Zootecnica International - World Poultry Journal

P.B. CRONJE
Nutrition consultant and editor
Buderim, QLD
Australia

The gut health of poultry is important not only because of its effects on productivity, but because impaired gut integrity can increase susceptibility to enteric pathogen invasion. Osmotic stress and heat stress can both damage the lining of the gut. Finely ground and pelleted diets of high digestibility exert considerable osmotic stress on the lining of the gut, which may exceed the ability of the cell to maintain water homeostasis and ionic balance. Many studies have shown that the ability of the gut to withstand osmotic stress can be improved by supplementation with betaine, an organic osmolyte, and that this results in decreased susceptibility to enteric pathogens. During heat stress, blood is diverted to the periphery at the expense of the gut, the integrity of which can be compromised by reactive oxygen species generated by an insufficient supply of nutrients and oxygen. If heat stress is sustained, the ability of the glutathione cycle to prevent reactive oxygen species from destroying cell membranes may be overwhelmed. Supplementation with selenium yeast has been shown to increase the activity of the glutathione cycle in heat stressed poultry, which may prevent heat-induced damage to the lining of the gut.

Introduction

The gut health of livestock species has received scant attention in the past, largely because the threat of pathogens that might have entered the body through an impaired gut lining was negated by the routine use of antibiotic growth promoters. That poultry industries in countries in which antibiotic growth promoters have been banned are now experiencing an increase in health problems and infections associated with enteric pathogens indicates that the gut health of poultry reared in modern production systems is far from optimal. Impaired gut health is economically significant for the industry not only because it affects the productivity of birds, but also because human outbreaks of food borne illnesses associated with poultry products affect the market for poultry products.
Chicken consumption has been identified as a major risk factor for human Campylobacter infections in the USA, New Zealand and the UK, and consumption of egg products has been identified as a major risk factor for human Salmonella infections (see review by Doyle and Erickson, 2006). Although pathogenic organisms are always present in the gut, beneficial gut bacteria and the gut immune system of the host normally restrict the numbers of pathogens present. High numbers of gut pathogens result when these defence mechanisms are impaired. This communication discusses two factors that can impair gut integrity: osmotic stress and heat stress.

Causes of impaired gut health

(a) Osmotic stress
Osmosis refers to the diffusion of water through semi-permeable membranes from solutions that have low concentrations of solutes to those that have high concentrations of solutes. The difference in osmolar concentration between two solutions determines the osmotic pressure exerted on the membrane separating them. Whereas the osmolarity of the interior of the cells lining the gut and that of the blood supply to the gut is relatively constant, the osmolarity of the contents of the gut varies substantially, increasing after a meal as the number of particles in solution (and hence the osmolar concentration) is increased by the action of digestive enzymes on food and decreasing as food particles are absorbed. During digestion, water in the spaces between the cells lining the gut is forced into the lumen of the gut by osmotic pressure and is replaced by water from the capillaries supplying blood to the gut. Water is thus drawn into the gut from the blood system shortly after a meal. As nutrients are absorbed, the osmolarity of digesta decreases and the osmolarity within and between the cells lining the gut increases because of the presence of the absorbed molecules. Consequently, water follows the nutrients back through the gut lining and into the blood. In poultry, the daily flow of water to and from the gut is double the mass of feed consumed (Hoerr, 1998).
The osmolarity of chicken plasma is about 300 mOsmol but the osmolarity of the gut in the fed bird varies from about 900 mOsmol in the duodenum to about 700 mOsmol in the caecum (Klasing et al., 2002), which exerts substantial osmotic pressure on the cells lining the gut and on the tight junctions that bind them together to form a barrier against entry of pathogens. Diets containing high concentrations of feedstuffs that have been processed by grinding or steam flaking to increase the rate and extent of hydrolysis of nutrients by digestive enzymes can increase the osmotic pressure of water moving from the blood into the digesta to such an extent that the junctions between the cells lining the gut give way, permitting microorganisms and toxins to enter the body. Pelleted and finely ground diets have been associated with gut lesions in pigs (Eiseman and Argenzio, 1999) and ruminants (Owens et al., 1998).
Impairment of the integrity of the gut lining of poultry would explain why pelleting is associated with a high incidence of Salmonella in the gut of broilers (Huang et al., 2006) and why the incidence of mortality from necrotic enteritis is greater in poultry fed finely ground feed than in those fed coarsely ground feed (Branton et al., 1987). It is also likely that high osmotic pressure contributes to problems associated with indigestible non¬starch polysaccharides present in barley, rye and wheat diets.

