The Gastrointestinal (GI) Tract
A key factor in the biological performance, health, welfare and efficiency of livestock is the health of the gastrointestinal (GI) tract. Within the poultry industry there are many initiatives aiming to understand gut health and how it can be influenced to improve production and reduce disease. These initiatives have shown that there is a need for further understanding of gut physiology and the relationship between the gut microbiota and the host. Decades of research in a wide range of animal species has shown that gut health can be influenced by a wide range of intrinsic and extrinsic factors on the farm, and understanding these factors is key for improving gut health as the poultry industry actively reduces antimicrobial use.
Optimal gut tissue development is critical for correct gut function; development of the gut starts during incubation and continues once the chick hatches out of the egg. During the first week of life, the gut tissues undergo rapid development and the villi at the gut surface elongate. During the initial villi growth the villi reach approximately 50% of their final length. If villus growth is impaired during the first two weeks of life it can have long term consequences for the absorptive capacity and efficiency of the gut in the adult bird. The intestinal tract is covered by a single layer of epithelial cells held together by cell junctions; together these create a physical barrier ensuring separation of the gut contents and the internal tissues separate. This barrier prevents bacteria from the gut from entering the body proper; failure of this barrier results in a leaky gut which allows bacteria to translocate into the gut tissues and blood stream the where they can cause disease. The result can be localized issues in the gut tissues; such as in the case of necrotic enteritis where Clostridium perfringens crosses the gut barrier resulting in severe, and sometimes fatal, damage to the gut tissues. Alternatively, when the bacteria enter the bloodstream and become dispersed around the body there can be pathology found in body systems other than the gut such as the liver, the heart and the skeletal structures. Bacterial chondronecrosis with osteomyelitis (BCO) is such a condition where failure of the gut barrier allows gut bacteria to enter the blood stream and gain access to osteochondrotic microfractures in the developing cartilage. Establishment of healthy gut architecture and maintenance of the gut barrier is therefore essential to broiler breeder performance and health. In the older breeder there is a greater risk of the gut barrier failing which increases the likelihood of bacterial translocation. As such the provision of gut health products designed to promote the gut barrier can help reduce the potential of bacterial translocation.
Development and Maintenance of Optimal Intestinal Health
Optimal intestinal health is heavily reliant upon the acquisition and maintenance of a balanced intestinal microbiota, this has become one of the key topics in poultry husbandry. Bacteria reside in all known habitats, therefore it is not surprising that evolution has resulted in symbiotic relationships between an animal and its microbial residents. The intestinal microbiome of an animal is a complex community of micro-organisms dominated by bacteria. The bacteria in the intestinal tract vary in density and species in the different compartments of the gut depending on the local environment. The gut microbiota plays a vital part in the health and well-being of its host by providing a number of benefits. The intestinal microbiota aids digestion, protects against pathogens, produces nutrients and plays a role in the development and maturation of the gut tissues and immune system. As such if the gut microbiota fails to properly develop then the gut tissues and gut immune system will also not develop correctly which will have long-term health implications for the bird.
Intestinal health is a complex area due to the wide range of management and health related factors which can impact upon the function of the gut. The gut and its resident microbiota are both dynamic entities that change as the bird ages, and understanding these changes is key to maintaining intestinal integrity to ensure optimal bird performance. Optimal brooding conditions with good access to feed and clean water are essential for optimal post-hatch gut development and long term gut health of the flock. Another key factor in the development of the villi is stimulation by the intestinal microbiota; villus length has been shown to be stimulated by lactobacilli sp. which are the bacteria which dominate the small intestine. Even though the GI tract of the developing chick in ovo is not completely sterile, the majority of colonization of the gut of the chick occurs post hatch. Sources of bacteria include the farm environment, the feed, the water supply and the litter onto which the birds are placed. The application of probiotics and competitive exclusion agents to day old chicks has been shown to aid the maturation of the intestinal microbiota and enhance the development of the gut tissues.
The Balance of Intestinal Microbiota
There is a delicate balance between the host, the intestinal microbiota, the intestinal environment and dietary compounds. An imbalance in this relationship can alter the composition of the intestinal microbiota and impact upon the integrity of the gut barrier. The shift in microbial populations can have a negative effect on the host leading to poor growth and poor performance – this is seen in cases of dysbacteriosis. Dysbacteriosis is a digestive condition of poultry and has been broadly described as an overgrowth of the intestinal microbiota leading to non-specific enteritis which can result in wet litter. Its aetiology is not fully understood but it is often seen following events which can cause physical or physiological stress on the birds such as feed changes and handling or moving the birds. Stress induced enteric upset is seen in many different animal species and has been shown to be linked to the increase in growth of certain bacterial species. The hormones released during stressful events enter the gut and the activity of certain bacteria, such as E. coli, increases which can lead to enteritis and increased susceptibility to disease. Environmental conditions also play a major role in intestinal health with a key example being the impact of heat stress on the gut. During heat stress blood is diverted to the surfaces of the bird to aid cooling, this results in reduced blood flow to the gut which can result in hypoxia and a build-up of metabolic waste in the gut tissues. The result of this is a failure of the gut barrier which reduces the efficiency of the gut and increases the risk of bacterial overgrowth and disease. Bacteria have a preferred range of nutrient sources; as such the composition of the intestinal microbiota is influenced heavily by dietary formulation. During the life of a flock there are a number of feed changes where there is a shift in energy and protein densities of diets; this invokes a change in the micronutrients available for the gut microbiota. In the event of a feed change, the gut microbiota can become unbalanced as the response to the change varies across the bacterial population and the result can be overgrowth of certain bacteria leading to diarrhea. The use of direct fed microbial products or organic acids over a feed change can inhibit the overgrowth of the less favorable bacteria during these periods and maintain gut integrity to reduce the likelihood of dysbacteriosis occurring.
Key to the Maintenance of Intestinal Health
Understanding the requirements of the developing gut to promote optimal tissue and immune system development. Furthermore being aware of how the intestinal microbiota changes at key points throughout a bird’s life and how it is possible to prepare the bird for these changes can reduce the likelihood of serious intestinal imbalances.
Dr. Anand,AGM-Technology and Product Development
Advanced Bio-Agro Tech Limited (ABTL).