Association between the use of avilamycin for growth promotion and the occurrence of resistance among Enterococcus faecium from broilers: epidemiological study and changes over time.

This study describes the changes in the occurrence of resistance to avilamycin among Enterococcus faecium from broilers in Denmark and the epidemiological association between usage of avilamycin for growth promotion and the occurrence of avilamycin-resistant E. faecium on broiler farms. The consumption of avilamycin for growth promotion increased from 10 kg in 1990 to 2,740 kg 1996 and decreased in the following years to only 7 kg in 1998. Most of this has been used for broilers. As part of the nationwide monitoring program for antimicrobial resistance, a total of 473 E. faecium isolates from broilers and 290 isolates from pigs have been tested for their susceptibility to avilamycin from 1995 to 1998. A very limited number of isolates from pigs were resistant to avilamycin, whereas the occurrence of resistance among isolates from broilers increased from 63.6% at the end of 1995 to a maximum of 80.7% during the last half of 1996. Since then, the occurrence of resistance has decreased to 23.3% in the last half of 1998. The epidemiological association between consumption of avilamycin and occurrence of resistant E. faecium fecal droppings were examined on 10 poultry farms that had not used avilamycin for growth promotion during 1996 or 1997 and eight farms that had used avilamycin during 1997. We tested a total of E. faecium isolates from the exposed farms and 104 from the nonexposed farms for their susceptibility to avilamycin. Resistant isolates were found on all eight exposed farms, and on seven of 10 nonexposed farms. Sixty-four isolates (72%) from the exposed farms were resistant, compared with 24 (23%) of the isolates from nonexposed farms. The adjusted chi-square p value equaled 0.01065, and showed a significant association between use of avilamycin and occurrence of resistance. The national monitoring program showed a decrease in the occurrence of resistance following a decreased use of avilamycin in Denmark, and the epidemiological study showed a statistically significant association between the use of avilamycin for growth promotion and the occurrence of avilamycin-resistant E. faecium on broiler farms.