Immunity, immunomodulation, and antibiotic alternatives to maximize the genetic potential of poultry for growth and disease response

Multiple challenges confront the increasing demand for wholesome poultry food products, including governmental restrictions on the use of antibiotic growth promoters (AGPs), nutritional requirements to obtain maximum growth potential, understanding crosstalks among the immunity–microbiota–neuroendocrine system in the gut to maximize intestinal efficiency, high-density production conditions, waste management, and the emergence of infectious pathogens, particularly those that emerge in the antibiotic-free animal production environment. Although in-feed antibiotics have dramatically increased the efficiency of commercial poultry production over the last 50 years, we are now faced with an increasing global crisis concerning the heightened use of antibiotics in animal agriculture and the emergence of multidrug-resistant superbugs that threaten disease management in animals and humans. Therefore, much interest has focused on the development of alternative, antibiotic-free methods of commercial poultry production. Initially, alternatives to antibiotics included any strategies that replace AGPs, but now include any feed additives or treatment that will allow antibiotic-free animal production to prevent and/or treat diseases. These newer disease control strategies can be classified broadly into those that are directly cytotoxic against infectious agents or remove pathogenic toxins, including vaccines, hyperimmune antibodies, antimicrobial peptides, and bacteriophages, and those that augment non-specific host immunity and gut health, including phytochemicals, adjuvants, prebiotics, and probiotics. Furthermore, because the gut microbiota influences various physiological aspects of the immune response, brain function, and gut health, most antibiotic alternatives are expected to promote beneficial microbes that will benefit host physiological responses. However, there is a timely need to better understand the role of the microbiota in gut health if we want to use microbes to modulate the host response to enhance growth performance. This review will highlight current knowledge of host immunity in poultry, and various strategies to modulate host immunity, growth performance, and disease responses to guide the development of alternatives to reduce the use of antibiotics, using a few selected alternatives and a description of their efficacy and modes of action.