(b) Heat stress
In recent years, medical researchers have adopted a new paradigm to explain the aetiology of heat stroke in humans. Heat stroke has been redefined as a systemic inflammatory response caused by damage to the lining of the gut, which precipitates multi¬organ dysfunction (Bouchama and Knochel, 2002).
One of the first responses of animals to a heat load is to increase the supply of blood to the periphery so that heat can be dissipated by radiation, convection and conduction. If insufficient heat is dissipated by this route, active cooling mechanisms are invoked. In birds, blood flow to the mouth and upper part of the throat is increased and heat is dissipated by panting and gular flutter. Redirection of blood to the periphery, throat and mouth is accomplished by dilation of the blood vessels at these sites. The blood supply to the gut decreases simultaneously to prevent blood pressure from decreasing. If exposure to high environmental temperatures is sustained, the reduced supply of oxygen and nutrients to the gut results in cell damage. Eventually, the gut barrier becomes compromised to such an extent that endotoxin, a component of bacteria in the gut, enters the body. The presence of endotoxin induces an exaggerated inflammatory response from the gut immune system, involving secretion of high levels of tumour necrosis factor and interleukin-1. These cytokines result in several harmful effects, including leakage of blood from capillaries, blood clotting and cell death, which precipitate multiple organ failure and death.
Evidence for the involvement of gut integrity in heat stress and its applicability to livestock species was reviewed by Cronje (2005), who concluded that available evidence on the pathology of heat stress in livestock species is consistent with this paradigm. The potentially additive nature of gut damage induced by osmotic stress and heat stress suggests that strategies to strengthen the immune system of the gut would not only increase productivity but also decrease susceptibility to pathogen invasion and heat stress.

Improvement of gut health

(a) Protection against osmotic stress
Post-prandial osmotic pressure not only induces movement of water from the fluid surrounding the cells of the gut into the lumen of the digestive tract, but also exerts similar pressure on water inside the cells lining the gut. Loss of intracellular water causes cells to shrink, which decreases nutrient absorption and cell membrane transport and impairs important intracellular processes such as the metabolism of amino acids, ammonia, carbohydrates and fatty acids (Häussinger 1996). In extreme cases, the cells shrink to such an extent that they pull away from each other, destroying the tight junctions that bind them together and allowing pathogens and toxins entry to the body.
Cells adapt to normal variations in external osmolarity by transporting ions into or out of the cell to reduce osmotic pressure. This is accomplished by ion pumps, the most important of which is the Na+/K+ pump. Although the efflux of water from gut cells can be prevented by increasing the concentration of ions within the cell, the extent to which Na+ ions can be imported into the cells is limited because high Na+ concentrations inhibit the uptake of nutrients into the cell, which exacerbates the problem by increasing the osmolarity of the digesta still further. The extent to which ions other than Na+ can be imported into the cell is also limited because charged particles such as potassium, magnesium, and phosphate inhibit the action of enzymes necessary for the metabolism of nutrients within the cell.
Because of the limited ability of ion pumps to cope with extreme variations in osmotic pressure, organisms such as marine invertebrates, plants that grow in saline environments and bacteria effectively reduce osmotic pressure by transferring organic osmolytes into and out of their cells. These organisms use organic osmolytes because they can be imported into cells at high concentrations without impairing nutrient absorption or metabolism.
Betaine is an osmolyte that occurs naturally in plants such as sugarbeet and is readily absorbed by the gut, liver and kidney cells of poultry (Kettunen et al. 2001a, b). Because the transport of betaine into the cell is coupled to the diffusion of Na+ and Cl- across the cell wall (Burg, 1995), the uptake of betaine increases when the osmolarity of the fluid outside the cell increases, making it an ideal supplement for improving the resilience of gut cells against osmotic stress.
In in vitro trials, osmolyte supplementation was shown to increase the resilience of cells to osmotic stress by 42% (Moeckel et al., 2002). Kettunen et al. (2001b) showed that betaine supplementation of chickens decreased the movement of water out of intestinal cells when osmotic pressure was increased. Betaine supplementation also improves the morphology of the cells lining the small intestine of poultry and stabilizes the structure of the gut mucosa (Kettunen et al. 2001c). Several studies have shown that dietary betaine supplementation increases nutrient absorption in poultry (reviewed by Eklund et al., 2005), which suggests that the osmotic challenge presented by modern production diets exceeds the ability of the ion pumps of gut cells to maintain normal function. This concept is supported by the results of Moeckel et al. (2002), who showed that betaine reduces the activity of the Na+/K+ pump by 64% and that of the Ca++ pump by 73%. Reduction of ion-pump activity of this magnitude would have a beneficial effect on energy expenditure, as ion pumps consume considerable amounts of energy. In pigs, maintenance energy requirements were reduced by 5.5% by betaine supplementation (Schrama et al., 2003).
The beneficial effects of supplementation with a compound whose main effect is to alleviate osmotic pressure in the gut constitutes compelling support for the hypothesis that modern poultry diets exert excessive osmotic pressure on the gut of poultry and thus compromise gut integrity. Furthermore, the practical benefits of improving gut health using betaine supplementation are reflected in studies showing that it attenuates the decrease in gut villus height associated with coccidiosis, enhances the destruction of coccidia by the immune cells of the gut, reduces the invasiveness of coccidia and decreases the incidence of coccidia¬related gut lesions, (Augustine et al. 1997; Augustine and Danforth 1999; Kettunen et al. 2001c; Klasing et al. 2002).

