Role of Physiology, Immunity, Microbiota, and Infectious Diseases in the Gut Health of Poultry

Abstract

"Gut health" refers to the physical state and physiological function of the gastrointestinal tract and in the livestock system; this topic is often focused on the complex interacting components of the intestinal system that influence animal growth performance and host-microbial homeostasis. Regardless, there is an increasing need to better understand the complexity of the intestinal system and the various factors that influence gut health, since the intestine is the largest immune and neuroendocrine organ that interacts with the most complex microbiome population. As we face the post-antibiotic growth promoters (AGP) era in many countries of the world, livestock need more options to deal with food security, food safety, and antibiotic resilience to maintain agricultural sustainability to feed the increasing human population. Furthermore, developing novel antibiotic alternative strategies needs a comprehensive understanding of how this complex system maintains homeostasis as we face unpredictable changes in external factors like antibiotic-resistant microbes, farming practices, climate changes, and consumers' preferences for food. In this review, we attempt to assemble and summarize all the relevant information on chicken gut health to provide deeper insights into various aspects of gut health. Due to the broad and complex nature of the concept of "gut health", we have highlighted the most pertinent factors related to the field performance of broiler chickens.

Keywords: alternatives to antibiotics; chicken; gut diseases; gut health; gut integrity; gut–brain axis; immunity; microbiota; oxidative stress.

Figure 1

Proposed gut–brain axis model for chickens. The gut–brain axis refers to the bidirectional communication system between the central nervous system and the GI tract. When chickens encounter enteric stress or inflammation, the intestinal epithelium, enteric muscles, and associated immune cells transmit signals to the brain via the central nervous system (vagus nerve). Further, it induces the leucocytes to release cytokines into circulation from the intestine. Cytokines trigger the activation of the central nervous system. Neurotransmitters produced by chicken gut microbes also induce the central nervous system. All these stimuli from the intestine via the vagus nerve activate the hypothalamic–pituitary–adrenal (HPA) axis and increase the serum corticosterone levels. Corticosterone thereby modulates heterophile migration into the inflammation site of the GI tract to attune inflammation. Activation of the HPA axis leads to sickness behavior in chicken, owing to elevated cortisol secretion.

Figure 2

Major intestinal infectious diseases in poultry. Based on the nature of the causative agent, intestinal infections can be divided into three categories: parasitic, bacterial, and viral diseases.

Figure 3

Factors affecting chicken gut health. A wide range of factors affecting chicken gut health can be categorized into three categories: host factors, feed and feeding factors, and environment and biosecurity factors. A single factor or many factor combinations can significantly affect the structure and physiological function of the chicken gut, altering the gut health and performance.