Invisible Killer in High Temperatures -Heat Stress in Chickens Ⅲ

Invisible Killer in High Temperatures-Heat Stress in ChickensⅢ

Chicken Gut Health

The chicken's intestine is both a digestive and absorptive organ and the largest immune organ in the body. Therefore, maintaining intestinal health is a prerequisite for ensuring the normal growth and development of chickens.

In order to ensure the normal physiological function of chickens under heat stress, the gut will bear more burden. The depletion of intestinal cells caused by a large amount of extra energy supply, the depletion of immune cells that activate their work in response to damage from harmful substances or stress in the intestine.

In the face of stressors, the intestine is not only an organ that is damaged, but also an organ that aggravates damage to the intestine and other tissues. Once the intestinal mucosal barrier function is damaged and the immune function is disordered, conditions will be created for bacteria and toxins to translocate and invade the body. The invasion of bacteria and toxins will lead to systemic multi-organ dysfunction. The consequence will be an imbalance of the internal environment of the body and the intestinal maintenance. Barriers and normal defenses become initiating a systemic inflammatory response.

1. The structure of the intestinal mucosa

The intestines of chickens are the main place for digestion and absorption of nutrients: the jejunum is mainly responsible for absorbing nutrients, and the ileum plays a vital role in fermentation. Mucosal morphology is key to nutrient absorption and reflects the overall health of the intestine (Ducatelle et al., 2018).

There are many intestinal villi and folds on the intestinal mucosa of chickens. Small intestinal villi are finger-like protrusions of the small intestinal epithelium and lamina propria that bulge toward the intestinal lumen. They have the function of absorbing nutrients. The larger their surface area and the more contact area with nutrients, the stronger the absorption capacity.

The roots of the small intestinal villi and the muscularis mucosae form small intestinal crypts. There are a large number of intestinal cells in the small intestinal crypts, such as goblet cells that synthesize and secrete mucin, Paneth cells that are similar to neutrophils, as well as various immune cells and endocrine cells.

Tight junctions between intestinal epithelia are the main way to maintain the normal structure of cells and play an important role in regulating cell permeability. Intestinal epithelial cells, tight junctions, adherens junctions, gap junctions and desmosome junctions together constitute the mechanical barrier of the intestine.

The intestinal flora has a healthy, mutually beneficial relationship with the host, and plays an important role in
the absorption and utilization of nutrients, the development of the immune system, and the regulation of metabolic homeostasis

2. Gut microbes

Intestinal microorganisms refer to the general term for microorganisms such as bacteria, fungi, and viruses that live in the intestines of chickens. According to the living time of microorganisms in the intestine, they can be divided into native bacteria (also called indigenous microorganisms, most of which are the dominant intestinal flora), symbiotic bacteria (symbiosis depends on the existence of native bacteria) and foreign bacteria (commonly known as passing bacteria, which cannot survive in the body for a long time). The existence of indigenous intestinal microbiota is conducive to maintaining normal host mucosal immune function, while the latter two types have certain potential pathogenicity.

Through 16S sequencing, it was found that the dominant microbial flora in the chicken intestines are mainly Firmicutes, Actinobacteria, Proteobacteria, and Bacteroidetes, accounting for approximately 88% or more. Studies have found that certain intestinal microbiota can stimulate the secretion of antimicrobial peptides, induce the production of immunoglobulins, or directly compete for space and nutrients from intestinal pathogenic bacteria, thereby protecting the intestines and even improving intestinal immunity.

Intestinal microorganisms play a crucial role in promoting the development of intestinal mucosa and intestinal immune system, maintaining the integrity of intestinal mucosal structure, and regulating the host intestinal innate and acquired immune responses. Moreover, intestinal microorganisms can also affect the functions and immune responses of other tissues in the body through the microbiome - gut - brain axis.

Studies have found that under heat stress conditions, the composition of the intestinal microflora of chickens changes. The number of beneficial intestinal bacteria decreases and the number of harmful bacteria increases, leading to the destruction of the intestinal microecological balance. Intestinal microbial imbalance will lead to the destruction of the intestinal mucosal structure of animals, change the natural immune function of the intestine, and increase the susceptibility to intestinal pathogenic bacteria. In this case, the intestinal resistance of chickens decreases and the occurrence of inflammatory diseases increases.

The study found that the intestinal tract of 30-week-old laying hens contains more abundant Firmicutes and Bacteroidetes than those of 8-week-old laying hens. The dominant bacteria in the intestinal tract of broiler chickens are Lactobacillus, Enterococcus, Bacteroidetes, etc. Environmental temperature is one of the important factors affecting the intestinal flora of poultry, and different temperatures and processing times also have different effects on the intestinal flora.

