Avian Pathogenic Escherichia coli (APEC): An Overview of Virulence and Pathogenesis Factors, Zoonotic Potential, and Control Strategies

Abstract

Avian pathogenic Escherichia coli (APEC) causes colibacillosis in avian species, and recent reports have suggested APEC as a potential foodborne zoonotic pathogen. Herein, we discuss the virulence and pathogenesis factors of APEC, review the zoonotic potential, provide the current status of antibiotic resistance and progress in vaccine development, and summarize the alternative control measures being investigated. In addition to the known virulence factors, several other factors including quorum sensing system, secretion systems, two-component systems, transcriptional regulators, and genes associated with metabolism also contribute to APEC pathogenesis. The clear understanding of these factors will help in developing new effective treatments. The APEC isolates (particularly belonging to ST95 and ST131 or O1, O2, and O18) have genetic similarities and commonalities in virulence genes with human uropathogenic E. coli (UPEC) and neonatal meningitis E. coli (NMEC) and abilities to cause urinary tract infections and meningitis in humans. Therefore, the zoonotic potential of APEC cannot be undervalued. APEC resistance to almost all classes of antibiotics, including carbapenems, has been already reported. There is a need for an effective APEC vaccine that can provide protection against diverse APEC serotypes. Alternative therapies, especially the virulence inhibitors, can provide a novel solution with less likelihood of developing resistance.

Keywords: APEC; antibiotic resistance; infections; pathogenesis; vaccines; virulence; virulence inhibitors; zoonosis.

Scheme 1

Schematic diagram showing overview of Avian pathogenic Escherichia coli (APEC) infection in chickens along with infection control checkpoints. After entry through oral, nasal, or cloacal routes, APEC colonizes the mucosal sites of gastrointestinal, respiratory, and reproductive tracts without causing disease in chickens. In the presence of concurrent viral or mycoplasma infections or under immunosuppressive or stressed conditions, APEC invades the mucosal layers and reach extra-intestinal organs (heart, liver, lung, spleen, kidney, reproductive organs, etc.) resulting in multi-systemic infections, which are commonly referred to as colibacillosis. Colibacillosis leads to high morbidity and mortality, production losses, and condemnation of carcasses as well as foodborne transmission risk to humans. Colibacillosis can be prevented by the management of stressors, biosecurity measures, and vaccination against APEC or associated viral infections. Chicken breeds with high intrinsic resistance to APEC can be developed through genetic technologies. Vertical transmission of APEC from breeders through contaminated eggs should be monitored to prevent APEC entry into chicken flocks. Antibiotics are commonly used to treat chicken flocks affected with colibacillosis.