(b) Protection against heat stress
During heat stress, deprivation of oxygen and energy generates reactive oxygen species within gut cells (Hall et al., 1999). These destructive molecules attack cell membranes, initiating chain-reactions that quickly impair cell structure and membrane integrity. As reactive oxygen species also impair the ability of ion pumps to maintain cellular ion homeostasis, betaine supplementation could also help alleviate susceptibility to the adverse consequences of heat stress. The cell has two main defence mechanisms against reactive oxygen species: glutathione (GSH) and its selenium-containing enzymes, which destroy reactive oxygen species, and heat shock proteins, which repair proteins that have been damaged by reactive oxygen species.
GSH is the most important antioxidant in the body and is essential for normal intestinal function. GSH not only protects gut cells against reactive oxygen species, but is also required for activation of heat shock proteins during heat stress (Rokutan et al., 1996). In order to provide sufficient protection, the GSH cycle requires an adequate supply of selenium and sulfur. It would appear that normal dietary selenium intakes are inadequate for heat stressed poultry because supplementation of broiler chickens with selenium yeast has been shown to increase the activity of the glutathione cycle during heat stress (Mahmoud and Edens, 2003, 2005).
It has also been proposed that sulfur containing amino acids such as methionine may exert beneficial influences on the health and survival of gut cells through their effects on glutathione (Shoveller et al., 2005). Selenomethionine, an organic product derived from selenium-enriched yeast, has been shown to protect cells against oxidative stress (Wan et al., 2006) and may confer greater resilience against damage to the gut and against heat stress than selenium or methionine alone.

Conclusions

The extent of the adverse effect of modern poultry production systems on gut health and poultry productivity is emerging in countries in which antibiotic supplementation has been banned. Available evidence indicates that nutritionally induced osmotic stress and heat stress both contribute to impairment of gut integrity. However, it is unrealistic to expect that attempts to improve gut health by altering the form and composition of diets or by increasing expenditure on cooling systems would be adopted by the poultry industry.
An alternative and more readily acceptable strategy is to increase the resilience of the gut against both these stresses by supplementing the diet with compounds that complement and enhance the natural defence mechanisms of the gut.



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From Proceedings of the “19th Australian Poultry Science Symposium”, New South Wales,