Under stress conditions, the general trend is for Lactobacilli to decrease and E. coli to increase. Stress causes hormonal changes, which in turn affect the secretion of mucus, which in turn reduces the composition of the microorganisms that adhere to it. When the stress exceeds its physiological range, it will cause the microecological imbalance of the digestive tract, and harmful bacteria will multiply in large numbers to produce bacterial toxins, which will damage the tissues and organs of the whole body, and then show pathological conditions.

Chicken intestinal mucosal barrier

The intestine is not only an important organ for digesting and absorbing nutrients, but also an important place for immune function. The intestinal immune system is mainly composed of intestinal epithelial cells, intraepithelial lymphocytes, lamina propria lymphocytes and their secretory immunoglobulin A (sIgA), cytokines (such as growth factors, interleukins, interferons, etc.) .

Intestinal barrier function refers to the function of intestinal mucosa to block intestinal substances and prevent pathogenic antigens from invading the body through the mechanical barrier, chemical barrier, immune barrier and microbial barrier of intestinal epithelium. Ensuring the integrity of intestinal mucosa is crucial for maintaining animal health.

1. Mechanical barrier (physical barrier) function of intestinal mucosa

Mechanical barriers, also known as physical barriers, are particularly important in the intestinal mucosal barrier. The mechanical barrier is a complete intestinal epithelial structure composed of intestinal mucosal epithelial cells and their tight junctions. It can prevent the entry of pathogens, microorganisms and other harmful substances inside and outside the cavity, and at the same time serves as a selective filter to allow nutrients, electrolytes and water to pass.

Mechanical barrier function is determined by the permeability of tight junctions between intestinal epithelial cells, and intestinal permeability increases during high temperature stress.

2. Intestinal mucosal chemical barrier function

The chemical barrier is composed of mucus and digestive juice secreted by intestinal epithelial cells and antibacterial substances secreted by normal bacteria. It can effectively inhibit harmful bacteria and prevent damage to the intestinal mucosa by adverse factors.

The intestinal mucosal epithelium is mixed with a large number of goblet cells, which secrete mucus. The mucus layer is translucent and covers the surface of the intestinal mucosa. Its main components are mucin (MUC) and immunoglobulin, which can combine with bacteria and be excreted in the feces; gastric acid is acidic and can kill most bacteria entering from the mouth. The acidic environment can protect the intestinal tract from pathogenic bacteria. The role of infection; bile salts and bile acids in bile secreted by the liver can combine with bacterial endotoxins or directly degrade endotoxins.

3.  Intestinal mucosal immune barrier function

The intestinal mucosal immune barrier is mainly composed of gut-associated lymphocyte tissue (GALT) and the diffuse immunoglobulins and cytokines secreted by it. Gut-associated lymphoid tissue includes intraepithelial lymphocytes, lamina propria lymphocytes, and intestinal lymph nodes. A large number of lymphoid tissues and cells are distributed among the epithelial cells of the intestinal mucosa, in the lamina propria and in the submucosa, which can promptly identify abnormal antigens and induce the body to produce immune responses to protect the intestines from external invasion and prevent abnormal immune responses.

4. Intestinal mucosal microbiota barrier

Intestinal flora mainly includes mucosal flora and intestinal luminal flora.

The mucosal flora are mainly Bifidobacterium and Lactobacillus; the intestinal flora are mostly Escherichia coli and Enterococcus, which adhere to the intestinal mucosa layer and form a multi-layered intestinal microbial barrier.

Under normal circumstances, the number and distribution of intestinal microorganisms are relatively constant, and the microbial flora is relatively balanced and stable. However, factors such as feed, disease, immunity, and stress may lead to the colonization and invasion of harmful bacteria, and the structure and diversity of the intestinal flora may change. Changes can damage the intestinal mucosa and reduce immunity.

Deng Wen et al. found that heat stress reduced the contents of cecal lactobacilli and bifidobacteria. Song et al. found that heat stress can reduce the number of bifidobacteria and lactobacilli in the intestines of broiler chickens and increase the contents of salmonella and Escherichia coli.

The number and types of microbial flora in the intestinal tract of poultry are easily affected by individual, age, environment and other factors.

In summary, the intestinal mucosal immune system is an important part of the body's immune system, and ensuring the normal physiological state and complete morphological structure of the intestinal mucosa is the basis and key to ensuring normal mucosal immune function.

Original Source: Huimu Power