The impact of antimicrobial use in broiler chickens on growth performance and on the occurrence of antimicrobial-resistant Escherichia coli

The manipulation of intestinal microflora of food producing animals with antimicrobials has been recognized as an important tool for improving growth performance and feed efficiency. However, worldwide concern about the biological consequences regarding antimicrobial-resistance development and persistence associated with antimicrobial use in animal husbandry, has led to a more accurate examination of the effects of these drugs on commensal flora. The objective of this study was to investigate, under field conditions, the influence of preventive antimicrobial administration on growth performance and resistance in faecal Escherichia coli isolated from growing broilers. The study took place over three consecutive broiler flocks reared in a controlled environment poultry house (1600 m2) that was previously divided into two equal compartments. In each trial, 32 000 day-old broiler chicks were randomly allocated to each compartment. For a three days period, the birds in the medicated group (n = 16 000) were given therapeutic doses in their drinking water of enrofloxacin, gentamicin and amoxicillin when they were 1, 19 and 26 days old, respectively. The birds in the control group (n = 16 000) shared exactly the same rearing conditions. E. coli strains isolated from medicated group showed higher antimicrobial-resistance rates compared with those recovered from the control group. Multi-resistant strains were transient in control broilers, whereas in medicated broilers they were more prevalent and persisted along the rearing period. Influence of antimicrobial use on broiler chickens performance was also examined. In general, medicated broilers tended to consume more feed (P < 0.01) and grew faster (P < 0.001) than those on the control group. Cumulative feed conversion rate was lower for medicated broiler chickens (P < 0.001). It was also observed that medicated broilers had lower water/feed ratio and cumulative mortality than the non-medicated ones (P < 0.001). These positive effects on performance and viability of chickens are much more perceptible than the emergence and spreading of antimicrobial-resistant strains.