Antibiotics are used in broiler poultry production both for the prevention and treatment of infectious diseases. However, antibiotic use is a driver of antibiotic resistance. The World Health Organization has published numerous reports urging all stakeholders concerned with both food-producing animals and humans to establish recommended steps to enhance the prudent use of antimicrobials (WHO, 2015). Similarly, the World Animal Health Organization has also published recommendations and position statements regarding prudent use and risk management related to antimicrobial use in animals (OIE, 2017).

Colibacillosis is an important bacterial pathogen of poultry, and a costly disease for the industry resulting in multimillion dollar losses annually through morbidity, mortality or carcass condemnation at slaughter. Colibacillosis refers to any localized or systemic infection caused entirely or partly by the organism avian pathogenic Escherichia coli (APEC). These bacteria may be isolated as the sole pathogen or contribute to a disease complex with mixed viral and bacterial infections (Guabirara and Schouler, 2015). Two main disease processes important in the broiler industry are early mortality and cellulitis. Early mortality is defined by chicks under a week of age experiencing a higher than normal percentage of deaths in a flock. Early mortality can be caused by many things, for example chilling, overheating, or dehydration, however E.coli infection, or colibacillosis, is one of the main culprits. Colibacillosis can present with omphalitis, yolk sacculitis, enteritis, pasty vents, pericarditis, perihepatitis, polyserositis, congested lungs, splenomegaly and darkened proventriculus or any these combinations (Guabiraba and Schouler, 2015; Geetha and Palanivel, 2018). Many chicks succumb to an early and severe infection or are culled due to excessive morbidity. Antibiotics are typically used to reduce early mortality (Chauvin et al., 2005; Dziva and Stevens, 2008). Those with severe infection are unlikely to survive, however appropriate treatment reduces transmission between birds and improves the suitability of those with a mild infection. Not every labelled drug for E.coli is efficacious, resistance is common (Kabir, 2010) and effectiveness can vary from flock to flock, even within a flock, with more than one strain and more than one treatment.

Understanding the efficacy of antibiotics used to prevent colibacillosis in broiler chickens is essential to optimizing their use; ineffective antibiotics should not be used for prevention or, if there are multiple efficacious antibiotics, their importance to human medicine should be considered when making decisions on antibiotic use. Systematic reviews of randomized controlled trials, and network meta-analysis to provide input on relative antibiotic efficacy, will yield the highest level of evidence for efficacy of treatments under field conditions (Sargeant and O’Connor, 2014).