Vet. Res. 36 (2005) 723-734
DOI: 10.1051/vetres:2005027

Assessing the effect of a single dose florfenicol treatment in feedlot cattle on the antimicrobial resistance patterns in faecal Escherichia coli

Anna Catharina B. Berge^a, William B. Epperson^{b, c} and Robbi H. Pritchard<sup>d

a</sup>  Veterinary Medicine Teaching and Research Center, University of California Davis, 18830 road 112, Tulare, CA, 93274, USA
^b  Department of Veterinary Science, College of Agriculture and Biological Sciences, South Dakota State University, Brookings, South Dakota, 57007, USA
^c  Present address: Department of Veterinary Preventive Medicine, College of Veterinary Medicine, The Ohio State University, 1920 Coffey road, Columbus, Ohio, USA
^d  Department of Animal and Range Science, College of Agriculture and Biological Sciences, South Dakota State University, North Campus drive, Brookings, South Dakota, 57007, USA

(Received 10 August 2004; accepted 21 February 2005)

Abstract - The objective of this clinical trial was to examine the effect of a single dose of florfenicol on antimicrobial resistance patterns in faecal E. coli of feedlot steers. Steers (n = 370), were purchased from two sources and housed in outdoor concrete floored pens. Two cattle from each pen (n = 42 pens, 84 cattle) were randomly selected for faecal sampling at study day 1, 14, 28, and 42. One sampled animal from each of 21 pens was randomly selected to receive a single 39.6 mg/kg dose of florfenicol subcutaneously at study day 11. Ten lactose positive colonies were isolated from faecal swabs and tested for antimicrobial resistance to 11 antimicrobials using the disk diffusion method. Zones of inhibition were grouped using cluster analysis and clusters were ordered by increasing multiple resistance. A cumulative logistic regression model using generalized estimating equations was used to assess factors associated with increasing levels of multiple resistance. Immediately post-treatment, all isolates obtained from treated cattle belonged to multiple resistant clusters containing chloramphenicol resistance. Though less pronounced in later sampling, resistance to chloramphenicol and other antimicrobials persisted. Antimicrobial treatment, sampling time and animal source, as well as interactions between these variables, were important predictors of the odds of E. coli belonging to a more resistant cluster. A very clear but transitory shift to increasingly multiple resistant faecal E. coli in response to florfenicol treatment was observed. There was no indication of horizontal transfer of resistant E. coli between steers. Level of resistance was influenced by complex interaction of animal source and previous management.

Key words: antimicrobial resistance / food animal / florfenicol / Escherichia coli

Corresponding author: William B. Epperson Epperson.1@osu.edu

© INRA, EDP Sciences 2005