effect of treating dairy cows systemically with ceftiofur on antimicrobial resistance of this important pathogen. Ceftiofur, a third-generation cephalosporin, is an attractive treatment for certain diseases in dairy cattle such as metritis and interdigital necrobacillosis since withholding times for meat are short and milk does not need to be withheld from marketing. Third-generation cephalosporins are also used in human medicine, for example in the treatment of systemic non-typhoidal salmonellosis in children. Therefore, the emergence and spread of resistance towards this class of antimicrobials has been a focus of concern. Effects of ceftiofur treatment on a group of cattle were compared to a penicillin treated group and a healthy control group not receiving treatment. Reduction of susceptibility of E. coli was tested for ampicillin, tetracycline and three different concentrations of ceftiofur. A significant decrease in the concentration of fecal E. coli on days 2 and 7 post-treatment was detected in animals treated with ceftiofur. Treatment did not have a significant effect on the proportion of isolates expressing reduced susceptibility to ceftiofur. The result of the reduction of total E. coli count in the samples during and after ceftiofur use is consistent with results reported by other authors. The large number of isolates analyzed provided a high possibility of detecting major effects of ceftiofur treatment on E. coli susceptibility. However we did not observe a net increase in reduced susceptible E. coli isolates with the exception of decreased tetracycline susceptibility in the ceftiofur treated group on day 2. The results of this work lead to the conclusion that the emergence and spread of resistance-encoding genes in a bacterial population under the influence of ceftiofur are rare and that the effect could not be measured even though we were able to draw information on a large number of isolates analyzed compared to other investigations previously published on this topic. The results are applicable to commercial dairy herds with typical antimicrobial treatment practices comparable to those used in the US, including prior use of ceftiofur for treating individual sick animals.