Vancomycin-resistant enterococci (VRE) in broiler flocks 5 years after the avoparcin ban

The glycopeptide growth promoter avoparcin was banned from animal production in Denmark in 1995. In this study, we investigated the occurrence of vancomycin-resistant enterococci (VRE) in broiler flocks in the absence of the selective pressure exerted by the use of avoparcin. One hundred sixty-two broiler flocks from rearing systems with different histories of avoparcin exposure were investigated for the presence of VRE. Using a direct selective plating procedure, VRE were isolated from 104 of 140 (74.3%) broiler flocks reared in broiler houses previously exposed to avoparcin on conventional and extensive indoor broiler farms. In contrast, only 2 of 22 (9.1%) organic broiler flocks reared on free-range farms with no history of previous exposure to avoparcin were VRE-positive. Furthermore, the occurrence of VRE over time in flocks reared in broiler houses previously exposed to avoparcin was investigated. Results obtained by direct selective plating showed no significant decrease in the proportion of VRE-positive flocks during the study period (1998-2001). This study demonstrated the extensive occurrence of VRE in broiler flocks more than 5 years after the avoparcin ban in Denmark, and indicates that VRE may persist in the absence of the selective pressure exerted by avoparcin. The results differ markedly from previously published Danish surveillance data on VRE in broilers. This may reflect differences in isolation procedures.