Effect of avilamycin fed to chickens on E. faecium counts and on the selection of avilamycin-resistant E. faecium populations.

This study was conducted to investigate the effect of avilamycin used as a growth promoter on the number of E. faecium and on avilamycin-resistant E. faecium in the intestines of broilers over time. Avilamycin was added at 13.6 ppm to the feed of chickens during 28 days or during a typical growth period of 42 days; a nonmedicated group was included. Three hundred twenty-four Ross broiler chickens were equally distributed over the different groups in a treatment trial and kept in three isolation rooms. In each room, two replicates of the three experimental groups were kept in separate pens. At various time points, samples from different intestinal compartments or the feces were serially diluted and plated on avilamycin-supplemented and on unsupplemented Slanetz and Bartley (SL) media, and E. faecium counts were recorded. Only in the feces and only on the last sampling day was a significant decrease noted in the E. faecium counts in chickens treated with avilamycin for 42 days. Intermediate resistant (MIC 4-8 microg/ml) and resistant strains (MIC>or=16 microg/ml) were isolated from all groups, and there was a rise in prevalence over time. Pulsed-field gel electrophoresis profiles of these strains indicated clonal spread from one pen to another within the same room. The ratio between the counts of E. faecium isolated on antibiotic-supplemented to unsupplemented plates was significantly higher at the end of the trial in the feces samples from the group fed avilamycin for 42 days compared to the other groups, indicating a selective effect of avilamycin.