A framework for assessing the efficacy of antimicrobials in the control of necrotic enteritis in broiler chickens

This review presents a framework for assessing the efficacy of antimicrobials used to control necrotic enteritis (NE) caused by Clostridium perfringens (CP) in the context of susceptibility testing and clinical efficacy, and their potential interactions with the intestinal microbiota of poultry. Practitioners have traditionally based their choice of antimicrobial agent on antimicrobial susceptibility testing, but there appears to be a lack of correlation with clinical efficacy for in-feed antimicrobials (particularly bacitracin and virginiamycin). Resistance patterns of CP and antimicrobials have been monitored using epidemiological cutoffs for minimal inhibitory concentration (MIC), which are not intended to guide therapy. Also, most data have been determined using CP isolates from healthy birds (i.e., potentially from commensal strains not known to be clinically relevant). It is believed that NE is caused by specific virulent CP strains (and potentially other bacteria) that proliferate and displace these commensals. The presence of resistant commensals is not necessarily detrimental (and may even be beneficial) if they inhibit the single CP strain dominance effect observed in acute NE. The choice of antimicrobial therapy in a clinical setting should thus be based on a variety of factors, including an accurate diagnosis, results of efficacy studies, prior experience at the premises in question, and interpretation of MIC data, recognizing that it is not necessarily well correlated with clinical efficacy.